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Quasi-decadal signals of Sahel rainfall and West African monsoon since the mid-twentieth century
Abstract
Sahel rainfall shows pronounced decadal variability and a negative trend between wet conditions in the 1950s–1960s and dry ones in the 1970s–1980s. Using continuous wavelet transform the quasi-decadal variability (QDV) of rainfall reveals zonal contrasts. The highest QDV is identified in the 1950s–1960s over western Sahel and in the 1970s–1980s over eastern Sahel. The quasi-decadal atmospheric anomalies has been reconstructed using Fourier transform for the 1950s–1960s and the 1970s–1980s, respectively, and assessed by the composite analysis of the QDV phases for the periods before and after 1968. Over western Sahel, the rainfall QDV in the 1950s–1960s is related to the north Atlantic sea-surface temperature (SST) variability, as highlighted by the wavelet coherence. A southward shift trend of the Inter-Tropical Convergence Zone (ITCZ) is identified through an enhancement of north-easterly fluxes and moisture convergence over the western part of West Africa. A decrease (increase) of southern (northern) subtropical sinking motions seems to be involved. In the 1970s–1980s, a strengthening of cross-equatorial Atlantic SST and pressure gradients is related to an increase of monsoon flow from lower troposphere up to the mid-troposphere and to the northward shift of the ITCZ, mainly over eastern Sahel. The Pacific SST influence is also identified, which involves changes in the global zonal circulation.
... Over West Africa, rainfall variability is driven by the West African Monsoon (WAM) system, which strongly depends on variations in global and regional sea-surface temperature (SST) and regional land-surface conditions at different timescales (e.g., Nicholson et al., 2000;Giannini et al., 2005;Lu and Delworth, 2005;Balas et al., 2007;Rodríguez-Fonseca et al., 2011;Dieppois et al., 2013;Dieppois et al., 2015a). At the interannual timescale, for instance, the WAM dynamic is mainly driven by the El Niño-Southern Oscillation (ENSO; e.g., Giannini et al., 2005;Rodríguez-Fonseca et al., 2015), the Atlantic Equatorial Mode (also referred to as Atlantic Nino; e.g., Losada et al., 2010;Rodríguez-Fonseca et al., 2011) and SST in the Mediterranean Sea (e.g., Rowell, 2003;Gaetani et al., 2010;Fontaine et al., 2011). ...
... At the interannual timescale, for instance, the WAM dynamic is mainly driven by the El Niño-Southern Oscillation (ENSO; e.g., Giannini et al., 2005;Rodríguez-Fonseca et al., 2015), the Atlantic Equatorial Mode (also referred to as Atlantic Nino; e.g., Losada et al., 2010;Rodríguez-Fonseca et al., 2011) and SST in the Mediterranean Sea (e.g., Rowell, 2003;Gaetani et al., 2010;Fontaine et al., 2011). At decadal timescales, West African rainfall variability is mainly driven by the Inter-decadal Pacific Oscillation (IPO) and/or the Pacific Decadal Oscillation (PDO), as well as the Atlantic Multi-decadal Oscillation (AMO) patterns (Biasutti et al., 2008;Mohino et al., 2011;Dieppois et al., 2013Dieppois et al., , 2015b. Persistent behaviour in rainfall across the Sahel has prompted research on development of multi-year rainfall forecasts (Sheen et al., 2017Sheen et al., 2017 and exploration of their application in various sectors (Ward andConway, 2020Ward andConway, 2020). ...
... Nevertheless, according to Berntell et al. (2018), 20CRv2 is better representing decadal rainfall variability in the Sahel, as compared to ERA-20C and CERA-20C, in which decadal signals are significantly negatively correlated to the observation. In addition, this study can be compared to other studies based on 20CRv3 to analyse the atmospheric dynamic associated with interannual to decadal variability in Sahel rainfall (Fontaine et al., 2011;Dieppois et al., 2013) and streamflow (Sidibe et al., 2019). ...
West Africa exhibits decadal patterns in the behaviour of droughts and floods, creating challenges for effective water resources management. Proposed drivers of prolonged shifts in hydrological extremes include the impacts of land-cover change and climate variability in the region. However, while future land-degradation or land-use are highly unpredictable, recent studies suggest that prolonged periods of high-flows or increasing flood occurrences could be predicted by monitoring sea-surface temperature (SST) anomalies in the different ocean basins. In this study, we thus examine: i) what ocean basins would be the most suitable for future seamless flood-prediction systems; ii) how these ocean basins affect high-flow extremes (hereafter referred as extreme streamflow); and iii) how to integrate such nonstationary information in flood risk modelling. We first use relative importance analysis to identify the main SST drivers modulating hydrological conditions at both interannual and decadal timescales. At interannual timescales, Pacific Niño (ENSO), tropical Indian Ocean (TIO) and eastern Mediterranean (EMED) constitute the main climatic controls of extreme streamflow over West Africa, while the SST variability in the North and tropical Atlantic, as well as decadal variations of TIO and EMED are the main climatic controls at decadal timescales. Using regression analysis, we then suggest that these SST drivers impact hydrological extremes through shifts in the latitudinal location and the strength of the Intertropical Convergence Zone (ITCZ) and the W alker circulation, impacting the West African Monsoon, especially the zonal and meridional atmospheric water budget. Finally, a nonstationary extreme model, with climate information capturing regional circulation patterns, reveals that EMED SST is the best predictor for nonstationary streamflow extremes, particularly across the Sahel. Predictability skill is, however, much higher at the decadal timescale, and over the Senegal than the Niger catchment. This might be due to stronger impacts of land-use (-cover) and/or catchment properties (e.g. the Inner Delta) on the Niger River flow. Overall, a nonstationary framework for floods can also be applied to drought risk assessment, contributing to water regulation plans and hazard prevention, over West Africa and potentially other parts of the world.
... Precipitation variability over this region is governed by several factors, which interact in a complex way and present strong interannual to multi-decadal fluctuations Dieppois et al., 2013Dieppois et al., , 2015. For instance, the wet period of the 1950s and the early 1960s was followed by unprecedented droughts in the 1970s and 1980s (e.g. ...
... All this system is marked by fluctuations at the interannual to multi-decadal timescales (Dieppois et al., 2013(Dieppois et al., , 2015, which result in different precipitation anomaly patterns (Janicot, 1992;Nicholson, 2008Nicholson, , 2013: North/South dipole (i.e. +/-or -/+) and monopole (i.e. ...
... Previous studies suggested that different patterns of precipitation variability over West and Central Africa primarily result from fluctuations in global and regional SST at interannual to multi-decadal timescales (e.g. Fontaine et al., 1998;Ward, 1998;Rowell, 2001;Mohino et al., 2011;Rodriguez-Fonseca et al., 2011, Dieppois et al., 2013, 2015Suarez-Moreno et al., 2018). ...
This thesis examined past hydroclimatic trends and variability across West and Central Africa from 1950 to 2005, and their interactions with both catchment properties and large-scale climate patterns. The understanding gained from this study was then used to assess the impact of near-term climate change on hydrological regimes. Two imputation methods were successfully applied to assemble a new reconstructed streamflow dataset and time series analyses revealed high non-stationarity in annual streamflow, with two step-changes occurring at the regional scale in 1970 and 1993 respectively. The results also indicated good agreement between precipitation and streamflow fluctuations from one decade to another. Moreover, detailed analysis of streamflow variability modes highlighted significant interannual to multi-decadal fluctuations which were found to be associated with large-scale climate variability and modulated by catchment physical properties. Building upon the detected streamflow-sea surface temperature teleconnections, a multi-timescale linear regression model was built and applied in combination with two hydrological models (GR2M and IHACRES) to examine the
potential impacts of climate change on hydrological systems by the mid-21st century. The results highlighted a zonal contrast in future precipitation between western (dry) and eastern (wet) Sahel, and a robust signal in rising temperature, suggesting an increase in potential evapotranspiration, which are likely to induce a slight significant increase in discharge (~+5%) at the regional scale. More importantly, the findings indicated that uncertainties in streamflow predictions inherent to models and gridded observational datasets quality in Central Africa, could be narrowed by the teleconnections-based regression model.
... Over West and Central Africa, rainfall variability is dominated by the West Africa Monsoon (WAM) system, which strongly depends on fluctuations in global sea surface temperature (SST) and regional land surface conditions at different timescales (e.g. Nicholson et al., 2000;Giannini et al., 2005;Lu and Delworth, 2005;Balas et al., 2007;Dezfuli and Nicholson, 2013;Dieppois et al., 2013Dieppois et al., , 2015. These linkages between global SST and West and Central African rainfall are mainly established through modulations of regional atmospheric circulation features, namely the upper-level Tropical Easterly Jet (TEJ), the midlevel African Easterly Jet (AEJ), low-level westerlies (LLW) and the Inter-Tropical Convergence Zone (Nicholson and Grist, 2001;Nicholson, 2013;Dezfuli, 2017). ...
... At decadal to multi-decadal timescales, West African rainfall variability is mainly driven by the Inter-decadal Pacific Oscillation (IPO) and the Atlantic Multi-decadal Oscillation (AMO) patterns (e.g. Biasutti et al., 2008;Mohino et al., 2010;Dieppois et al., 2013). Central African rainfall variability and associated SST teleconnections remain relatively less understood at these timescales. ...
... In Section 5, similarly to Dieppois et al. (2013Dieppois et al. ( , 2016Dieppois et al. ( , 2019, composite analyses are performed to investigate teleconnections between streamflow and large-scale climate variability at multiple timescales. Two sets of large-scale climate variables, i.e. ...
... Other sources claimed that the region was entering a more humid period (e.g., Ozer et al., 2003). Complicating the issue was the fact that "recovery" showed strong geographical variations and that a handful of very wet years occurred during what was a relatively dry post-1980 interval (Nicholson, 2005;Lebel and Ali, 2009;Dieppois et al., 2013;Diatta and Finkc, 2014;Hastenrath and Polzin, 2014). In some years, devastating flood conditions were reported (Ozer et al., 2003;Paeth et al., 2011), consistent with the recent increase in extreme rainfall events (Panthou et al., 2014;Sanogo et al., 2015). ...
... This indicates notable geographical variations in the recovery, with the strongest recovery taking place in the east. This is in agreement with previous findings of Nicholson (2005), Lebel and Ali (2009), and Dieppois et al. (2013), based on a considerably shorter period of record. ...
... Except at 0 W, the difference is significant at the 1 or 2% level. This agrees with the finding of Dieppois et al. (2013) for the 1970s and 1980s. Except at 8 E, the rainfall maxima (Figure 12, right) were significantly further south (one-half to one degree of latitude) post-1968 than pre-1968. ...
The Sahel region is known for the multi-decadal occurrence of severe drought that
commenced in the late 1960s. A still open question is whether or not the region’s
rainfall has returned to “normal.” This paper provides a compelling answer to that
question by examining the longest and most comprehensive gauge series for the
region ever published. It extends from 1854 to 2014 and is based on 602 gauge
records. A comparative series for the Guinea Coast region to the south is also presented,
as the two regions collectively provide insight into the long-term variability
of the West African monsoon. In contrast to many previous studies, here the
question of recovery and regime change is not restricted to the core of the Sahelian
rainy season (July–September), but is separately discussed also for the coastal
phase (April–May), transition phase (June), Sahelian phase (July–September) and
the second transition phase (October–November) of the West African monsoon.
These analyses suggest that full recovery from the droughts of the 1970s and
1980s has not occurred that a major change in the rainfall regime occurred around
1968 and that since that time large-scale teleconnections have also changed markedly.
The shift post-1968 is evident in all phases of the monsoon except the
coastal phase, in which a change to drier conditions occurred a decade later. Overall,
recovery has been greater in the east than in the west, creating a change in the
climatological east–west rainfall gradient. The drier post-1968 conditions appear
to be associated with a general weakening of the intensity of the West African
monsoon and only a small southwards displacement of the rainfall maximum.
These changes have strong implications for the future of this region and for seasonal
prediction.
... This highlights a potential hiatus in the regional drying trend during the 1970s and 1980s, supporting earlier studies (Le Barbé and Lebel, 1997;D'amato and Lebel, 1998). These could result from increasing quasi-decadal rainfall variability as suggested in Dieppois et al. (2013Dieppois et al. ( , 2015. In the post-1993 period, we note an increase of annual precipitation compared to the previous period (trends significant for 11% of the study area), corroborating previous findings (Biasutti, 2013;Lebel and Ali, 2009;Nicholson et al., 2000). ...
... According to Mahé et al. (2013), Cluster 1, which is located at the outlet of the Congo Basin at Brazzaville, could be more sensitive to changes in the thermal gradient between the Atlantic and Indian oceans resulting in a unique runoff variability. Such decadal fluctuations have also been reported for eastern Sahel rainfall in Dieppois et al. (2013Dieppois et al. ( , 2015, suggesting that differences between clusters should at least partly be related to different interactions with catchment properties (e.g. reduction in soil water holding capacity) and water management. ...
... Therefore, rather than describing the "Sahelian paradox" as an increase in runoff despite reduced rainfall since 1970, it should be considered as enhancing runoff response to positive rainfall anomalies, as a result of changes in land-surface properties. If flow trends can be largely explained by decadal variability in rainfall (Dieppois et al., 2013), influence of other driving factors should also be considered at the catchment level (such as geology, soils, agricultural land use change, water consumption and urbanization). For instance, large dams constructed in the 1980s in Nigeria (e.g. the Dadin Kowa Dam and the Kiri dam, on a main tributary of the Benue river), might have affected to some extent the variability of the lower Niger river, but this is beyond the scope of the present paper. ...
Over recent decades, regions of West and Central Africa have experienced different and significant changes in climatic patterns, which have significantly impacted hydrological regimes. Such impacts, however, are not fully understood at the regional scale, largely because of scarce hydroclimatic data. Therefore, the aim of this study is to (a) assemble a new, robust, reconstructed streamflow dataset of 152 gauging stations; (b) quantify changes in streamflow over 1950 – 2005 period, using these newly reconstructed datasets; (c) significantly reveal trends and variability in streamflow over West and Central Africa based on new reconstructions; and (d) assess the robustness of this dataset by comparing the results with those identified in key climatic drivers (e.g. precipitation and temperature) over the region. Gap filling methods applied to monthly time series (1950-2005) yielded robust results (median Kling-Gupta Efficiency >0.75). The study underlines a good agreement between precipitation and streamflow trends and reveals contrasts between western Africa (negative trends) and Central Africa (positive trends) in the 1950s and 1960s. Homogenous dry conditions of the 1970s and 1980s, characterized by reduced significant negative trends resulting from quasi-decadal modulations of the trend, are replaced by wetter conditions in the recent period (1993-2005). The effect of this rainfall recovery (which extends to West and Central Africa) on increased river flows are further amplified by land use change in some Sahelian basins. This is partially offset, however, by higher potential evapotranspiration rates over parts of Niger and Nigeria. Crucially, the new reconstructed streamflow datasets presented here will be available for both the scientific community and water resource managers.
