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Spatio‐temporal patterns of rainfall trends and the 2015‐2017 drought over the winter rainfall region of South Africa
Abstract
We analyse long‐term (1900‐2017) rainfall data in the southern part of the winter rainfall region of southern Africa to understand the spatial patterns of recent and long‐term trends and contextualise the 2015‐2017 rainfall anomalies which led to the so‐called “Day Zero” drought in Cape Town. Our analyses reveal cohesive spatial patterns and seasonal differences in rainfall trends across a range of time scales. These suggest that rainfall is subject to regional driving mechanisms, predominantly manifested at the 20‐50 year time scale, but the influence of these mechanisms is modified by sub‐regional and seasonally specific processes, frequently resulting in trends of different magnitudes and even sign. Trend patterns are consistent with multi‐decadal scale quasi‐periodicity, with only the most recent phase (post‐1981 drying) corresponding to the expected regional response to hemispheric processes linked to anthropogenic climate change. The spatial and seasonal patterns of drying observed since 1981 alone do not explain the pattern of 2015‐2017 drought anomalies, although they share a strong autumn and weak mid‐winter signal. These results have implications to the interpretation of drought in the context of observed rainfall trends. Furthermore, we identify directions for improvement of the conceptual understanding of drivers of rainfall variability and the role of anthropogenic climate change in the winter rainfall region of South Africa. This article is protected by copyright. All rights reserved.
... Between 2015 and 2017 South Africa's winter rainfall zone (WRZ) saw its most severe 3-year drought in more than a century of rainfall records -with a return period of more than 300 years in the southern WRZ (SWRZ) Wolski et al., 2020). This event -commonly referred to as the "Day Zero Drought" after the consequent water crisisexposed the WRZ's vulnerability to drought stress, leading 64 W. S. Conradie et al.: SA winter rainfall drought heterogeneity ...
... However, observed long-term rainfall trends are weak or absent over most of the region (MacKellar et al., 2007(MacKellar et al., , 2014Hoffman et al., 2011;Kruger and Nxumalo, 2017;Burls et al., 2019). The centennial-scale records are dominated by quasi-oscillatory behaviour with a period of ≈ 40 years (Wolski et al., 2020;Ndebele et al., 2019;Du Plessis and Schloms, 2017). The significant trends observed in Climatic Research Unit (CRU) rainfall series (Otto et al., 2018;Seager et al., 2019) appear to be the result of a bias introduced to the dataset by dramatically reduced available station numbers in recent decades predominantly affecting wetter stations (Wolski et al., 2020). ...
... The centennial-scale records are dominated by quasi-oscillatory behaviour with a period of ≈ 40 years (Wolski et al., 2020;Ndebele et al., 2019;Du Plessis and Schloms, 2017). The significant trends observed in Climatic Research Unit (CRU) rainfall series (Otto et al., 2018;Seager et al., 2019) appear to be the result of a bias introduced to the dataset by dramatically reduced available station numbers in recent decades predominantly affecting wetter stations (Wolski et al., 2020). ...
The 2014–2018 drought over South Africa's winter rainfall zone (WRZ) created a critical water crisis which highlighted the region's drought and climate change vulnerability. Consequently, it is imperative to better understand the climatic characteristics of the drought in order to inform regional adaptation to projected climate change. In this paper we investigate the spatio-temporal patterns of drought intensity and the recent rainfall trends, focusing on assessing the consistency of the prevailing conceptual model of drought drivers with observed patterns. For this we use the new spatial subdivision for the region encompassing the WRZ introduced in our companion paper (Conradie et al., 2022).
Compared to previous droughts since 1979, the 2014–2018 drought in the WRZ core was characterised by a markedly lower frequency of very wet days (exceeding the climatological 99.5th percentile daily rainfall – including dry days) and of wet months (SPI1>0.5), a sub-seasonal attribute not previously reported. There was considerable variability in the spatial footprint of the drought. Short-term drought began in the south-western core WRZ in spring 2014. The peak intensity gradually spread north-eastward, although a spatially near-uniform peak is seen during mid-2017. The overall drought intensity for the 2015–2017 period transitions radially from most severe in the WRZ core to least severe in the surroundings. During 2014 and 2015, the drought was most severe at those stations receiving the largest proportion of their rainfall from westerly and north-westerly winds; by 2018, those stations receiving the most rain from the south and south-east were most severely impacted. This indicates an evolving set of dynamic drivers associated with distinct rain-bearing synoptic flows.