... In West Africa, shifts in time series of rainfall have been observed: a wet period occurred between 1930 and 1960, a drought from 1970 to 1980 and gradual return of normal rains in the period from 1990 to 2000. Change in precipitation frequency, intensity, duration and consequently on the hydrological cycle has then been notified by many authors [4] [10]. In Senegal, this style of agricultural practice employs 77% of working people and supplies 12.4% of the daily food [11]. ...
... The quasi-decadal variability of the rainfall in the Sahel has been studied. Results show a zonal contrast in rainfall behavior, but also a downward trend between the wet period ranging from 1950 to 1960 and the dry ones from 1970 to 1980 [10]. The variation of the volume of the Lake Naivasha, trend, flow rate and local rainfall variability has been studied. ...
The main purpose of this study is to assess the climate variability and change using statistical processing tools. The climate variability and change are assessed through highlighting the annual and monthly rainfall behavior between 1970 and 2010 in six strategical locations of Senegal according to the three climate bands: one in the warm desert zone (Saint-Louis), four in the warm semi-arid band (Dakar, Kaolack, Bakel and Tambacounda) and one in the tropical savanna (Ziguinchor). Further, differences in sensitivity of the statistical tests are exhibited applying several tests rather than a single one to check for a feature. The dependency of the results from statistical tests to studied sequences of the time series is also shown by applying the same statistical tests on two sequences (1970-2010 and 1960-2010). Between 1970 and 2010, exploratory data analysis (EDA) is made to give in a visible manner a first idea on the repartition and the behavior of the rainfall through the study area. Then, statistics characteristics such as the mean, the variance, the standard deviation, the coefficient of variation, the skewness and kurtosis are calculated. Subsequently, the statistical tests are applied to time series from all retained raingauges. The Kendall and the Spearman rank correlation tests allow to verify the independency of the annual rainfall from all raingauges. The Hubert procedure for times series segmentation, the Pettitt, the Lee Heghinian and the Buishand tests allow to check for shift in annual rainfall series. The presence of trend in rainfall is studied by first employing the Mann-Kendall test at annual, seasonal and the monthly scales. At annual scale, Sen’s slope estimator is used to estimate the magnitude of the trends. Finally, all above tests are applied to time series from 1960 to 2010 in order to highlight the dependency of the statistical tests to studied sequence of the time series. Explanatory data analysis indicates upwards trends for rainfall in northern and central (warm desert and North site of the warm semi-arid) Senegal and absence of trend for those in Southern (tropical savanna and South site of the warm semi-arid zone). The Kendall and Spearman rank correlation tests have led to same conclusions and confirm assumptions from EDA. They also reveal the fact that the Kendall tau test can be used as alternative to the Spearman rho test. According to the two tests, independency was only found for rainfall at Ziguinchor and Tambacounda. The difference in the sensitivity of the statistic tests was more exhibited by issues from tests for shift detection. Indeed, tests for shift detection lead to different results either in the dates of the shifts or in the significance of the shift. A synthesis analysis of the results of the tests was carried out to conclude about shift in rainfall. Thus, rainfall in Southern are homogeneous (Tambacounda and Ziguichor), while in Northern and Central shift occurs in rainfall series. Tests for trend confirms the upward trends in Northern and central from EDA. Sen’s slop estimator shows that all retained trend can’t be consider as of linear type. This seems to show a return to a wet period as noticed in many studies. Tests applied to time series ranging from 1960 to 2010 have led to different results in term of independency, shift or trend in comparison to their application between 1970 and 2010. Therefore, the results of the statistical tests may depend on the time series sequence.
... At the multi-decadal scale, warmer TSA SSTs are likely associated with observed drier conditions at stations over the eastern Sahel and a part of the central Sahel ( Fig. 10(a)). Northeastward latitudinal modulations of the ITCZ might be involved in agreement with Losada et al. (2010) and Dieppois et al. (2013). At the quasidecadal scale, variations of the inter-hemispheric SST gradient (i.e. ...
... This favoured latitudinal modulations in the ITCZ and therefore the existence of successive drier (1969-1975 and 1981-1990) and wetter (1976)(1977)(1978)(1979)(1980) conditions at Mopti, eastern Sahel ( Fig. 10(b)). This is consistent with the findings of Dieppois et al. (2013), showing that a strengthening of cross-equatorial Atlantic SST and pressure gradients is related to an increase of monsoon flows and a northward shift of the ITCZ over the eastern Sahel. ...
This study systematically examines teleconnections between Atlantic sea surface temperature (SST) and the west–east distribution of Sahel rainfall throughout the 20th century, taking non-stationarity into account. Sahel rainfall variability of six selected rain gauges displays three dominant time-scales: multi-decadal (>20 year), quasi-decadal (8–18 year) and interannual (2–8 year). Regarding their patterns of low-frequency scales, three coherent Sahelian sub-regions can be identified: the Atlantic Coast (Dakar), western-central Sahel (Nioro and Mopti) and eastern Sahel (Niamey, Maradi, Maine-Soroa). Cross-analyses combining spectral and multivariate analysis of 20 station-based data and West-African gridded rainfall data statistically confirm dissimilarities between the western and eastern Sahel. Western and eastern Sahel rainfall data are correlated with SSTs from different regions of the Atlantic Ocean, especially in the North and tropical South Atlantic. As determined by wavelet coherence and phase, in phase relationship with North Atlantic SSTs only occurs in wet periods and at the multi- and quasi-decadal scales. This teleconnection depends on the time-period and the time-scale, displaying a NW-SE pattern, which suggests non-uniform modulations of meridional displacements of the ITCZ. Tropical South Atlantic SST variability is often related to opposite patterns between the Gulf of Guinean Coast (in phase) and Sahel region (out of phase).
... Using numerous filtering methods (FFT band-pass; MODWT: maximum overlap discrete wavelet transform; EEMD: ensemble empirical mode decomposition), the standard deviations of QDV in Sahel rainfall are approximately of 50 mm.year -1 with time-space fluctuations over West-Africa (Dieppois et al., 2013b). As an illustration, the QDV signals of eastern Sahel are reinforced (+/-9 mm.month −1 ) in the 1970/80s, which led to wet anomalies (W) in 1976–1981 and dry anomalies (D) in 1968–1974 and 1983–1989 (Fig. 1c ). ...
... In the 1970s/80s wet life-cycles (and conversely during the dry life-cycles), a strengthening of both equatorial Atlantic SST (Dieppois et al., 2013b) and the pressure gradient can be related to the enhancement of monsoon flow and moisture convergence up to the mid-troposphere, and therefore a northward shift of the ITCZ. African Easterly Jet speed variations could also be identified (Nicholson and Webster, 2007). ...
West-African Sahel rainfall shows a large decadal signal, transitioning from wet conditions in the 1950s to dry conditions in the 1970s/80s. Only the quasi-decadal time-scale is significantly detected in the Sahel rainfall index during the 1970s and 1980s. This time-scale of rainfall variability appears to be related to two periods of dry conditions and one of relatively wet conditions in the mid-70s. The dominant quasi-decadal anomalies of West-African Monsoon have been reconstructed using Fourier transform for the 70s–80s, and subsequently assessed by the differences in wet and dry states. A strengthening of cross-equatorial Atlantic SST and pressure gradients can be related to an enhancement of monsoon flow and the northward shift of the ITCZ.
... For example, several earlier studies [33][34][35] used the DWT to identify the dominant periodic components in the data with trend analysis. The general applications of WT-based studies are significantly increasing in studying the hydrological process [36][37][38][39], water quality [40], streamflow prediction [41,42]; rainfall [43][44][45]; drought [46][47][48][49]; and trend analysis [50]. The scalogram is the absolute value of the continuous wavelet transform (CWT) of a signal, plotted as a function of time and frequency. ...
Over time, the initial algorithms to derive atmospheric density from accelerometers have been significantly enhanced. In this study, we discussed one of the accurate accelerometers—the Earth’s Magnetic Field and Environment Explorers, more commonly known as the Swarm satellites. Swarm satellite-C level 2 (measurements from the Swam accelerometers) density, solar index (F10.7), and geomagnetic index (Kp) data have been used for a year (mid 2014–2015), and the different types of temporal (the diurnal, multi-day, solar-rotational, semi-annual, and annual) atmospheric density variations have been investigated using the statistical approaches of correlation coefficient and wavelet transform. The result shows the density varies due to the recurrent geomagnetic force at multi-day, solar irradiance during the day, appearance and disappearance of the Sun’s active region, Sun–Earth distance, large scale circulation, and the formation of an aurora. Additionally, a correlation coefficient was used to observe whether F10.7 or Kp contributes strongly or weakly to annual density, and the result found a strong (medium) correlation with F10.7 (Kp). Accurate density measurement can help to reduce the model’s bias correction, and monitoring the physical mechanisms for the density variations can lead to improvements in the atmospheric density models.
... Such relationships are similar to those detected for rainfall over the region (e.g. Mohino et al. 2011;Rodríguez-Fonseca et al. 2015;Dieppois et al. 2013Dieppois et al. , 2015a. Building upon these teleconnections, we thus use multiple regressions of annual streamflow on empirical orthogonal functions (EOFs) of SST fields following the modelling strategy developed by Benestad (2001). ...
Climate change is expected to significantly impact on the availability of water resources in West and Central Africa through changes in rainfall, temperature and evapotranspiration. Understanding these changes in this region, where surface water is fundamental for economic activity and ecosystem services, is of paramount importance. In this study, we examine the potential impacts of climate variability and change on hydrological systems by the mid-21st century in West and Central Africa, as well as the uncertainties in the different climate-impact modelling pathways. Simulations from nine global climate models downscaled using the Rossby Centre Regional Climate model (RCA4) are evaluated and subsequently bias-corrected using a nonparametric trend-preserving quantile mapping approach. We then use two conceptual hydrological models (GR2M and IHACRES), and a regression-based model built upon multi-timescale sea surface temperatures and streamflow teleconnections, to understand hydrological processes at the subcontinental scale and provide hydrological predictions for the near-term future (2020-2050) under the RCP4.5 emission scenario.
The results highlight a zonal contrast in future precipitation between western (dry) and eastern (wet) Sahel, and a robust signal in rising temperature, suggesting an increase in potential evapotranspiration, across the multi-model ensemble. Overall, across the region, a significant increase in discharge (~+5%) is expected by the mid-21st century, albeit with high uncertainties reported over most of Central Equatorial Africa inherent to climate models and gridded observation data quality. Interestingly, in this region, teleconnections-based regression models tend to be an alternative to hydrological models.
... Fontaine and Janicot (1996) later simplified this classification as they found that the western and continental Sahel rainfall anomalies were of the same sign for the majority of stations they analyzed. More recently, east-west contrasts have also been noted at decadal timescales (Dieppois et al., 2013) as well as at longer timescales, related to the long-term trends in response to climatic forcing (e.g., Lebel et al., 2009;Monerie et al., 2012;Biasutti, 2013). ...
The ability of the Artificial Neural Network (ANN) and the Multiple Linear Regression (MLR)
in reproducing the area-average observed daily precipitation during the rainy season
(Feb–Mar–Apr) over the north of the Northeast of Brazil (NEB) is examined. For the present
climate of Dec-Jan-Feb from 1963 to 2003 period these statistical models are developed
and validated using the observed daily precipitation and simulated from the historical
outputs of four models of the fifth phase of the Coupled Model Intercomparison Project
(CMIP5). The simulations from all the models during DJF and FMA seasons have an
anomalous intensification of the ITCZ and southward displacement in comparison with
the climatology. Correlations of 0.54, 0.66, and 0.66 are found between the simulated
daily precipitation of the CCSM4, GFDL_ESM2M, and MIROC_ESM models during
DJF season and the observed values during FMA season. Only the CCSM4 model
displays a slightly reasonable agreement with the observations. A comparison between
the statistical downscaling using the nonlinear (ANN) and linear model (MLR) to identify
the one most suitable for the analysis of daily precipitation was made. The ANN technique
provides more ability to predict the present climate when compared to MLR technique.
Based on this result, we examined the accuracy of the ANN model in project the changes
for the future climate period from 2055 to 2095 over the same study region. For instance,
a comparison between the daily precipitations changes projected indirectly from the ANN
during Feb–Mar–Apr with those projected directly from the CMIP5 models forced by RCP
8.5 scenario is made. The results suggest that ANN model weights the CMIP5 projections
according to the each model ability in simulating the present climate (and its variability).
In others, the ANN model is a potentially promising approach to use as a complementary
tool to improvement of the seasonal numerical simulations.
... Fontaine and Janicot (1996) later simplified this classification as they found that the western and continental Sahel rainfall anomalies were of the same sign for the majority of stations they analyzed. More recently, east-west contrasts have also been noted at decadal timescales (Dieppois et al., 2013) as well as at longer timescales, related to the long-term trends in response to climatic forcing (e.g., Lebel et al., 2009;Monerie et al., 2012;Biasutti, 2013). ...
Chapter: Refinement of the daily precipitation simulated by the CMIP5 models over the north of the Northeast of Brazil (da Silva, G. A. M and Mendes D.)
The ability of the Artificial Neural Network (ANN) and the Multiple Linear Regression (MLR) in reproducing the area-average observed daily precipitation during the rainy season (Feb–Mar–Apr) over the north of the Northeast of Brazil (NEB) is examined. For the present climate of Dec-Jan-Feb from 1963 to 2003 period these statistical models are developed and validated using the observed daily precipitation and simulated from the historical outputs of four models of the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The simulations from all the models during DJF and FMA seasons have an anomalous intensification of the ITCZ and southward displacement in comparison with the climatology. Correlations of 0.54, 0.66, and 0.66 are found between the simulated
daily precipitation of the CCSM4, GFDL_ESM2M, and MIROC_ESM models during DJF season and the observed values during FMA season. Only the CCSM4 model displays a slightly reasonable agreement with the observations. A comparison between the statistical downscaling using the nonlinear (ANN) and linear model (MLR) to identify
the one most suitable for the analysis of daily precipitation was made. The ANN technique provides more ability to predict the present climate when compared to MLR technique. Based on this result, we examined the accuracy of the ANN model in project the changes for the future climate period from 2055 to 2095 over the same study region. For instance, a comparison between the daily precipitations changes projected indirectly from the ANN during Feb–Mar–Apr with those projected directly from the CMIP5 models forced by RCP 8.5 scenario is made. The results suggest that ANN model weights the CMIP5 projections according to the each model ability in simulating the present climate (and its variability).In others, the ANN model is a potentially promising approach to use as a complementary tool to improvement of the seasonal numerical simulations.