No evidence is found to support the suggestion that the drought was more severe in the mountain catchments of Cape Town's major supply reservoirs than elsewhere in the core nor that rain day frequency trends since 1979 are more negative in this subdomain. Rainfall and rain day trend rates also exhibit some connections to the spatial seasonality structure of the WRZ, although this is weaker than for drought intensity. Caution should be applied in assessing South African rain day trends given their high sensitivity to observed data shortcomings. Our findings suggest an important role for zonally asymmetric dynamics in the region's drought evolution. This analysis demonstrates the utility of the spatial subdivisions proposed in the companion paper by highlighting spatial structure in drought intensity evolution linked to rainfall dynamics.
... Differences in seasonality within the region have not been considered explicitly. Focusing on standardised variability, Wolski et al. (2021) detect three spatially continuous subregions of the WRZ, displaying a high degree of internal coherence that remains underexplored, since most of the northern WRZ was not considered and relatively few stations were used. Of the Day Zero drought studies, only Abba Omar and Abiodun (2020) have shown a link between spatial rainfall variability patterns and variability in rainfall drivers (cut-off lows -COLs). ...
... Of the Day Zero drought studies, only Abba Omar and Abiodun (2020) have shown a link between spatial rainfall variability patterns and variability in rainfall drivers (cut-off lows -COLs). It has been proposed that spatial differences in recent rainfall variability and trends may be related to wind directions from which rainfall is primarily received (Deacon et al., 1992;Wolski et al., 2021), but this hypothesis has not been systematically tested. ...
... This, in turn, would allow for more robust, locally relevant, climate change signal projections in this water-sensitive region. Hence, to complement the trend focus of Wolski et al. (2021), here we look at variability and seasonality and relate them to spatial and seasonal heterogeneity in rain-bearing winds. Section 2 provides a brief overview of the study domain, Sect. 3 describes the methods and data employed, Sect. 4 presents the primary findings of the study, Sect. 5 introduces a new spatial subdivision of the region, Sect. ...
A renewed focus on southern Africa's winter rainfall zone (WRZ) following the Day Zero drought and water crisis has not shed much light on the spatial patterns of its rainfall variability and climatological seasonality. However, such understanding remains essential in studying past and potential future climate changes. Using a dense station network covering the region encompassing the WRZ, we study spatial heterogeneity in rainfall seasonality and temporal variability. These spatial patterns are compared to those of rainfall occurring under each ERA5 synoptic-scale wind direction sector. A well-defined “true” WRZ is identified with strong spatial coherence between temporal variability and seasonality not previously reported. The true WRZ is composed of a core and periphery beyond which lies a transition zone to the surrounding year-round rainfall zone (YRZ) and late summer rainfall zone. In places, this transition is highly complex, including where the YRZ extends much further westward along the southern mountains than has previously been reported. The core receives around 80 % of its rainfall with westerly or north-westerly flow compared to only 30 % in the south-western YRZ incursion, where below-average rainfall occurs on days with (usually pre-frontal) north-westerly winds. This spatial pattern corresponds closely to those of rainfall seasonality and temporal variability. Rainfall time series of the core and surroundings are very weakly correlated (R2
... Many studies have investigated climatological factors driving the 2015-2017 drought. Although poor watermanagement practices and infrastructure deficiencies exacerbated this water crisis (Muller 2018), the 2015-2017 rainfall deficit was the main driver (Otto et al. 2018;Wolski et al. 2021). Several studies demonstrate that this rainfall deficit, particularly during the transition seasons, corresponds to trends of annual and seasonal rainfall declines, which are more pronounced for recent decades, from ~ 1980 (du Plessis and Schloms 2017; Sousa et al. 2018;Mahlalela et al. 2019;Jury 2020;Ndebele et al. 2020;Wolski et al. 2021). ...
... Although poor watermanagement practices and infrastructure deficiencies exacerbated this water crisis (Muller 2018), the 2015-2017 rainfall deficit was the main driver (Otto et al. 2018;Wolski et al. 2021). Several studies demonstrate that this rainfall deficit, particularly during the transition seasons, corresponds to trends of annual and seasonal rainfall declines, which are more pronounced for recent decades, from ~ 1980 (du Plessis and Schloms 2017; Sousa et al. 2018;Mahlalela et al. 2019;Jury 2020;Ndebele et al. 2020;Wolski et al. 2021). Odoulami et al. (2021) demonstrate that the 2015-2017 rainfall deficits correspond to a decrease in winter rain-bearing circulation types, including cold fronts, and an increase in dry circulation types, including the South Atlantic Anticyclone (SAA) and associated ridging anticyclones. ...