... Thus, according to our simulations, the simultaneous increase in annual totals and interannual rainfall variability would lead to antagonistic effects on the resilience of the system and, therefore, an uncertain future. The red noise model used to assess the effect of rainfall variability poorly represents the strong decadal structure of Sahelian rainfall (Dieppois et al 2013), and the effect of the temporal structure of rainfall may be stronger than estimated. Furthermore, a larger intraseasonal variability is also anticipated in a warmer climate (Martin 2018), which could also affect the potential for resilience. ...
The Sahel (semi-arid fringe south of the Sahara) experienced a long and prolonged drought from the 1970s to the mid-1990s, with a few extremely severe episodes that strongly affected ecosystems and societies. Long-term observations showed that surface runoff has increased during this period, despite the rainfall deficit. This paradox stems from the soil degradation that was induced by various factors, either directly linked to the drought (impact on vegetation cover), or, in places, to human practices (land clearing and cropping). Surface runoff is still increasing throughout the region, suggesting that Sahelian eco-hydrosystems may have shifted to a new hydrological regime. In order to explore this issue, we have developed a simple system dynamics model incorporating vegetation-hydrology interactions and representing in a lumped way the first order processes occurring at the hillslope scale and the annual time-step. Long term observations on a pilot site in northern Mali were used to constrain the model and define an ensemble of plausible simulations. The model successfully reproduced the vegetation collapse and the runoff increase observed over the last 60
years. Our results confirmed that the system presents two alternative states and that during the drought it shifted from a high-vegetation/low-runoff regime to the alternative low-vegetation/high-runoff one where it has remained trapped until now. We showed that the mean annual rainfall deficit was sufficient to explain the shift. According to the model, vegetation recovery and runoff reduction are possible in this system, but the conditions in which they could occur remain uncertain as the model was only constrained by observations over the collapse trajectory. The study shows that the system is also sensitive to the time variability of rainfall, and that larger variability leads to higher runoff. Both mean rainfall and rainfall variability may increase in central Sahel under climate change, leading to antagonist effects on the system, which makes its resilience uncertain.
... Moreover, similar results were also obtained by Mahé and Paturel (2009) who indicated that Sahelian rainfall has increased since the end of the 1990s, but the annual average rainfall is still as low as during the drought of the 1970s. The existence of great interannual variability in all variables (MAXAN, annRAIN and DURATION) at all stations was raised by most of the studies that have been carried out on climate variability in West Africa (Conway et al., 2009;Dieppois et al., 2013;Le Barbé et al., 2002;Nicholson, 2013;Paeth et al., 2005). Rainfall in West Africa is generally generated by the West African monsoon regime. ...
Study region: The Kara River basin, northern Togo and Benin, West Africa.
Study focus: This study investigated long-term trends in annual rainfall (annRAIN), annual rainfall duration (DURATION) and annual maximum rainfall (MAXAN) for seven stations between 1950 and 2010. A Bayesian trend analysis was performed by fitting the Lognormal, Normal and Generalized Extreme Value (GEV) distributions to annRAIN, DURATION and MAXAN, respectively, with a time covariate for both the location and scale parameters. Spatio-temporal variation of the mean decadal rainfall and the seasonality of the mean monthly rainfall were also analyzed.
New hydrological insights for the region: The results indicate that the interannual variability of annRAIN is decreasing over time at all stations, and the average annRAIN is also markedly decreasing at several stations. However, DURATION is increasing at most stations suggesting that in those parts of the basin where annRAIN is decreasing, rainfall occurs more frequently but with less intensity. For MAXAN, evidence for decreasing trend is found in two stations, and for increasing trend in one station. It is also shown that the peak of the rainy season shifted from September to August since the 1980s. Furthermore, changes in the spatio-temporal distribution of the mean decadal rainfall are also observed. This study provides valuable new insights into trends affecting rainfall variables in the Kara River basin.
... Le climat actuel de la zone d'étude est tropical de mousson caractérisé par l'alternance de 2 saisons: une saison pluvieuse dont la durée diminue du sud vers le nord suivant l'installation de la mousson. Le fonctionnement de cette mousson ouest africaine ainsi que sa variabilité temporelle ont été étudiés par plusieurs chercheurs (Dieppois, 2013;Jury, 2002;Weldeab, 2007). La variabilité de cette mousson a comme conséquence la variation des limites des différentes zones agroécologiques du nord vers le sud. ...
The reconstruction climate changes using paleoclimatic methods based on sunspots and the major historical events that characterized Western Sudan from 850 to 1900 have been related in this research. The objective was to identify the importance of climatic fluctuations on the stability of the political entities of the area and to verify certain oral tradition statements showing the role of successive years of drought in the decline of empires especially of the empire of Ghana . The results show that the birth periods of the West African empires correspond to phases of climatic improvement characterized by a return of wet phases while the declines correspond to periods of drought. This confirms the data of the oral tradition when it states that the empire of Ghana was destroyed by 7 years, 7 months, 7 weeks of drought. It is clear that the consequences of these climatic minima on sociopolitical equilibrium have been very important: weakening of the empires with population migrations to more favorable areas such that in the end the empires were no longer able to resist to external enemies ( case of almoravids invasion). Therefore, added to the politico-economic causes generally cited to explain the political events of the region, it is important to give an importance to the natural conditions including climate change and its consequences.
... The Atlantic Ocean's effect (Fig. 3l) acts throughout the entire year with distinct dipole patterns and intermittent pulses but is relatively weak. The role of Atlantic SST in producing this dipole has been discussed in several studies (e.g., Knight et al. 2006;Mohino et al. 2011;Dieppois et al. 2013). The Indian Ocean SST (Fig. 3m) has the largest impact in the winter and early spring during the period when the ITCZ moves northward. ...
The second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II) is designed to improve understanding of the possible roles and feedbacks of sea surface temperature (SST), land use land cover change (LULCC), and aerosols forcings in the Sahel climate system at seasonal to decadal scales. The project’s strategy is to apply prescribed observationally based anomaly forcing, i.e., “idealized but realistic” forcing, in simulations by climate models. The goal is to assess these forcings’ effects in producing/amplifying seasonal and decadal climate variability in the Sahel between the 1950s and the 1980s, which is selected to characterize the great drought period of the last century. This is the first multi-model experiment specifically designed to simultaneously evaluate such relative contributions. The WAMME II models have consistently demonstrated that SST forcing is a major contributor to the twentieth century Sahel drought. Under the influence of the maximum possible SST forcing, the ensemble mean of WAMME II models can produce up to 60 % of the precipitation difference during the period. The present paper also addresses the role of SSTs in triggering and maintaining the Sahel drought. In this regard, the consensus of WAMME II models is that both Indian and Pacific Ocean SSTs greatly contributed to the drought, with the former producing an anomalous displacement of the Intertropical Convergence Zone before the WAM onset, and the latter mainly contributes to the summer WAM drought. The WAMME II models also show that the impact of LULCC forcing on the Sahel climate system is weaker than that of SST forcing, but still of first order magnitude. According to the results, under LULCC forcing the ensemble mean of WAMME II models can produces about 40 % of the precipitation difference between the 1980s and the 1950s. The role of land surface processes in responding to and amplifying the drought is also identified. The results suggest that catastrophic consequences are likely to occur in the regional Sahel climate when SST anomalies in individual ocean basins and in land conditions combine synergistically to favor drought.
... General applications of wavelet based approaches are being increasingly used in studying hydrological processes, such as time series analysis (Pan et al., 2005;Westra and Sharma, 2006;Karthikeyan and Nagesh Kumar, 2013;Sang et al., 2016), water quality in agricultural watershed (Kang and Lin, 2007), stream flow prediction (Smith et al., 1998;Bayazit et al., 2001), rainfall-runoff relation (Labat et al., 2000), drought forecasting (Ozger et al., 2011;Özger et al., 2012), trend analysis of evapotranspiration and its relation to draught (Madhu et al., 2015), prediction of meteorological drought (Kim and Valdés, 2003), spatial and temporal variability of drought (Ujeneza and Abiodun, 2014;Wang et al., 2015), variability in monsoon rainfall in West Africa (Dieppois et al., 2013) etc. ...
... Fontaine and Janicot (1996) later simplified this classification as they found that the western and continental Sahel rainfall anomalies were of the same sign for the majority of stations they analyzed. More recently, east-west contrasts have also been noted at decadal timescales (Dieppois et al., 2013) as well as at longer timescales, related to the long-term trends in response to climatic forcing (e.g., Lebel et al., 2009;Monerie et al., 2012;Biasutti, 2013). ...
A paleoreconstruction of the length and intensity of the rainy season over western Africa has been recently proposed, using analysis of fossil mollusk shells from the Saloum delta region, in western Senegal. In order to evaluate the significance of local long-term reconstructions of precipitations from paleoclimate proxies, and to better characterize the spatial homogeneity of rainfall distribution in northern Africa, we analyze here the spatial representativeness of rainfall in this region, from seasonal to decadal timescales. The spatial coherence of winter episodic rainfall events is relatively low and limited to surrounding countries. On the other hand, the summer rainfall, associated with the West African Monsoon, shows extended spatial coherence. At seasonal timescales, local rainfall over the Saloum is significantly correlated with rainfall in the whole western half of the Sahel. At interannual and longer timescales, the spatial coherence extends as far as the Red Sea, covering the full Sahel region. This spatial coherence is mainly associated to the zonal extension of the Inter Tropical Convergence Zone. Coherently, summer rainfalls appear to be driven by SST anomalies mainly in the Pacific, the Indian Ocean, the Mediterranean basin, and the North Pacific. A more detailed analysis shows that consistency of the spatial rainfall coherence is reduced during the onset season of the West African Monsoon.
... mm in 1976). Moreover, between the dry periods (i.e., in the late 1970s), the two rainfall stations recorded a brief wet anomaly, as described by Dieppois et al. (2015). ...
Using three daily measurements of wind speed and direction from synoptic weather station data in SE Niger, we examined the diurnal, seasonal and interannual time-scale of Sahel climate variability between 1950 and 1992. The seasonal wind patterns are closely related to the temperatures and West African monsoon dynamics. The transitions between the two seasons are marked by an important increase in calms (wind speed <0.5 m.s-16 ). Such variations are related to meridional shifts of the Inter Tropical Discontinuity (ITD) and Inter Tropical Convergence Zone (ITCZ). Interannual fluctuations of annual wind speeds are consistent with Sahel rainfall variability. Dry years, such as in 1969-1973 and 1983-1986 periods, are associated with negative anomalies in wind speeds mainly due to an increase in calms and dry conditions. Nevertheless, we note several differences: the first period is associated with a yearly increase in the annual mean speed, while the second is associated with a decrease. Differences could be related to changes in atmospheric circulation, especially regarding the strength and latitudinal position of Tropical and African Easterly jets.
... The above results contrast with the findings reported by Peel et al. (2004, p. 818), who concluded that Sahelian precipitation is strongly influenced by low frequency oscillations with periods generally longer than 7 or 8 years. Dieppois et al. (2013) also found 12-to18-year cycles in their time series of Sahel rainfall index defined over latitudes 10-20 ∘ N and longitudes 20 ∘ W-20 ∘ E using CRU data (version TS 3.10.1). We suspect these differences are likely due to the fact that we spatially aggregated rainfall and temperature data based on sub-basins of the Niger Basin because the focus of the larger study from which our work originated is on water resources planning and risk management. ...
Hydroclimatic variability manifests as abrupt shifts, trends, runs, and recurrent cyclical phenomena, collectively referred to as components. In this article, we tested for the presence and magnitude of each component in the Niger Basin (West Africa), using the 0.5° by 0.5° gridded annual rainfall and temperature data produced by the Climate Research Unit (CRU) at the University of East Anglia, UK, for the period 1901–2006. The streamflow data was also analysed for different sub-basins of the Niger Basin. Abrupt shifts were tested using a Bayesian and nonparametric approach. Trends were analysed using the Mann-Kendall trend test. Runs were extracted for dry, neutral, and wet conditions, simulated 1000 times based on the skew normal distribution, and used to investigate various run characteristics. Cyclical behaviour was investigated using continuous wavelet analysis. The results show that an abrupt change point occurred in 1969 in the rainfall and streamflow (but not temperature) time series in all subwatersheds of the Niger Basin. The magnitude of the shift in the mean rainfall varied between 16 and 24%. The temperature time series exhibit strong positive trends in all watersheds. Post change point, the rainfall and streamflow time series show positive, though statistically non-significant trends at α = 0.1. In contrast, disregarding the change point, all subwatersheds show significant negative trends at α = 0.05 and the maximum run lengths are about 4 years long for both dry and wet conditions. Finally, wavelet analysis showed that both rainfall and streamflow in the Niger Basin fluctuate on cycles that are predominantly 2–4 years long, with a few occurrences in the 6–8 years range. Wavelet activity diminished noticeably when a series appeared strongly dominated by trends. The results provide a more comprehensive view of climatic variability than would be obtained from only one or a few components.
Understanding hydrological variability is of crucial importance for water resource
management in sub-Saharan Africa (SSA). While existing studies typically focus on
individual river basins, and suffer from incomplete records, this study provides a new
perspective of trends and variability in hydrological flood and drought characteristics
(frequency, duration, and intensity) across the entire SSA. This is achieved by: i)
creating a 65-year long, complete daily streamflow dataset consisting of over 600
gauging stations; ii) quantifying changes in flood and drought characteristics between
1950 and 2014; iii) evaluating how decadal variability influences historical trends.
Results of daily streamflow reconstructions using random forests provide satisfactory
performance over most of SSA, except for parts of southern Africa. Using change-point
and trend analyses, we identify three periods that characterise historical variations
affecting hydrological extremes in western and central Africa, and some parts of
southern Africa: i) the 1950s–60s and after the 1980s–90s, when floods (droughts)
tend to be more (less) intense, more (less) frequent and more (less) persistent; and ii)
the 1970s–80s, when floods (droughts) are less (more) intense, less (more) frequent
and less (more) persistent. Finally, we reveal significant decadal variations in all flood
and drought characteristics, which explain aperiodic increasing and decreasing trends.
This stresses the importance of considering multiple time-periods when analysing
recent trends, as previous assessments may have been unrepresentative of long-term
changes.
Au Sahel, la variabilité climatique a d’abord été analysée à partir de la pluviométrie. Mais celle-ci ne concerne que quelques dizaines de jours dans l’année, alors que les jours sans vent sont peu fréquents. La variabilité climatique est donc analysée ici à partir de l’analyse de la dynamique éolienne au sol et de ses conséquences en lien avec la dégradation environnementale entre 1950 et 2009. - Appréhender les variations climatiques à partir de la direction et de la vitesse du vent mesurées par les stations synoptiques (TCM). Les variations journalières reflètent le rôle de la turbulence thermique. Les cycles saisonniers sont distingués par les directions et par les vitesses du vent en lien avec les mouvements de la ZCIT et du FIT. Les périodes de forte aridité sont mises en évidence par des baisses significatives des vitesses moyennes. Les vents au sol distinguent différents types de sécheresses, ce que la pluviométrie ne permettait pas. Ainsi donc, les mesures d’observation du vent fournies par les stations synoptiques aboutissent à des résultats homogènes et en conformité avec les connaissances antérieures issues principalement de l’analyse d’autres paramètres. Ces mesures météorologiques sont donc fiables et d’un intérêt certain. Comparer les observations météorologiques des vents avec les réanalyses 20CR, NNR-1, ERA-40 et ERA-Interim. Les séries mensuelles présentent de bonnes corrélations à Niamey et Maïné-Soroa, mais pas à Bilma. L’analyse du cycle annuel montre de fortes sur/sousestimations des vents par les réanalyses. L’évolution interannuelle des vents en été (JJA) et hiver (DJF) montre que ces discordances varient selon les vents, les stations, les périodes et les réanalyses. Au total, ce sont donc ERA-40 et NNR-1 qui sont les plus proches des observations alors que 20CR s’écarte le plus fortement. La comparaison des observations avec les mesures météorologiques assimilées par NCAR (SYNOP) indique que les faibles corrélations des réanalyses avec les mesures TCM pourraient être liées aux fichiers SYNOP en raison de l’importance des données manquantes ou erronées.- Analyser les variations de la conséquence combinée des variations pluviométriques et des vents à travers la visibilité horizontale et les types de temps liés aux basses visibilités (chasses-sable, brumes de sable et brumes-sèches). Les variations interannuelles indiquent des baisses importantes de la visibilité en lien avec les variations de la pluviométrie. Mais l’augmentation des phénomènes météorologiques liés aux basses visibilités indique aussi un changement climatique plus précoce que ce qui est généralement admis sur la base de la seule évolution pluviométrique. Ces modifications concernent la zone saharienne, plus sensible à l’érosion éolienne, dès la fin des années 1950. Pour les stations sahéliennes, une première dégradation est visible vers 1970, puis une seconde au début des années 1980. Il est possible qu’une nouvelle dégradation observable uniquement à Niamey dans les années 1990 et 2000 soit d’origine anthropique. - Analyser la relation vitesse du vent-visibilité horizontale au cours du cycle annuel. Compte tenu du gradient pluviométrique et de la position amont-aval des stations sur la trajectoire éolienne d’échelle continentale NE-SW, nous observons une évolution qui laisse supposerqu’interviennent d’autres facteurs. En effet, on passe progressivement d’un véritable cycle d’hystérésis à Maïné-Soroa en période humide à une forte relation binaire que vient à peine perturber une courte saison des pluies à Nguigmi en période aride. On peut donc supposer unerelation binaire : s’agit-il de la végétation ou d’un facteur anthropique ?
Since the industrial revolution, an anthropogenic forcing occurs over all time-scales of natural variability. This change resulted in significant variations of climate on the regional scale. Therefore, based on spectral analysis, this dissertation revisits the question of the spatiotemporal non-stationnarity of teleconnections between the global climatic variability in the Atlantic and the regional climates of NW Europe and West-Africa. The study of long-term climatic records back to the pre-industrial period reveals similar trends in temperatures of England and Northern France, contrary to trends of rainfalls. Multi- and interdecadal variabilities display several periods of enhanced amplitude for both temperature and rainfall that may be related to large-scale climate control. On these scales, the relationship between the Atlantic Multidecadal Oscillation (AMO) and temperatures is marked by phase changes over the 19th century. For rainfall, coherence with AMO is observed for scales at around 30-60-year, whereas coherence with the North Atlantic Oscillation (NAO) is detected on 50-80-year and interdecadal 16-23-year scales. However, NAO/rainfall teleconnections are highly unstable due to spatial fluctuations of NAO patterns. After the Little Ice Age, the NAO pattern seems shifted south-westwards in winter and spring. Moreover, the results of an analysis of North Atlantic sea-level pressure (SLPs) at these co-oscillation time scales highlight not only NAO regimes, but also other patterns explaining a non-negligible amount of variance. Over West-Africa, zonal contrasts are identified through a study of time-evolutions of Sahel rainfall variability. These patterns are more pronounced at quasi-decadal scale. The teleconnections with Atlantic sea-surface temperatures (SSTs), which depend on time-scales, also show east-west contrasts. During the 20th century, a synchronous Sahel rainfall teleconnection with North Atlantic and Tropical South Atlantic SSTs is rarely observed. In-phase relationship with North Atlantic SSTs only occurs in wet periods (e.g. 1950s) and across multi- and quasi-decadal scales. This teleconnection display a NW-SE pattern suggesting non-uniform modulations of meridional displacements of the ZCIT. The Tropical South Atlantic SSTs variability is often related to a dipole-type configuration between the in-phase rainfall along Gulf of Guinea and the phase opposition in the Sahel. Nevertheless, this teleconnection depends on the time-period, the time-scale, and the mean state of Pacific SSTs. The involved quasi-decadal anomalies of such zonal contrasts vary over time. In the 1950/60s, the western flank of West-Africa displays fluctuations of Harmattan flux, rising motions over sahelian region and of the subtropical sinking motions. In the 1970/80s, variations of the cross-equatorial Atlantic SST and pressure gradient can be related to modulations of monsoon fluxes, and more generally northward-southward shift of the ZCIT.
During the last decades much has been gained on how the oceans can influence the climate of Southen Africa at the interannual and also decadal scales. This offers predictability at the seasonal scale. This also indicates that the future climate of Southern Africa will be influence by the future state of the oceans under natural and anthropogenic forcing. The most severe droughts are happening in Southern Africa during the mature phase of El Niño when the central and eastern Pacific and the Indian Ocean are warmer than normal. El Niño and La Niña have also an impact on streamflows, vegetation, fire and the fluxes of nutrients into the ocean. On top of this modes of variability are long term trends. For instance ocean's temperature has risen globally by not uniformly during the last 50 years due to the anthropogenic increase in CO2. The IPCC scenarios for Southern African Climate are based on coupled Ocean Atmosphere models called CMIP5. CMIP5 models were run on the period 1950-2005 to allow comparison with actual climate data and to verify the adequacy of those models to reproduce contemporary climate. Here, we analyse 27 IPCC CMIP5 coupled model with a focus on the teleconnection ENSO/Southern African climate and the adequacy of those models to represent the physical mechanisms linking the two. The summer teleconnections between ENSO and South African rainfall trough the CMIP5 models can be impact by biases in the SST mean state and an erroneous rainfall response over Southern Africa. Overestimations of summer rainfall are detected from the south-eastern coastal regions to Botswana and Namibia while underestimations occur over the Western Cape Province. Westward extensions of ENSO patterns and warm biases in the equatorial Pacific considerably affect the summer interannual to decadal ENSO variability. From the near-surface to the mid-troposphere, the CMIP5 experiments show biases in tropical part of the South Atlantic SLP dipole which could affect the eastward ridging of the Santa Helena High and its onshore flow over South Africa. At 500 hPa, fluctuations of continental Angola-Botswana low and their effects on easterly waves and cloud-bands development are however correctly reproduced. Although large-scale tropical anomalies of suppressed deep-convection over the maritime continent and enhanced convections from the central to eastern Pacific are correctly simulated, regional biases occur from Africa to Indian Ocean. Anomalies of the SICZ location, as well as the TTT development, can therefore lead to opposite
conditions to reality depending on models. Moreover, due to discrepancy between models regarding the future of ENSO. it is not yet possible to say whether ENSO activity will be enhanced or damped, or if frenquency of events with change in coming decades with aggravate the problem identified aloft.
Since the industrial revolution, an anthropogenic forcing occurs over all time-scales of natural variability. This change resulted in significant variations of climate on the regional scale. Therefore, based on spectral analysis, this dissertation revisits the question of the spatio-temporal non-stationnarity of teleconnections between the global climatic variability in the Atlantic and the regional climates of NW Europe and West-Africa.
The study of long-term climatic records back to the pre-industrial period reveals similar trends in temperatures of England and Northern France, contrary to trends of rainfalls. Multi- and interdecadal variabilities display several periods of enhanced amplitude for both temperature and rainfall that may be related to large-scale climate control. On these scales, the relationship between the Atlantic Multidecadal Oscillation (AMO) and temperatures is marked by phase changes over the 19th century. For rainfall, coherence with AMO is observed for scales at around 30-60-year, whereas coherence with the North Atlantic Oscillation (NAO) is detected on 50-80-year and interdecadal 16-23-year scales. However, NAO/rainfall teleconnections are highly unstable due to spatial fluctuations of NAO patterns. After the Little Ice Age, the NAO pattern seems shifted south-westwards in winter and spring. Moreover, the results of an analysis of North Atlantic sea-level pressure (SLPs) at these co-oscillation time scales highlight not only NAO regimes, but also other patterns explaining a non-negligible amount of variance.
Over West-Africa, zonal contrasts are identified through a study of time-evolutions of Sahel rainfall variability. These patterns are more pronounced at quasi-decadal scale. The teleconnections with Atlantic sea-surface temperatures (SSTs), which depend on time-scales, also show east-west contrasts. During the 20th century, a synchronous Sahel rainfall teleconnection with North Atlantic and Tropical South Atlantic SSTs is rarely observed. In-phase relationship with North Atlantic SSTs only occurs in wet periods (e.g. 1950s) and across multi- and quasidecadal scales. This teleconnection display a NW-SE pattern suggesting non-uniform modulations of meridional displacements of the ZCIT. The Tropical South Atlantic SSTs variability is often related to a dipole-type configuration between the in-phase rainfall along Gulf of Guinea and the phase opposition in the Sahel. Nevertheless, this teleconnection depends on the time-period, the time-scale, and the mean state of Pacific SSTs. The involved quasidecadal anomalies of such zonal contrasts vary over time. In the 1950/60s, the western flank of West-Africa displays fluctuations of Harmattan flux, rising motions over sahelian region and of the subtropical sinking motions. In the 1970/80s, variations of the cross-equatorial Atlantic SST and pressure gradient can be related to modulations of monsoon fluxes, and more generally northward-southward shift of the ZCIT.
Relationships between monthly West African rainfall anomaly patterns and monthly tropospheric wind changes are investigated for the 32 months of August (1958–1989) at an interannual time step. Regarding the Sahelian belt alone, results show that wet (dry) months are significantly linked to an increase (decrease) in both upper easterlies and lower southwesterlies, along with weaker (stronger) than usual midtropospheric easterlies south and under the main axis of the African Easterly Jet (AEJ). However, the most interesting signals are obtained with consideration of the West African rainfall anomaly patterns. Large droughts over Sahelian and Guinean areas are significantly associated with a less southward extension of upper easterlies and a decrease of the upper-meridional diffluence in the Hadley circulation. Contrasted rainfall anomaly patterns exhibiting both Sahelian droughts and Guinean floods are mainly linked to easterly anomalies in the equatorial stratosphere and enhanced easterlies south and under the AFJ, associated with a reduction of westerlies in the monsoon flow and midtropospheric southerly anomalies north of 15°N. The opposite-contrasted rainfall patterns (Sahelian floods and Guinean droughts) are characterized by a southward extension of upper easterlies, a decrease of the easterlies extending south of the AEJ, an increase of wind components in the monsoon flow, and an increased confluence of the meridional components in the midtroposphere above 5°N. Two indexes taking into account the vertical (200 hPa/850 hPa) and the 850 hPa latitudinal (10°N/20°N) relationships in the zonal wind component discriminate the two contrasted rainfall anomaly patterns. A third index, using the meridional components in high levels for documenting diffluence variability in the Hadley circulation, characterizes large-scale West African droughts.
Despite recent progress in the understanding of the West African monsoon
(WAM), its interactions with oceans, land, and aerosols, are still not
well understood. The West African Monsoon Modeling and Evaluation
project (WAMME) is a project comprised of both general circulation
models (GCMs) and regional climate models (RCMs) to collectively
investigate WAM processes and feedbacks between WAM and external
forcings. WAMME activities are closely coordinated with AMMA
activities. Recent observational evidence has supported the notion
that there are strong decadal climate variabilities in the West Africa
from the 1950s to the 2000s, not only in precipitation, but also in
spatial distributions of sea surface temperature (SST), vegetation
cover, land use and land cover (LULC) change, and aerosols. In WAMME-2,
multi-model intercomparison experiments are further designed to test
whether seasonal and decadal variability of WAM precipitation is
associated with these forcings. The WAMME-2 objectives are established
as: (a) to improve our understanding of impacts of these forcings on the
regional water cycle of the WAM, (b) to evaluate the sensitivity of the
seasonal and decadal variability of the West African climate to those
external forcings, and (c) to assess their relative contributions in
producing/amplifying the WAM seasonal and decadal climate variability.