... It is necessary, following data quality control and cleaning, to determine data homogeneity (Aguilar et al. 2003;Wolski et al. 2021). However, due to a lack of metadata, on changes in data collection methods or station relocation, for instance, and no suitable reference series prior to 1900, homogenization was not possible for 1841-1899 (Picas et al. 2019). ...
Mounting evidence across South Africa’s southwestern winter rainfall zone (WRZ) reflects consistent drying since ~ 1980, and projected trends suggest this will continue. However, limited evidence exists for changes in the region’s rainfall seasonality. To improve our understanding of these WRZ drying trends, especially within the context of Cape Town’s 2015–2017 “Day Zero” drought, it is necessary to explore long-term rainfall seasonality trends. Thus, we use the longest WRZ meteorological record from the South African Astronomical Observatory (SAAO) in Cape Town to investigate rainfall seasonality shifts during 1841–2020. Consistent with recorded poleward migrations of the subtropical high-pressure belt and mid-latitude westerlies, known drivers behind the drought and drying trends, calculated trends demonstrate strengthening of WRZ conditions, primarily from a later start-date trend leading to a shorter wet-season. Long-term drying trends are quantified for the wet and dry seasons; however, analysis of trend evolution reveals much variability, reflecting that drying has only persisted since ~ 1892. Comparative analyses of the first and last 59 years of 1841–2020 reveal a rainfall decline of almost 10% across both seasons—highlighting that the extreme “Day Zero” drought was not only driven by wet-season rainfall declines. Results demonstrate that these drying trends were consistently driven by a long-term decline in rain day counts and a more recent decline in average rainfall per rain day. Correspondence between our results and projected rainfall seasonality trends suggests the trends we quantified will likely continue; thus, improvements and continuation of existing water conservation and management strategies are imperative for Cape Town.
... It is necessary, following data quality control and cleaning, to determine data homogeneity (Aguilar et al. 2003;Wolski et al. 2021). However, due to a lack of metadata, on changes in data collection methods or station relocation for instance, and no suitable reference series prior to 1900, homogenization was not possible for 1841-1899 (Picas et al. 2019). ...
... Note that the rainfall seasonality trends quanti ed here tentatively extend across the southwestern Cape region, containing Cape Town's supply dams (Fig. 1). This is argued based on results from Wolski et al. (2021), as trends for stations across the southwestern Cape region they de ned, based on interannual rainfall variability patterns, are largely consistent, particularly in direction, throughout the various temporal periods they considered. As rainfall seasonality characteristics represent surface responses of weather systems (Lennard and Hegerl 2015), the changes quanti ed here are likely largely driven by weather system changes which are primarily driven by hemispheric-scale atmospheric circulation changes (Sousa et al. 2018;Burls et al. 2019). ...
... al. 2016; du Plessis and Schloms 2017; Dunning et al. 2018; Jury 2020; Ndebele et al. 2020;Wolski et al. 2021). Although rainfall declines detected, across the southwestern Cape, for spring (September-November) or months therein are argued to re ect earlier end-dates (du Plessis and Schloms 2017; Jury 2020; Ndebele et al. 2020), our results do not necessarily re ect end-date changes during 1841-2020, but instead the end-dates varied substantially; this variability has seemingly increased for recent decades. ...
Mounting evidence across South Africa’s southwestern winter rainfall zone (WRZ) reflects consistent drying since ~1980 and projected trends suggest this will continue. However, limited evidence exists for the region’s rainfall seasonality changes. To improve our understanding of these WRZ drying trends, especially within the context of Cape Town’s 2015-2017 “Day Zero” drought, it is necessary to explore long-term rainfall seasonality trends. Thus, we use the longest WRZ meteorological record from the South African Astronomical Observatory (SAAO) in Cape Town to investigate rainfall seasonality shifts during 1841-2020. Consistent with recorded poleward migrations of the subtropical high-pressure belt and mid-latitude westerlies, known drivers behind the drought and drying trends, calculated trends demonstrate strengthening of WRZ conditions, primarily from a later start-date trend leading to a shorter wet-season. Long-term drying trends are quantified for the wet- and dry-seasons, however, analysis of trend evolution reveals much variability, reflecting that drying has only persisted since ~1892. Comparative analyses of the first and last 59 years of 1841-2020 reveals a rainfall decline of ~10% across both seasons – highlighting that the extreme “Day Zero” drought was not only driven by wet-season rainfall declines. Results demonstrate that these drying trends were consistently driven by a long-term decline in rain day counts and a more recent decline in average rainfall per rain day. Correspondence between our results and projected rainfall seasonality trends suggests the trends we quantified will likely continue, thus improvements and continuation of existing water conservation and management strategies are imperative for Cape Town.