The WAMME-2 strategy is to apply observational data-based anomaly
forcing, i.e., "idealized but realistic" forcing, in GCM and RCM
simulations to test the relative impacts of such forcings. Ten GCMs and
six RCMs are participating in this experiment. African scientists are
also closely involved in this activity. In all these experiments, data
from the AMMA Project are used for evaluation and analyses. In the
SST experiment, in addition to the global SST effect, each ocean's role
is also evaluated. To test this, anomalies of SST forcing in each ocean
are removed sequentially from the global SST anomalies, which differs
from common practice, i.e., adding each ocean basin's SST anomalies one
by one. The preliminary results from most GCMs consistently indicate
that SST has a pronounced impact on the WAM decadal variability, and
that the effect of the Pacific Ocean is quite dominant. However, the
models differ in producing other oceans' contribution to WAM decadal
variability. In the LULC change experiment, a newly available land use
change map is applied. A consistent change in the vegetation maps is
proposed for each WAMME modeling group. The simulated LULC change
impact is also substantial, compatible to the SST forcing. The critical
factors in producing the LCLU change effect and the consensus among the
models are discussed. In the dust experiment, the direct impact of dust
on the radiation budget and its influence to the WAM rainfall are
evaluated using GOCCARD and MATCH dust data and are compared with other
external forcings. The discrepancy and consistency among the model
results are discussed.
Numerous analyses of relatively short (25-30 years in length) time series of the observed surface temperature of the tropical Atlantic Ocean have indicated the possible existence of decadal timescale variability. It was decided to search for such variability in 100-yr time series of sea surface temperature (SST) measured aboard ships and available in the recently published Global Ocean Surface Temperature Atlas (GOSTA). Fourier and singular spectrum analyses of the GOSTA SST time series averaged over 11 subregions, each approximately 1 x 10{sup 6}km{sup 2} in area, show that pronounced quasi-oscillatory decadal ({approximately}-20 yr) and multidecadal ({approximately}30-40 yr) timescale variability exists in the GOSTA dataset over the tropical Atlantic. Motivated by the above results, SST variability was investigated in a 200-yr integration of a global model of the coupled oceanic and atmospheric general circulations developed at the geophysical Fluid Dynamics Laboratory (GFDL). The second 100 yr of SST in the coupled model`s tropical Atlantic region were analyzed with a variety of techniques. Analyses of SST time series, averaged over approximately the same subregions as the GOSTA time series, showed that the GFDL SST anomalies also undergo pronounced quasi-oscillatory decadal and multidecadal variability but at somewhat shorter timescales than the GOSTA SST anomalies. Further analyses of the horizontal structures of the decadal timescale variability in the GFDL coupled model showed the existence of two types of variability in general agreement with results of the GOSTA SST time series analyses. One type, characterized by timescales between 8 and 11 yr, has high spatial coherence within each hemisphere but not between the two hemispheres of the tropical Atlantic. A second type, characterized by timescales between 12 and 20 yr, has high spatial coherence between the two hemispheres. 31 refs., 14 figs., 3 tabs.
Diagnostic analyses in the Pacific, have revealed distinct quasi-decadal (QD) and inter-decadal (ID) climate fluctuations with coherent varying patterns of sea surface temperature (SST) and sea level pressure (SLP) anomalies. From a 200-year CGCM simulation, distinct low-frequency fluctuations are also obtained: QD (i.e., 8-12 years period band) and ID (i.e., 18-25 years period band). Traced modeled QD evolution reveals equatorial SST anomalies resembling ENSO, while tropical recharge/discharge mechanisms seem to be operating. The traced modeled ID evolution reveals high latitudes spatially coherent patterns, with maximum SST anomalies in the vicinity of the subarctic frontal zone (SAFZ) and subtropical gyres through advection. Contrary to QD SST anomalies, the modeled ID SST anomalies peak away from the equator as observed. QD fluctuations may then be viewed as low-frequency ENSO phenomena, while ID fluctuations may set-up thermal background modulating frequency and intensity of ENSO.
The influences of decadal Indian and Atlantic Ocean SST anomalies on late-twentieth-century Sahel precipitation variability are investigated. The results of this regional modeling study show that the primary causes of the 1980s Sahel drought are divergence and anomalous anticyclonic circulation, which are associated with Indian Ocean warming. The easterly branch of this circulation drives moisture away from the Sahel. By competing for the available moisture, concurrent tropical Atlantic Ocean warming enhanced the areal coverage of the drought. The modeled partial recovery of the precipitation in the 1990s simulations is mainly related to the warming of the northern tropical Atlantic Ocean and an associated cyclonic circulation that supplies the Sahel with moisture. Because of the changes in the scale and distribution of the forcing, the divergence associated with the continued Indian Ocean warming during the 1990s was located over the tropical Atlantic, contributing to the recovery over the Sahel. In general, the influence of SSTs on Sahel precipitation is related to their modulation of the easterly flow and the associated moisture transport. Precipitation anomalies are further enhanced by the circulation patterns associated with local convergence anomalies. These convergence anomalies and circulation patterns are sensitive to the scale and distribution of the SST anomalies and the moisture.
An examination of analyses and model simulations is used to show that the African easterly jet forms over West Africa in summer as a result of strong meridional soil moisture gradients. In a series of GCM experiments, the imposition of realistic surface wetness contrasts between the Sahara and equatorial Africa leads to strong positive meridional temperature gradients at the surface and in the lower troposphere; the associated easterly shear in the atmosphere is strong enough to establish easterly flow-the African easterly jet-above the monsoon westerlies at the surface. Positive temperature gradients associated with the summertime distributions of solar radiation, SSTs, or clouds are not large enough to produce the easterly jet in the absence of soil moisture gradients. A thermally direct ageostrophic circulation is identified that can accelerate the largely geostrophic zonal flow and maintain the jet.While moisture converges throughout the lower troposphere over East Africa, moisture divergence between 600 and 800 mb overlies low-level convergence over West Africa to the south of the African easterly jet. This moisture divergence is important for determining the total column moisture convergence. Since the moisture divergence is closely tied to the jet dynamics, and the jet's magnitude and position are sensitive to SST and land surface conditions, a mechanism by which the West African precipitation field is sensitive to surface conditions is suggested.
This work presents a description of the 1979–2002 tropical Atlantic (TA) SST variability modes coupled to the anomalous West African (WA) rainfall during the monsoon season. The time-evolving SST patterns, with an impact on WA rainfall variability, are analyzed using a new methodology based on maximum covariance analysis. The enhanced Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) dataset, which includes measures over the ocean, gives a complete picture of the interannual WA rainfall patterns for the Sahel dry period. The leading TA SST pattern, related to the Atlantic El Niño, is coupled to anomalous precipitation over the coast of the Gulf of Guinea, which corresponds to the second WA rainfall principal component. The thermodynamics and dynamics involved in the generation, development, and damping of this mode are studied and compared with previous works. The SST mode starts at the Angola/Benguela region and is caused by alongshore wind anomalies. It then propagates westward via Rossby waves and damps because of latent heat flux anomalies and Kelvin wave eastward propagation from an off-equatorial forcing. The second SST mode includes the Mediterranean and the Atlantic Ocean, showing how the Mediterranean SST anomalies are those that are directly associated with the Sahelian rainfall. The global signature of the TA SST patterns is analyzed, adding new insights about the Pacific–Atlantic link in relation to WA rainfall during this period. Also, this global picture suggests that the Mediterranean SST anomalies are a fingerprint of large-scale forcing.
This work updates the results given by other authors, whose studies are based on different datasets dating back to the 1950s, including both the wet and the dry Sahel periods.
This idealized modeling study investigates how convectively triggered African easterly waves (AEWs) are influenced by the intraseasonal variability of the African easterly jet (AEJ). A set of 10-day averaged zonally varying basic states is constructed with theNCEP-2 reanalysis (1979-2006).Aprimitive equation model is used to simulate linear AEWs on each of these basic states using the same idealized convective heating localized over the Darfur mountains as an initial trigger. It is shown that the transient response depends strongly on the basic state. With the same trigger, many configurations of the AEJ fail to produce a wave disturbance, while others produce strong easterly wave structures. Necessary conditions for the development of strong waves can be characterized by a strong jet, a strong vertical shear, or a strong and extended potential vorticity reversal. In strong-wave cases the jet is extended to the south and west, and the jet core is aligned with the maximum of surface westerlies, maximizing the vertical shear. The pattern that is optimal for generating easterly waves also closely resembles the dominant mode of variation of the AEJ revealed by an empirical orthogonal function (EOF) analysis of the set of basic states.
Atmospheric circulation cells associated with the El Niño-Southern Oscillation (ENSO) are described and examined using the NCEP-NCAR reanalysis field and the NCEP sea surface temperature (SST) from January 1950 to December 1999. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. The warm phase of ENSO shows positive SST anomalies in the equatorial eastern Pacific and along the east coast of Asia and the west coast of North America, and negative SST anomalies in the off-equatorial western Pacific and in the central North Pacific. Associated with this SST anomaly distribution are variations of atmospheric zonal and meridional circulation cells over the Pacific. The equatorial zonal Walker circulation cell is weakened, consistent with previous schematic diagrams. The anomalous meridional Hadley circulation cell in the eastern Pacific shows the air rising in the Tropics, flowing poleward in the upper troposphere, sinking in the subtropics, and returning back to the Tropics in the lower troposphere. The anomalous Hadley cell in the western Pacific is opposite to that in the eastern Pacific. The divergent wind and vertical velocity also show a midlatitude zonal cell (MZC) over the North Pacific. The mean MZC is characterized by the air rising in the central North Pacific, flowing westward and eastward in the upper troposphere, descending in the east coast of Asia and the west coast of North America, then returning back to the central North Pacific in the lower troposphere. The anomalous MZC during the mature phase of El Niño shows an opposite rotation to the mean MZC, indicating a weakening of the MZC.
Using the comprehensively quality-controlled Meteorological Office
Historical Sea Surface Temperature data set (MOHSST)1,2 we
show for the first time that persistently wet and dry periods in the
Sahel region of Africa are strongly related to contrasting patterns of
sea-surface temperature (SST) anomalies on a near-global scale. The
anomalies include relative changes in SST between the hemispheres, on
timescales of years to tens of years, which are most pronounced in the
Atlantic. Experiments with an 11-level global atmospheric general
circulation model (AGCM) support the idea that the worldwide SST
anomalies modulate summer Sahel rainfall through changes in tropical
atmospheric circulation3-6. El Niño events may also
play a part. We do not discount the effects of soil moisture and albedo
changes in the Sahel7,8, although Courel et al.9
have questioned the importance of albedo changes, but we do suggest that
worldwide SST anomalies may have a more fundamental influence on Sahel
rainfall.
Two distinct low-frequency fluctuations are suggested from a joint frequency domain analysis of the Pacific Ocean (30°S-60°N) sea surface temperature (SST) and sea level pressure (SLP). The lowest frequency signal reveals a spatially coherent interdecadal evolution. In-phase SST and SLP anomalies are found along the subarctic frontal zone (SAFZ). It is symmetric about the equator, with tropical SST anomalies peaking near 15° latitudes in the eastern Pacific. The other low-frequency signal reveals a spatially coherent decadal evolution. It is primarily a low-latitude phenomenon. Tropical SST anomalies peak in the central equatorial ocean with evidence of atmospheric teleconnections. These interdecadal and decadal signals join the ENSO and quasi-biennial signals in determining dominant patterns of Pacific Ocean natural climate variability. Relative phasing and location of the SST and SLP anomalies for the decadal, ENSO, and the quasi-biennial signals, are similar to one another but significantly different from that of the interdecadal signal.
Using sea surface temperature (SST), precipitation, and atmospheric information, this statistical study revisits the questions of the July–September SST-Sahel teleconnection variability after removing impact of quasi-global SSTs over the period 1900–2008. The eastern Mediterranean and the Indian Ocean dominate the relationship, both in terms of intensity and time stability, with significant values in 52% and 47% of years, respectively. More than two thirds of the rainy seasons classified as dry (wet) and 16 out of 18 (12 out of 15) of those classified as very dry (very wet) are concomitant of negative (positive) differences between the Mediterranean and the Indian Ocean. Correlations with the tropical Atlantic, the Niño area, and the western Pacific region are generally lower and less robust, although, in some periods, they can be high with the southern tropical Atlantic. Teleconnection observed with continental precipitation and the 950 hPa moisture flux field confirmed these results. Positive SST differences between the eastern Mediterranean and the Indian Ocean are synchronous of in-phase rainfall excess over the whole Sudan-Sahel due to a strengthening of the convergence between the northeasterly moisture transport from the eastern Mediterranean and the monsoon southwesterly moisture transport from the eastern equatorial Atlantic. This is associated with changes in the atmospheric circulation along the meridional and zonal planes, mainly (1) a subsidence departure from midlevels above 10°N–18°N associated with air ascents above the Saharan thermal lows, (2) upward anomalies on the western and eastern Sahel reinforcing the atmospheric ascents in upper levels, (3) a low-level subsidence anomaly by 30°E–40°E in agreement with the Indian cooling weakening the normal uplifts, and (4) a reinforcement of the tropical easterly jet over 0°−20°E.
1] The sea surface temperatures (SSTs) of the tropical Indian Ocean show a pronounced warming since the 1950s. We have analyzed the impact of this warming on Sahelian rainfall and on the North Atlantic Oscillation (NAO) by conducting ensemble experiments with an atmospheric general circulation model. Additionally, we investigate the impact of the other two tropical oceans on these two climate parameters. Our results suggest that the warming trend in the Indian Ocean played a crucial role for the drying trend over the West Sahel from the 1950s to 1990s and may also have contributed to the strengthening of the NAO during the most recent decades.
1] Prominent multidecadal fluctuations of India summer rainfall, Sahel summer rainfall, and Atlantic Hurricane activity have been observed during the 20th century. Understanding their mechanism(s) will have enormous social and economic implications. We first use statistical analyses to show that these climate phenomena are coherently linked. Next, we use the GFDL CM2.1 climate model to show that the multidecadal variability in the Atlantic ocean can cause the observed multidecadal variations of India summer rainfall, Sahel summer rainfall and Atlantic Hurricane activity (as inferred from vertical wind shear changes). These results suggest that to interpret recent climate change we cannot ignore the important role of Atlantic multidecadal variability.
1] Biennial, interannual, and decadal signals in the Pacific basin are observed to share patterns and evolution in covarying sea surface temperature (SST), 18°C isotherm depth (Z18), zonal surface wind (ZSW), and wind stress curl (WSC) anomalies from 1955 to 1999. Each signal has warm SST anomalies propagating slowly eastward along the equator, generating westerly ZSW anomalies in their wake. These westerly ZSW anomalies produce cyclonic WSC anomalies off the equator which pump baroclinic Rossby waves in the western/central tropical North Pacific Ocean. These Rossby waves propagate westward, taking $6, $12, and $36 months to reach the western boundary near $7°N, $12°N, and $18°N on biennial, interannual, and decadal period scales, respectively. There, they reflect as equatorial coupled waves, propagating slowly eastward in covarying SST, Z18, and ZSW anomalies, taking $6, $12, and $24 months to reach the central/eastern equatorial ocean. These equatorial coupled waves produce a delayed-negative feedback to the warm SST anomalies there. The decrease in Rossby wave phase speed with latitude, the increase in meridional scale of equatorial SST anomalies with period scale, and the associated increase in latitude of Rossby wave forcing are consistent with the delayed action oscillator (DAO) model used to explain El Niño. However, this is not true of the western-boundary reflection of Rossby waves into slow equatorial coupled waves. This requires modification of the extant DAO model. We construct a modified DAO model, demonstrating how the various mechanisms and the size and sources of their delays yield the resulting frequency of each signal., A delayed action oscillator shared by biennial, interannual, and decadal signals in the Pacific Basin, J. Geophys. Res., 108(C3), 3070, doi:10.1029/2002JC001490, 2003.
Analysis of the annually varying regional circulations and their relationship to surface conditions and water vapour transport in the West African region is presented. The progression of the West African monsoon is described in terms of four key phases: (i) an oceanic phase between November and mid‐April when the rain band is broad with peak values just north of the Equator (∼1°N); (ii) a coastal phase between mid‐April and the end of June when the rainfall peak is in the coastal region around 4°N (over the ocean); (iii) a transitional phase during the first half of July when the rainfall peak decreases; and (iv) a Sahelian phase between mid‐July and September when the rainfall peak is more intense and established in the Sahelian region around 10°N. The annual evolution of the moisture fluxes, associated convergence, and rainfall is strongly impacted by the Atlantic cold tongue (cool water close to the Equator between boreal spring and summer) and the Saharan heat‐low. The cold tongue strongly regulates the timing and intensity of the coastal rainfall in spring. The heat‐low and its associated shallow meridional circulation strongly affect the profile in moisture flux convergence north of the main rain‐band maximum; in particular it is responsible for the establishment of a second peak in column moisture flux convergence there (approximately 8° poleward of the rainfall peak).
Particular emphasis is given to the coastal rainfall onset in April. A key aspect of this onset is acceleration of low‐level cross‐equatorial southerly winds, important for establishing the cold tongue, discouraging convection near the Equator and transporting moisture towards the coast. We argue that the rainfall peak is maintained at the coast, rather than steadily moving inland with the solar insolation, due to persistent warm water in the coastal region together with frictionally induced moisture convergence there. Copyright © 2011 Royal Meteorological Society
The Twentieth Century Reanalysis (20CR) project is an international effort to produce a comprehensive global atmospheric circulation dataset spanning the twentieth century, assimilating only surface pressure reports and using observed monthly sea‐surface temperature and sea‐ice distributions as boundary conditions. It is chiefly motivated by a need to provide an observational dataset with quantified uncertainties for validations of climate model simulations of the twentieth century on all time‐scales, with emphasis on the statistics of daily weather. It uses an Ensemble Kalman Filter data assimilation method with background ‘first guess’ fields supplied by an ensemble of forecasts from a global numerical weather prediction model. This directly yields a global analysis every 6 hours as the most likely state of the atmosphere, and also an uncertainty estimate of that analysis.
The 20CR dataset provides the first estimates of global tropospheric variability, and of the dataset's time‐varying quality, from 1871 to the present at 6‐hourly temporal and 2° spatial resolutions. Intercomparisons with independent radiosonde data indicate that the reanalyses are generally of high quality. The quality in the extratropical Northern Hemisphere throughout the century is similar to that of current three‐day operational NWP forecasts. Intercomparisons over the second half‐century of these surface‐based reanalyses with other reanalyses that also make use of upper‐air and satellite data are equally encouraging.
It is anticipated that the 20CR dataset will be a valuable resource to the climate research community for both model validations and diagnostic studies. Some surprising results are already evident. For instance, the long‐term trends of indices representing the North Atlantic Oscillation, the tropical Pacific Walker Circulation, and the Pacific–North American pattern are weak or non‐existent over the full period of record. The long‐term trends of zonally averaged precipitation minus evaporation also differ in character from those in climate model simulations of the twentieth century. Copyright © 2011 Royal Meteorological Society and Crown Copyright.
There are two major organized cloud configurations in the vicinity of the equator. Where there is a small cross-equatorial surface pressure gradient, convection is close to the equator and is generally tied to the location of the lowest sea-level pressure (SLP) and warmest sea-surface temperature (SST), in agreement with arguments based upon simple thermodynamical considerations. However, when there is a substantial cross-equatorial pressure gradient, such as occurs in the monsoon regions, organized convection appears off the equator in the summer hemisphere, equatorward of the SLP minimum and not necessarily collocated with the warmest SSTs. Thus, in this instance, simple thermodynamical considerations alone cannot explain the location of the convection. In this situation, the zero absolute vorticity contour (eta = 0) also lies in the summer hemisphere. Therefore, between the equator and the eta = 0 contour is a region of locally-anticyclonic absolute vorticity and an inertially unstable regime. It is argued that the convection results from the low-level divergence-convergence doublet centred about the eta = 0 contour which is the mitigating response to the inertial instability. The associated latitude-height secondary circulation should provide subsidence (suppressed convection) over the equator and rising motion (enhanced convection) to the north of the zero absolute vorticity contour. Signatures of the inertial instability predicted by theory are found in observations supporting the hypothesis. Wherever a strong cross-equatorial pressure gradient exists, the a = 0 contour bisects a maximum in the divergent wind field. Divergence is found equatorward of the zero contour and convergence on the poleward side. Latitude height cross sections show strong local meridional circulations with maximum rising motion on the poleward side of eta = 0. As the regions where the rising motions occur are conditionally unstable, there is deep convection and the vertical circulations extend throughout the troposphere. It is noted that the intensity of the off-equator convection is deeper (and probably stronger) than convection located at the equator. This is probably because the convection associated with the inertial instability is more efficient. Necessary conditions for the location of near-equatorial convection are listed. Arguments are presented whereby inertial instability is established as the cause, rather than an effect, of off-equatorial convection. These include an outline of the sequence of processes leading up to the convection. The factors that limit the encroachment of the eta = 0 contour into the summer hemisphere are discussed and an explanation for the existence of the low-level westerly monsoon wind maximum is suggested. The possible role played by the instability mechanism (or the lack of it) in coupled model simulations that produce seasonally migrating and/or double ITCZs in the eastern Pacific Ocean is discussed. Finally, it is proposed that the instability mechanism is important in the initiation of westward-moving disturbances found in the eastern Pacific and in determining active and break periods in the summer Indian monsoon.
This study uses models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to evaluate and investigate Sahel rainfall multidecadal variability and teleconnections with global sea surface temperatures (SSTs). Multidecadal variability is lower than observed in all historical simulations evaluated. Focus is on teleconnections with North Atlantic SST [Atlantic multidecadal variability (AMV)] as it is more successfully simulated than the Indian Ocean teleconnection. To investigate why some models successfully simulated this teleconnection and others did not, despite having similarly large AMV, two groups of models were selected. Models with large AMV were highlighted as good (or poor) by their ability to simulate relatively high (low) Sahel multidecadal variability and have significant (not significant) correlation between multidecadal Sahel rainfall and an AMV index. Poor models fail to capture the teleconnection between the AMV and Sahel rainfall because the spatial distribution of SST multidecadal variability across the North Atlantic is incorrect. A lack of SST signal in the tropical North Atlantic and Mediterranean reduces the interhemispheric SST gradient and, through circulation changes, the rainfall variability in the Sahel. This pattern was also evident in the control simulations, where SST and Sahel rainfall variability were significantly weaker than historical simulations. Errors in SST variability were suggested to result from a combination of weak wind-evaporation-SST feedbacks, poorly simulated cloud amounts and feedbacks in the stratocumulus regions of the eastern Atlantic, dust-SST-rainfall feedbacks, and sulfate aerosol interactions with clouds. By understanding the deficits and successes of CMIP5 historical simulations, future projections and decadal hindcasts can be examined with additional confidence.
1. Introduction to wavelets 2. Review of Fourier theory and filters 3. Orthonormal transforms of time series 4. The discrete wavelet transform 5. The maximal overlap discrete wavelet transform 6. The discrete wavelet packet transform 7. Random variables and stochastic processes 8. The wavelet variance 9. Analysis and synthesis of long memory processes 10. Wavelet-based signal estimation 11. Wavelet analysis of finite energy signals Appendix. Answers to embedded exercises References Author index Subject index.
A practical step-by-step guide to wavelet analysis is given, with examples taken from time series of the El NiñoSouthem Oscillation (ENSO). The guide includes a comparison to the windowed Fourier transform, the choice of an appropriate wavelet basis function, edge effects due to finite-length time series, and the relationship between wavelet scale and Fourier frequency. New statistical significance tests for wavelet power spectra are developed by deriving theoretical wavelet spectra for white and red noise processes and using these to establish significance levels and confidence intervals. It is shown that smoothing in time or scale can be used to increase the confidence of the wavelet spectrum. Empirical formulas are given for the effect of smoothing on significance levels and confidence intervals. Extensions to wavelet analysis such as filtering, the power Hovmöller, cross-wavelet spectra, and coherence are described. The statistical significance tests are used to give a quantitative measure of changes in ENSO variance on interdecadal timescales. Using new datasets that extend back to 1871, the Niño3 sea surface temperature and the Southern Oscillation index show significantly higher power during 1880-1920 and 1960-90, and lower power during 1920-60, as well as a possible 15-yr modulation of variance. The power Hovmöller of sea level pressure shows significant variations in 2-8-yr wavelet power in both longitude and time.
An analysis of the 130-year record of the Earth's global mean temperature reveals a significant warming trend and a residual consistent with an auto-correlated (“red”) noise process whose predictability decays with a timescale of two years. Thus global temperatures, in isolation, do not indicate oscillations at 95% confidence against a red noise null hypothesis. Weak signals identified in the global series can, however, be traced to significant sea surface temperature oscillations in the equatorial Atlantic (period ∼10 years) and the El Niño region of the Pacific (3–5 years). No robust evidence is found in this data for interdecadal oscillations, The 10-year Atlantic oscillation corresponds to a pattern of temperature anomalies which has been associated with interannual variations in West African rainfall and in U.S. hurricane landfall frequency.
A forcing mechanism is sought for the large-scale circulation changes in the Kuroshio Extension region of the western North Pacific Ocean as inferred by TOPEX/Poseidon sea surface height (SSH) data. The low- frequency signal of the Kuroshio Extension over the last decade was characterized by a modulation in its zonal mean flow intensity: the mean Kuroshio Extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997. The ability to simulate the major trends in the observed SSH signals with linear vorticity dynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by wind stress curl anomalies in the eastern North Pacific Ocean related to the Pacific decadal oscillations (PDOs). To be specific, the weakening (strengthening) trend in 1993-96 (1997-2001) was caused by westward expansions of negative (positive) SSH anomalies south of the Kuroshio Extension and positive (negative) SSH anomalies north of the Kuroshio Extension. Emergence of oppositely signed SSH anomalies on the two sides of the Kuroshio Extension jet is due to the different propagating speeds of the baroclinic Rossby waves, which carry the wind- induced SSH anomalies generated in the eastern North Pacific at different phases of the PDOs. Hindcasting the Kuroshio Extension jet strength over the last 45 years reveals that the jet modulation has a dominant timescale of ;12 yr. Given the location of the Kuroshio Extension jet relative to the maximum atmospheric forcing, it is found that this dominant timescale is consistent with the preferred timescale under a stochastic white-noise atmospheric forcing. It is hypothesized that this connection between the Kuroshio Extension strength and the latitudinally dependent baroclinic adjustment contributes to an increase in variance and persistence of the North Pacific midlatitude coupled system on the decadal timescale.
A compositing method is used to determine the average structure and properties of eight wave disturbances observed over west Africa and the eastern Atlantic during the period 23 August-19 September, 1974, a period marked by well-developed and regular wave activity. The disturbance centers propagated westward in the zone of cyclonic shear to the south of the 700 mb easterly jet, located at 16–17°N. The mean wave- length was about 25M km and the mean period 3.5 days. The mean zonal current satisfied the necessary condition for barotropic instability. The composite disturbance was most intense at 650 mb, being cold core below and warm core above. Two circulation centers were evident at the surface, one located below the upper center and the other displaced 10° to the north at about the latitude of the monsoon trough. When separate composites were constructed for land and ocean stations, the dual centers were found to be primarily a land phenomenon. Distinctive features of the high-level (200 inb) ci...
An idealized vertical–meridional zonally symmetric model is developed in order to recover a July typical monsoon regime over West Africa in response to surface conditions. The model includes a parameterization to account for heat and momentum fluxes associated with eddies. The sensitivity of the simulated West African monsoon equilibrium regime to some major processes is explored. It allows confirmation of the important role played by the sun’s latitudinal position, the aerosols, the albedo, and the SST’s magnitude in the Gulf of Guinea and in the Mediterranean Sea.
The important role of aerosols in warming the Saharan lower layers and their effect on the whole monsoon is underlined. Model results also stress the importance of the Mediterranean Sea, which is needed to obtain the extreme dryness of the Sahara. The use of this idealized model is finally discussed for studying the scale interactions and coupling involved in the West African monsoon as explored in a companion paper.
Space and time scales for analysis of the interannual variability of Sahelian rainfall are determined. Regionalizations of annual and monthly rainfall fields are performed in West Africa for the period 1948-78. Four coherent regions and a less coherent one are identified. Different type classifications derived from the regionalization results are built. The monthly type based on the rainfall anomaly signs north and south of 10°N suggests two major causes of the rainfall pattern variability, one resulting from an anomaly of rainfall amount and the other from a displacement of the intertropical convergence zone (ITCZ). The type based solely on the anomaly sign north of 10°N blends these factors and may give misleading analyses. The use of monthly rainfall fields over all of West Africa is then recommended.