... A drying is detected for the winter rainfall season over the recent past (Roffe et al., 2021), whereas a wettening was found over longer periods for the winter rainfall season alone and annual precipitation (MacKellar et al., 2014). Wolski et al. (2021) found negative trends of annual means for the long period 1900-2017 and the recent past 1981-2017, positive trends for the period 1933-2014, and mixed trends for the periods 1981-2014 and 1933-2017. This shows that trends over periods including the drought in 2015-2017 are generally negative. ...
... The findings of Onyutha (2018) with CRU data agree with our results of mainly wettening, except for areas directly at the coast over the period of 1901-2015 (annual precipitation) and more drying in coastal areas in austral summer. Analysis of station data for the WRZ showed a decrease for periods over the last 30 years (Roffe et al., 2021;Wolski et al., 2021). Also analysing precipitation trends over the last 30 years, Karypidou et al. (2022) found an underestimation of observed trends from CORDEX-Africa and CMIP5 models. ...
The southern African climate is strongly impacted by climate change. Precipitation is a key variable in this region , as it is linked to agriculture and water supply. Simulations with a regional atmospheric model over the past decades and the 21st century display a decrease in the past precipitation over some coastal areas of South Africa and an increase over the rest of southern Africa. However, precipitation is projected to decrease over the whole southern part of the domain in the future. This study shows that the Agulhas Current system, including the current and the leakage, which surrounds the continent in the east and south, impacts this precipitation trend. A reduction in the strength of the Agul-has Current is linked to a reduction in precipitation along the southeast coast. The Agulhas leakage, the part of the Agul-has Current that leaves the system and flows into the South Atlantic, impacts winter precipitation in the southwest of the continent. A more intense Agulhas leakage is linked to a reduction in precipitation in this region.
... Before analysis, raw data were scrutinised for outliers and errors, which were deleted, and cleaning was performed where necessary (Durre et al., 2010). Subsequently, we imputed all missing/deleted values, on the raw data temporal scales, using a weighted ratio considering nearby station records with a Pearson correlation coefficient of > 0.70 (Wolski et al., 2021). To produce daily values for analyses, daily maximum and minimum temperatures were averaged to quantify Fig. 1 Map of the study area depicting a Johannesburg (JHB) within South Africa's Gauteng province and Grassland biome, and b the location of the Johannesburg Botanical Gardens weather station (26.156°S 27.999°E at 1642 m asl) and the University of the Witwatersrand 7-day volumetric spore trap (26.193°S 28.030°E at 1758 m asl) sampling sites surrounded by a 30 km buffer around the spore trap (buffer size after Grinn-Gofroń et al., 2020). ...
Airborne fungal spores and pollen (aerospora), synergistic with air pollution, are key triggers of allergic respiratory diseases. Effective diagnosis and treatment requires up-to-date location-specific knowledge on the temporal variability of aerospora types and levels. Johannesburg is the largest city in South Africa and has grown substantially in three decades, with changes in ground cover, population density and air pollution, yet until now, no continuous aerospora sampling has occurred. We present a daily two-year (August 2019–July 2021) aerospora assemblage for Johannesburg and explore temporal characteristics of 13 dominant aerospora in relation to daily meteorological variables (pressure, rainfall, relative humidity, temperature and wind characteristics). February–July, July–September and January-July represent high-risk periods for fungal spores [(Alternaria alternata (Fries. ex Keissler), Ascospores, Aspergillus niger (Van Tieghem), Penicillium chrysogenum (Thom), Cladosporium graminum (Corda), Epicoccum nigrum (Link), Helminthosporium solani (Durieu and Montagne) Nigrospora sphaerica (Saccardo ex. Mason), Smuts Ustilago nuda (Jensen ex. Rostrup) and Torula herbarum (Link)], trees (Cupressus, Morus and Platanus) and grass (Poaceae), respectively. Using a generalised additive model, results show that daily meteorological characteristics explained 7–32% of daily aerospora variability, with the largest effect on tree pollen. Rainfall, relative humidity and temperature influenced daily fungal spore and Poaceae counts, with moderate/low rainfall (< 20 mm), higher/mid-ranging relative humidity (~ 40–60%) and temperatures of ~ 15–20 °C associated with higher counts during high-risk periods. Rainfall predominantly influenced tree counts during high-risk periods, with higher counts occurring on low rainfall (<10 mm) days. These results update the aerospora profile of Johannesburg, South Africa, providing important information to inform allergy care.