The summer climate of tropical North Africa exhibits strong decadal variability (the low frequency, LF) and also substantial variability within the decadal regimes (the high frequency, HF). Statistical analyses on raw climate data can confound processes on the HF and LF or be overwhelmed by the decadal scale. In this paper, the HF and LF are studied separately. In recent decades, the LF in tropical North Africa is dominated by decreasing rainfall, strongest in summer months, but not absent in the transition seasons. The known change in the north-south interhemispheric gradient of sea surface temperature (SST) has accompanied climate fluctuation not just in the Sahel, but through much of the Tropics, including a modest decline in July-September (JAS) Indian rainfall. These large-scale changes of the ocean and atmosphere are consistent with a coupled ocean-atmosphere phenomenon, though results are also discussed in terms of a possible role for land surface changes in tropical North Africa. On the HF, the JAS season in tropical North Africa is shown to be distinct in terms of large-scale connections within the climate system. The following comments apply to the JAS season. Tropical Atlantic SSTs are connected to out-of-phase rainfall anomalies in the Sahel and Guinea Coast regions. Focusing on those years with the same sign rainfall anomaly in the two regions (or applying coupled pattern techniques) reveals a clear North African rainfall connection with El Nino-Southern Oscillation (ENSO). Evidence is found for a degree of association, partly independent of the key SST indices, between HF Indian and Sahelian rainfall (positive correlation) and HF Sahelian and Guinea Coast rainfall (negative correlation). The independence from SST indices raises the possibility of teleconnection processes internal to the atmosphere or land-atmosphere system. A canonical correlation analysis showed that between 25% and 50% of the HF JAS rainfall variance (at large spatial scales) can be specified from the HF JAS SST. When April-June (AMJ) unfiltered SST is used, skill for the Sahel and Soudan is good at the LF, but near zero for the HF. This is likely an underestimate due to the conservative nature of the methods used, and the Nino3 tropical Pacific SST index in AMJ does show some predictive potential for the Sahel and Soudan regions on the HF. Nonetheless, the results indicate that a substantial part of the key SSTs, especially those related to ENSO, appear to evolve during late spring, a result previously also found for Indian rainfall. The Guinea Coast region in JAS exhibits little LF variance and tropical Atlantic AMJ SSTs yield hindcasts that explain about 30% of the rainfall variance. So for the Guinea Coast, forecasts can be expected to contain clear skill in predicting year-year fluctuations in rainfall.
Observations of sea surface and land-near-surface merged temperature anomalies are used to monitor climate variations and to evaluate climate simulations; therefore, it is important to make analyses of these data as accurate as possible. Analysis uncertainty occurs because of data errors and incomplete sampling over the historical period. This manuscript documents recent improvements in NOAA's merged global surface temperature anomaly analysis, monthly, in spatial 5° grid boxes. These improvements allow better analysis of temperatures throughout the record, with the greatest improvements in the late nineteenth century and since 1985. Improvements in the late nineteenth century are due to improved tuning of the analysis methods. Beginning in 1985, improvements are due to the inclusion of bias-adjusted satellite data. The old analysis (version 2) was documented in 2005, and this improved analysis is called version 3.
Future climate changes in African regions are model-dependent and there is no consensus regarding Sahelian rainfall by the end of this century. Using 12 atmosphere-ocean global climate models of the third Coupled Model Intercomparison Project (CMIP3) we propose a multi-model (MM) analysis contrasting the 1960–1999 period (20c3m integration) and the 2031–2070 period (A1B emission scenario). The analyses are based on MM response but also on the ‘one model-one vote’ concept to give the same weight to each model. The results show robust signals in the rainfall response, i.e., increasing (decreasing) amounts in central (western) Sahel associated with specific changes in atmospheric dynamics. The rainfall excesses expected in central Sahel are mainly linked to an enhancement of the northern Hadley-type cell and probably to its northward shift. At low-levels increasing temperature and evaporation strengthen the monsoon flux. Rainfall deficits predicted westward are due to a reinforcement of the African Easterly Jet and to anomalies in the zonal circulation between the Indian and the Atlantic Oceans, favouring air subsidence and moisture export outside the region. The weakening of the meridional circulation is also able to prevent rainfall amounts in the western part of the Sahel. More generally, these analyses show that in future works the Sahel region must not be considered as one consistent region but as two separate areas, both sides of the Greenwich meridian. The authors propose to consider this feature in the next Fifth Assessment cycle (AR5) in order to better understand the impacts of the climate change on West Africa. Copyright © 2012 Royal Meteorological Society
When analyzing pairs of time series, one often needs to know whether a correlation is statistically significant. If the data are Gaussian distributed and not serially correlated, one can use the results of classical statistics to estimate the significance. While some techniques can handle non-Gaussian distributions, few methods are available for data with nonzero autocorrelation (i.e., serially correlated). In this paper, a nonparametric method is suggested to estimate the statistical significance of a computed correlation coefficient when serial correlation is a concern. This method compares favorably with conventional methods.
We use an ensemble of experiments with GFDL's CM2.1 global coupled ocean-atmosphere model to assess the global response to observed multidecadal SST fluctuations in the Atlantic basin over the 20th century. In these experiments the fully dynamic ocean model over the Atlantic is replaced with a slab mixed-layer ocean model. Anomalous heat fluxes are added to the mixed layer which have spatial and temporal structures derived from observed SST fluctuations. The slab ocean model therefore simulates multidecadal patterns of SST anomalies in the Atlantic similar to those observed. In response to these realistic SST perturbations, the coupled model simulates multidecadal modulations of summer rainfall over India and the Sahel region of Africa, as well as modulations of vertical wind shear over the Main Development Region for Atlantic hurricanes. The modeled changes are quite similar to those observed. We interpret the vertical wind shear changes as one indicator of hurricane activity. The model also simulates a significant impact on summer surface air temperature over North America and western Europe. Thus, to interpret recent climate change we must consider both Atlantic multidecadal fluctuations and anthropogenic climate change, as well as their potential interactions.
This article investigates the African easterly jet (AEJ), its structure, and the forcings contributing to its maintenance, critically revisiting previous work that attributed the maintenance of the jet to soil moisture gradients over tropical Africa.
A state-of-the-art global model in a high-end computer framework is used to produce a three-member 73-yr ensemble run forced by observed SST to represent the control run. The AEJ as produced by the control is compared with the representation of the AEJ in the 40-yr ECMWF Re-Analysis (ERA-40) and other observational datasets and found to be very realistic.
Five experiments are then performed, each represented by sets of three-member 22-yr-long (1980–2001) ensemble runs. The goal of the experiments is to investigate the role of meridional soil moisture gradients, different land surface properties, and orography. Unlike previous studies, which have suppressed soil moisture gradients within a highly idealized framework (i.e., the so-called bucket model), terrestrial evaporation control is here achieved with a highly sophisticated land surface treatment and with an extensively tested and complex methodology. The results show that the AEJ is suppressed by a combination of absence of meridional evaporation gradients over Africa and constant vegetation, even if the individual forcings taken separately do not lead to the AEJ disappearance, but only its modification. Moreover, the suppression of orography also leads to a different circulation in which there is no AEJ. This work suggests that it is not just soil moisture gradients but a unique combination of geographical features present only in northern tropical Africa that causes and maintains the jet.
This study examines selected dynamical factors associated with wet and dry years in the West African Sahel. The approach is to evaluate the temperature, wind, and moisture fields and the dynamic instabilities for a 4-yr ''wet'' composite (1958-61) and a 4-yr ''dry'' composite (1982-85). The analysis, limited to the months of June through September, is carried out using the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) 40-Year Reanalysis Project dataset. Two upper-air datasets are also evaluated to confirm trends apparent in the NCEP-NCAR data. This study confirms some of the results of earlier studies, such as the weaker African easterly jet (AEJ) and stronger tropical easterly jet (TEJ) during wet years, but suggests a different interpretation of the wet-dry contrasts. In the Sahel, the most important characteristic of the AEJ appears to be its latitudinal location rather than its intensity. This governs the instability mechanisms. The AEJ is displaced northward during the wet years, thereby enhancing both the horizontal and vertical wind shear over the Sahel. Baroclinic instability is probably the dominant mechanism here. South of the Sahel, both the location and intensity of the AEJ appear to be important, and the dominant instability mechanism appears to be barotropic. Results presented here further suggest that equatorial westerlies significantly modulate inter- annual variability. These serve to displace the jet and associated disturbances poleward and to enhance convective activity. The TEJ may also play an important role by promoting ascent in the lower troposphere on the cyclonic side of the AEJ, where the disturbances develop. Contrasts in the moist layer between wet and dry years may be consequences of these other changes. On an interannual basis, the rainfall maximum and the surface position of the ITCZ are effectively decoupled, with the ITCZ keeping a relatively stable location from year to year despite large latitudinal shifts in the rain belt. As a consequence, the length of the rainy season does not change markedly. Instead, wet years in the Sahel are characterized by more intense rainfall (mm month 21). This is linked to a northward shift in the belt of maximum rainfall, which is coincident with the northward shift in the AEJ.
This study investigates the origin and structure of easterly waves that
form in the lower troposphere of North Africa and have a periodicity of
3-5 days,. From June to early October these waves propagate across the
Atlantic and occasionally reach the eastern Pacific. Although only a few
of these disturbances actually intensify after reaching the Atlantic,
they account for approximately half of the tropical cyclones that form
in the Atlantic.Spectral analysis of five years of upper air data shows
that African waves produce a spectral peak of the meridional wind at
periods of 3-5 days with a maximum amplitude of 1-2 m sec1
near 700 mb. These waves normally originate between Khartoum (32E) and
Ft. Lamy (I5E) and affect a greater depth of the atmosphere as they
propagate westward.Wind statistics at stations flanking the mountains in
Ethiopia indicate that airflow over these mountains is not the cause of
the easterly waves. This study shows that the African waves are directly
related to the mid-tropospheric easterly jet that is found within the
baroclinic zone to the south of the Sahara. During the same season that
the waves are observed, the gradient of the monthly mean potential
vorticity vanishes along the isentropic surfaces. Charney and Stem have
shown that this is a necessary condition for instability of the jet
provided that the amplitude of the waves is negligible at the ground.
Results show that the horizontal and vertical shear of the mean zonal
wind are acting as nearly equal sources of energy for the perturbations.
The role of convection in the origin of these waves has not yet been
determined.
Annual rainfall anomalies over subequatorial and northern tropical Africa are analysed for interannual and interdecadal variability over the time interval 1900–1994. Then, the main modes of variation in the annual rainfall field are related to the same frequencies of variation in several SST indexes chosen in key-areas. First, empirical orthogonal function (EOF) analysis is performed on the annual rainfall anomalies field for extracting the main dominant spatio-temporal modes, and leading principal components are analysed through multi-singular spectrum analysis (M-SSA). Regional rainfall anomaly indices, constructed from EOF analysis and describing, respectively, the temporal variability of Sahel, Guinea Gulf area, equatorial East Africa and Gabon–Congo area, are analysed individually through Monte Carlo SSA for assessing the level of significance of spectral peaks. Low-frequency variability, strongest north of 5°N, is a combination of a non-linear trend mixed with an irregular oscillation near 25–40 years. Together, these modes contribute about 35–40 per cent of the Sahelian regional variance and about 15–20 per cent of the Guinean one. The low-frequency variability shows weak negative values till 1915–1920, then positive ones till 1967–1970, with a relative minimum near 1940–1945, and lastly, strong negative values. This behaviour must be placed in a larger context of at least Atlantic-basin scale, or at a global scale in which the strongest signal seems to be the reversing of the interhemispheric thermal gradient around 1970. A quasi-decadal pulse (near 12–13 years) is also observed over Sahel and explains about 12–14 per cent of the regional rainfall index variance. Even if associated SSA components are not significant when a red noise null-hypothesis is considered, some physical consistency is found between this mode and a similar oscillation concerning the thermal gradient between tropical northern and equatorial Atlantic. At the interannual scale, significant oscillations of 5·1–5·8 years (quasi-quinquennial oscillation—QQO) (contributing less than 5 per cent of the Sahelian variance to almost 20 per cent of the East African one), and of 3·2–3·6 years (quasi-triennial oscillation—QTO) (explaining from 5 to 10 per cent of the Sahelian variance to 20–25 per cent of the East African one) are the first ones before quasi-biennial oscillations (2·0–2·8 years) which explain in general less than 5 per cent of the variance and at maximum, near 10 per cent of the variance in Equatorial Africa. The QQO and QTO seem to be modulated frequencies of the same phenomenon connected to central and eastern tropical Pacific (CETP): (i) QQO, which is characterized by an out-of-phase pattern between East Africa (more/less rainfall than normal when CETP and western Indian Ocean is anomalously warm/cold) and central Africa seems to be associated mainly with in-phase behaviour between the CETP and western Indian Ocean. The relation is stable during the twentieth century; (ii) QTO, which is characterized by an in-phase pattern from East Africa to the Sahel (less/more rainfall when CETP is anomalously warm/cold) seems to be related only to CETP and this relation is strongest at the end of the period (after 1960–1970). Relationships between QQO and QTO of rainfall with the same oscillations of the Atlantic SST index are weak. © 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 785–805 (1977) (No. of Figures: 13. No. of Tables: 3. No. of References: 64.)
This paper provides an eight-year high-resolution climatology of Sahelian mesoscale convective systems (MCSs) during the summer. MCSs are defined as convective cloud clusters larger than 5000 km2. They are extensively tracked from METEOSAT full-resolution infrared images (time resolution 0.5 h and spatial resolution about 5 km). The method enables every MCS to be tracked throughout its entire lifetime. For each time step, the MCS location and its morphological and radiative characteristics are computed for three different brightness temperature thresholds. The methodology is presented, evaluated and compared with previous studies using low-resolution data.
Statistical MCS distributions, diurnal cycle and spatial variability of MCS characteristics are analysed on the basis of this high-resolution tracking. It is shown that a few large and long-lived cloud clusters contribute most of the total cloud cover. Sahelian cloud clusters propagate westward at a greater speed when very deep convection is well developed. The diurnal organization of the convection has been analysed, and has proved that the merging of MCSs is partly explained by the actual merging of independent convective entities, whereas the splitting of MCSs is mostly associated with weakening of convection. The importance of mesoscale convective complexes for the total MCS coverage has also been studied.
A 47-year record (1951–1997) of gridded data covering Africa south of the Sahara was used to document the spatial and seasonal patterns of the correlation between precipitation and sea-surface temperatures (SST) in key tropical areas, as depicted by the NIÑO3, South Atlantic and North Atlantic indices. El Niño–Southern Oscillation (ENSO) is confirmed as playing a dominant part in northeastern, eastern and southern Africa. However, its impact is also found over the Sahel during the northern summer, and other parts of the Gulf of Guinea region outside this season, a hitherto poorly documented feature. Over these two areas, ENSO and Atlantic SST (predominantly South Atlantic) contribute to different parts of the rainfall variance. The correlation with South Atlantic SST appears as a south–north dipole (positive/negative correlation) which shifts northward following the Inter-tropical Convergence Zone (ITCZ) translation between the northern low-sun and high-sun periods. A typing of the seasonal correlation patterns and a mapping of the multiple correlation coefficients are carried out in order to synthesize the space–time impacts of the three SST indices. Decadal-scale changes affect the strength of the teleconnections with both Atlantic and East Pacific SST, as reflected for instance by a small rise of the correlation with the NIÑO3 index since 1970–1975 in the Sahel and southern Africa, and additional shifts for the Atlantic Ocean, but the main patterns remain generally apparent over the whole period.
African free air wind velocity data for wet and dry periods in the Sahel have been analysed objectively by the Cressman scheme and show that the lower troposphere easterly jet over west Africa is stronger in the dry period whereas the analogous easterly jet to the south of the equator exhibits greater vertical shear in the wet period. Two possible explanations, based on modulations of the meridional temperature gradients across the jet, are offered.
The El Niño–Southern Oscillation (ENSO) and Indian monsoon are shown to have undergone significant interdecadal changes in variance and coherency over the last 125 years. Wavelet analysis is applied to indexes of equatorial Pacific sea surface temperature (Niño3 SST), the Southern Oscillation index, and all-India rainfall. Time series of 2–7-yr variance indicate intervals of high ENSO–monsoon variance (1875–1920 and 1960–90) and an interval of low variance (1920–60). The ENSO–monsoon variance also contains a modulation of ENSO– monsoon amplitudes on a 12–20-yr timescale. The annual-cycle (1 yr) variance time series of Niño3 SST and Indian rainfall is negatively correlated with the interannual ENSO signal. The 1-yr variance is larger during 1935–60, suggesting a negative correlation between annual-cycle variance and ENSO variance on interdecadal timescales. The method of wavelet coherency is applied to the ENSO and monsoon indexes. The Niño3 SST and Indian rainfall are found to be highly coherent, especially during intervals of high variance. The Niño3 SST and Indian rainfall are approximately 180 out of phase and show a gradual increase in phase difference versus Fourier period. All of the results are shown to be robust with respect to different datasets and analysis methods.
A reference case of a Sahelian weather system observed during the Hydrological Atmospheric Pilot Experiment, HAPEX-SAHEL, in August 1992, is described from a seasonal viewpoint as well as from synoptic and convective system viewpoints. It is shown that the case-study is representative of the climatology at all these scales and presents many interacting scales and physical processes. At intraseasonal scale, the monsoon onset is characterized by an abrupt shift of precipitation together with a latitudinal migration of the African easterly jet (AEJ) and convection. At the month and day scales, the convective activity occurs in an apparent zonal break of the tropical easterly jet. The month of August 1992 exhibits intense synoptic activity. The vorticity field is characterized by northerly (dry) and southerly (wet) components located at 850 hPa on each side of the AEJ. Their intraseasonal modulation on a period of 20 to 40 days leads to active and break phases of the synoptic activity. Around 21 August, the 700 hPa vorticity field features the propagation of a typical easterly wave with a westward propagation of a cyclonic circulation followed by an anticyclonic circulation. Convective activity occurs mainly ahead of the 700 hPa vorticity maximum with the formation of a squall line on Aïr mountains propagating south-westward at 15 m s−1. The convective system propagates about twice as fast as the vortex core, in contrast with the convection in the European Centre for Medium-Range Weather Forecasts re-analysis which stays in phase with the vorticity. The squall line corresponds to the largest contributor to the systems passing in August 1992 over the HAPEX-SAHEL region; its environmental conditions and its effects on the atmosphere including the surface parameters are presented. Copyright © 2002 Royal Meteorological Society
Using station rainfall data extracted from two comprehensive data sets, we show that large decreasing rainfall trends were widespread in the Sahel (10–20°N and 18°W–20°E) from the late 1950s to the late 1980s. Thereafter, Sahel rainfall has recovered somewhat through 2003, although the drought conditions have not ended in the region. These results confirm the findings of many previous studies. We also found that large multi-year oscillations appear to be more frequent and extreme after the late 1980s than previously. Analyses of Sahel regional rainfall time series derived from a fixed subset of stations and from all available stations show that the decreasing trend in Sahel rainfall is not an artifact of changing station networks. The rainfall model used by Chappell and Agnew (2004 International Journal of Climatology24: 547–554) is incorrect and their modelled rainfall time series is totally unrepresentative of Sahel average rainfall. Their conclusion about the Sahel rainfall trends being an artifact of changing station locations is emphatically wrong and their speculative statements about the implications of their results for other studies and other regions of the world are completely unfounded. Copyright © 2004 Royal Meteorological Society
The surface ocean explains a significant part of the inter-annual variability of the West African monsoon (WAM). The present paper explores the role of Gulf of Guinea sea surface temperatures (SST): how is the ocean–atmosphere observed relationship reproduced by state-of-the-art coupled models? The ‘Atlantic Niño’ is the main mode of inter-annual variability in the Gulf of Guinea. SST anomalies are maximum in June–July, and are associated with a convective anomaly in the marine Intertropical Convergence Zone (ITCZ), which spreads over the Guinean coast. In most of the studied CMIP3 simulations, the inter-annual variability of SST is very weak in the Gulf of Guinea, especially along the Guinean Coast. As a consequence, the influence on the monsoon rainfall over the African continent is hardly reproduced. Interestingly, many models exhibit a dipolar response of the marine ITCZ to the Atlantic Niño. In the observations and reanalyses, the absence of any evident shift in the position of the monsoon rainbelt is associated with a collapse of the correlations between Gulf of Guinea SST and Sahel rainfall at the end of the twentieth century. It is suggested that this may be due to the counteracting effects of the Pacific and Atlantic basins over the last decades. In CMIP3 simulations, the Atlantic Niño is often correlated with the El Niño Southern Oscillation (ENSO). However, only one simulation catches the observed evolution of the Pacific–Atlantic relationship at the end of the century. Copyright © 2009 Royal Meteorological Society
This study documents simulated precipitation and circulation changes through the 20C3M and A1b scenarios. It portrays a robust pattern, associating rainfall deficits in subtropical regions with rainfall excesses over West Africa, except in Northern Senegal and Mauritania, with a significant enhancement of both the April–June rainy season in 10/12 models and of the July–September rainy season in 8/12 models. Eastward to 5°W a northward shift in the latitude of the moisture flux convergence at 850 hPa is evident in 10/11 models (+0.58° in mean) and a southward shift in 6/11 models in the western region (−0.24°) is observed. Copyright © 2011 Royal Meteorological Society
From joint sea surface temperature/sea level pressure (SST/SLP) EOF analyses, low-frequency variability modes are compared. The multi-decadal oscillation (MDO) changed phases twice during the 20th century, with its north Atlantic SST patterns resembling the Atlantic multi-decadal oscillation (AMO). The quasi-decadal oscillation (QDO) SST patterns displayed a double tripole configuration over the entire Atlantic basin, leading to tropical inter-hemispheric out-of-phase relationship. From the mid-1960s onward, while SST anomalies were negative to the north (negative phases of MDO/AMO), the Sahelian drought persisted with a weaker hurricane power dissipation index (PDI). During that period, the QDO modulated the intensity of the Sahelian drought. Copyright © 2010 Royal Meteorological Society
The strong interannual‐to‐decadal variability of the West African Monsoon is subject to an active field of climate research that tries to disentangle its influencing factors and explore its predictability. Reliable observation‐based data over a preferably long period are arguably the most important basis for such efforts. Here, we try to explore the quality of several data products available for the earlier period of upper‐air observations (1940–1957): the Comprehensive Historical Upper‐Air Network (CHUAN), the NCEP/NCAR Reanalysis (NNR), and the Twentieth Century Reanalysis (20CR). To do so, we compare wind soundings from 37 pilot balloon stations contained in CHUAN (10°S–30°N, 20°W–20°E) with the reanalyses.
The comparison with the NNR reveals seasonally and diurnally varying significant differences relative to the observations over West and Central Africa. The differences reach absolute values of several metres per second, and their spatially coherent structure strongly points to a deficiency of the reanalysis rather than observational errors. The difference fields indicate an overestimation of the strength and thickness of the low‐level monsoon in all seasons and an underestimation of the Harmattan winds over the Sahel in winter. At higher levels, they point to an overestimation of the mid‐tropospheric monsoon return flow and African easterly jet.
For the 20CR, the fields reveal again significant differences up to several metres per second on all levels and in all seasons. However, the direction relative to the observed monsoon flow and Harmattan/trades is opposite at the low levels. Additionally, the differences tend to be smaller and more confined to the coastal region.
Further analysis demonstrates that the observed interannual variability is only insufficiently modelled in both reanalyses. Together with the diurnal cycle of the differences, this precludes a simple correction of the reanalyses and demonstrates that, depending on the purpose of a study, one should be extremely cautious when using reanalysis products. Copyright © 2011 Royal Meteorological Society
There are two major organized cloud configurations in the vicinity of the equator. Where there is a small cross‐equatorial surface pressure gradient, convection is close to the equator and is generally tied to the location of the lowest sea‐level pressure (SLP) and warmest sea‐surface temperature (SST), in agreement with arguments based upon simple thermodynamical considerations. However, when there is a substantial cross‐equatorial pressure gradient, such as occurs in the monsoon regions, organized convection appears off the equator in the summer hemisphere, equatorward of the SLP minimum and not necessarily collocated with the warmest SSTs. Thus, in this instance, simple thermodynamical considerations alone cannot explain 2he location of the convection. In this situation, the zero absolute vorticity contour (η = 0) also lies in the summer hemisphere. Therefore, between the equator and the η = 0 contour is a region of locally‐anticyclonic absolute vorticity and an inertially unstable regime. It is argued that the convection results from the low‐level divergence‐convergence doublet centred about the η = 0 contour which is the mitigating response to the inertial instability. The associated latitude‐height secondary circulation should provide subsidence (suppressed convection) over the equator and rising motion (enhanced convection) to the north of the zero absolute vorticity contour.
Signatures of the inertial instability predicted by theory are found in observations supporting the hypothesis. Wherever a strong cross‐equatorial pressure gradient exists, the η = 0 contour bisects a maximum in the divergent wind field. Divergence is found equatorward of the zero contour and convergence on the poleward side. Latitude‐height cross sections show strong local meridional circulations with maximum rising motion on the poleward side of η = 0. As the regions where the rising motions occur are conditionally unstable, there is deep convection and the vertical circulations extend throughout the troposphere. It is noted that the intensity of the off‐equator convection is deeper (and probably stronger) than convection located at the equator. This is probably because the convection associated with the inertial instability is more efficient. Necessary conditions for the location of near‐equatorial convection are listed.
Arguments are presented whereby inertial instability is established as the cause, rather than an effect, of off‐equatorial convection. These include an outline of the sequence of processes leading up to the convection. The factors that limit the encroachment of the η = 0 contour into the summer hemisphere are discussed and an explanation for the existence of the low‐level westerly monsoon wind maximum is suggested. The possible role played by the instability mechanism (or the lack of it) in coupled model simulations that produce seasonally migrating and/or double ITCZs in the eastern Pacific Ocean is discussed. Finally, it is proposed that the instability mechanism is important in the initiation of westward‐moving disturbances found in the eastern Pacific and in determining active and break periods in the summer Indian monsoon.
The existence of subtropical deserts, such as the Sahara, has often been attributed to the annual-mean, zonal-mean Hadley circulation which shows strong descent in the subtropics. However, the zonal-mean Hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter, but practically no zonal-mean subtropical descent during summer when rainfall over the eastern Sahara and the Mediterranean is least. Charney (1975) proposed a biosphere-albedo feedback mechanism whereby local anthropogenic effects related to over-grazing could affect the radiative balance, enhancing summertime diabatic descent and leading to desertification of the subtropics in general. The present study, which uses an idealized model, suggests a monsoon-desert mechanism for desertification whereby remote diabatic heating in the Asian monsoon region can induce a Rossby-wave pattern to the west. Integral with the Rossby-wave solution is a warm thermal structure that interacts with air on the southern flank of the mid-latitude westerlies causing it to descend. This adiabatic descent is localized over the eastern Sahara and Mediterranean, and over the Kyzylkum desert to the south-east of the Aral Sea, by the mountains of north Africa and south-west Asia. Trajectories indicate that the monsoon-desert mechanism does not represent a simple ‘Walker-type’ overturning cell. Instead, the descending air is seen to be mainly of mid-latitude origin. It is speculated that the monsoon-forced adiabatic descent may result in clear air and, therefore, a local diabatic enhancement which effectively doubles the strength of descent. With this mechanism, desertification can be forced by remote changes in monsoon strength rather than by local effects. This conclusion is supported by the observed dramatic strengthening of descent over the Mediterranean and east Sahara during the onset of the Asian monsoon and, on the longer timescale, by relating prehistoric lake-levels to Milankovitch-monsoon forcing. The latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern Sahara and the strength of a ‘tropic-wide’ monsoon. The monsoon-desert mechanism may not be confined to the Asian monsoon alone and the existence of other monsoon-climate regions over the globe may, in a similar way, explain the observed summertime strengthening of the oceanic sub-tropical anticyclones and the existence of western continental deserts and ‘Mediterranean-type’ climate regions.
A major factor in rainfall variability over Sahelian West Africa is the latitudinal location of the tropical rainbelt. When it is displaced abnormally far northward, the Sahel experiences a wet year. An anomalous southward displacement results in drought. In this paper we examine the question of what controls the location during the boreal summer, hypothesizing that inertial instability plays a role. An analysis of surface pressure and temperature fields, wind fields, divergence and vertical motion show that the criteria for inertial instability are satisfied in wet Augusts but not in dry ones. The key determinant appears to be the surface pressure gradient between the continent and the equatorial Atlantic. When this is large, inertial instability results in the development of a low-level westerly jet. The presence of this jet enhances the horizontal and vertical shear, and displaces the African Easterly Jet northwestward. Associated with this situation is strong vertical motion over the Sahel and subsidence over the Guinea Coast, producing dry conditions over the latter. The result is a rainfall dipole, one of two major modes of variability over West Africa. Important factors include sea surface temperatures (SSTs) in the equatorial Atlantic and pressure in the South Atlantic. The first of these factors suggests a link with the Atlantic Niño mode of tropical Atlantic variability; while the second suggests a possible link with the Pacific and the extratropical South Atlantic. Overall, our study relates the well-known SST influence on Sahel rainfall to atmospheric dynamics over the continent. Copyright © 2007 Royal Meteorological Society