... The SBRIDI observed the presence of several drought monitoring indicators, the regional dissemination of drought-related information, and seasonal variations. Several studies used geostatistical techniques to examine the geographical and spatiotemporal features of the drought (Ji et al., 2021;Jiang et al., 2022;Wolski et al., 2021). ...
Droughts and heat waves are currently recognized as two of the most serious threats associated with climate changes. Drought is characterized by prolonged dry periods, low precipitation, and high temperature, while heat wave refers to an extended period of exceptionally high temperature, surpassing the region’s average for that time of year. There is a close relationship between droughts and heat waves, as both are often caused by similar weather patterns and can exacerbate each other’s impacts. Therefore, it is crucial to monitor and quantify both droughts and heat waves jointly at a regional level in order to develop sustainable policies and effectively manage water resources. This article develops a new index, the standardized composite index for climate extremes (SCICE), for joint monitoring and probabilistic quantification of extreme climate events at regional level. The procedure of SCICE is mainly based on the joint standardization of standardized precipitation index (SPI) and standardized temperature index (STI). In the application of SCICE, results reveal that the long-term probabilities of the joint occurrence of dry and hot events are significantly greater than those of wet and cold events. Furthermore, the outcomes of the comparative assessment support the validity of using SCICE as a compact statistical approach in regional drought analysis. In summation, the study demonstrates the capability of SCICE to effectively characterize and assess the joint monitoring of drought and heat waves at a regional level, providing a comprehensive approach to understanding the joint impact of climate extremes.
... This is particularly true for southern Africa, where precipitation extremes have already had enormous socioeconomic impacts. For instance, a multi-year drought in the southwest region of South Africa between 2015 and 2017 caused water supply reservoirs in Cape Town to deplete, leading to a serious water crisis that cut off the city's approximately 4 million residents' access to water (Wolski et al., 2021). In Mozambique, >1400 km 2 of farmlands were submerged in floodwaters, the head of livestock drowned in 2000, and >500 fatalities were reported during the flood event that occurred across the Limpopo basin in 2000 (Mosase and Ahiablame, 2018). ...
This study examines the change in daily characteristics of seasonal precipitation under 1.5 °C, 2.0 °C, and 3.0 °C global warming levels (GWLs) above the pre-industrial period over four southern African basins. We compare the projected changes among large ensembles of climate models during sub-seasons of the rainy season. In particular, we used future projections under the highest emission scenario from the Coupled Model Intercomparison Project (CMIP) versions 5 and 6, the Coordinated Regional Climate Downscaling Experiment (CORDEX), and the Coordinated Output for Regional Evaluations (CORE). The results show that all ensemble projects increase in the length of dry spells (CDD) and daily rainfall intensity (SDII), while wet spell duration (CWD) and the number of rainy days (RR1) will decrease under all GWLs. However, discrepancies are found in the signs of change for indices of heavy precipitation (R10mm and R20mm), especially among regional climate model (RCM) ensembles (CORDEX and CORE). The discrepancies among RCM ensembles may be associated with the difference in spatial resolution, number of downscaled global climate models (GCMs) and downscaling RCMs. Spatial inconsistencies are also found in terms of the magnitude and robustness of future changes under all GWLs. Overall, the projected changes are uncertain over most parts of the basins in CMIP6, whereas CORE projects a stronger signal and larger areas of robust changes over the basins under all GWLs compared to other ensembles. Stronger signals and larger areas of robust change in RCMs may be associated with an improved ability to capture topographical characteristics in RCMs compared to GCMs. Our results show that even if the Paris Agreement goals are met, some areas of the basin will be exposed to robust changes in extreme precipitation. However, our findings highlight that limiting global warming at 1.5 °C will reduce the exposure of southern Africa's major basins to global warming-induced risks. These findings are useful for Southern African policymakers to refine adaptation and mitigation strategies to account for warming-induced extreme precipitation.
... This is particularly true for southern Africa, where extreme precipitation events have already had enormous socioeconomic impacts. For instance, a multi-year drought in the southwest region of South Africa between 2015 and 2017 caused water supply reservoirs in Cape Town to deplete, leading to a serious water crisis that cut off the city's approximately 4 million residents' access to water (Wolski et al. 2021). In Mozambique, more than 1400 km² of farmlands were submerged in oodwaters, the head of livestock drowned in 2000, and more than 500 fatalities were reported during the ood event that occurred across the Limpopo basin in 2000. ...
We compare the consistency of the future response of extreme precipitation to 1.5°C, 2.0°C, and 3.0°C global warming levels (GWLs) above the pre-industrial period (1850–1900) over four major basins in southern Africa. In particular, we compare the results from large ensembles of global climate models (GCMs) and regional climate models (RCMs) from the Coupled Model Intercomparison Project (CMIP5 and CMIP6) and the COordinated Regional-climate Downscaling EXperiment (CORDEX and CORE). The results show that the signal of the changes under all GWLs is generally consistent across the ensembles, particularly for the spatially averaged medians over each basin. Generally, all the ensembles project an increase in maximum consecutive dry days (CDD) and simple daily rainfall intensity (SDII) and a decrease in maximum consecutive wet days (CWD) and the total number of rainy days (RR1) over the basins under all GWLs. However, there are differences in the signal and robustness of the projected changes among the ensembles. For instance, CORDEX projects a robust increase in heavy precipitation days over most of the Zambezi basin under 2.0°C and 3.0°C GWLs, while CORE projects a robust decrease over most of the basins. Additionally, the projected decrease in RR1 is robust over most parts of the basins in CMIP5, CORDEX, and CORE under all GWLs, but uncertain over most parts in CMIP6. Overall, CORE projects larger areas of robust changes in extreme precipitation compared to other ensembles, with uncertain changes over most parts of the basins in CMIP6. For the avoided impacts by limiting global warming at 1.5°C, all the ensembles agree that the impacts of extreme precipitation will be reduced in general over basins under 1.5°C GWLs compared to 2.0°C and 3.0°C GWLs.
... ; Wolski et al. 2021). In recent years, the perpetual increase in climate warming threatens to deteriorate the role of flood and drought mitigation, climatic, ecological, and regional environmental policies (Perkins-Kirkpatrick & Gibson 2017). ...
Climate change is one of the main consequences of anthropogenic activities. Since the 1950s, gradual changes and an increase in climate warming have been observed. Previous research has been indicating potential associations between climate warming and spatiotemporal changes in precipitation. Moreover, the regional patterns of precipitation have a key role in the continuous monitoring of climate characteristics and natural hazards such as floods and droughts. Therefore, precise and accurate measurements of precipitation concentration and spatiotemporal variability in their patterns are very crucial. In this study, a new method for measuring precipitation concentration is developed and applied to 54 meteorological stations in Pakistan. Furthermore, to assess the precipitation patterns, the proposed method provides solid evidence for considering the effect of temperatures under climate warming. Furthermore, using the spatial correlation between the proposed method and its competitor, a comparative analysis is made to evaluate the performance of the proposed method. Moreover, the spatial variability structure in various precipitation patterns is assessed and compared using spatial predictive maps. Outcomes associated with this research show significant deviations between the proposed method and the existing one. In this paper, regression analysis revealed that the additional input can potentially improve the precipitation estimates under the appropriate sampling estimator. This is the first study that has documented the impact of climate warming on measuring precipitation concentration. These findings can contribute to a better understanding of precipitation concentration in relation to climate warming.
HIGHLIGHTS
Develop a new method (RCTIPC) for precipitation modelling.;
Comparison between PCI and RCTIPC for precipitation modelling.;
Analyzing regional precipitation variability with climate change impact.;
Spatial variability structure in various precipitation patterns is assessed and compared using spatial predictive maps.;
... Constraining precipitation patterns is crucial for identifying hydrological extremes (i.e. floods and droughts) (Muñoz-Jiménez et al. 2018;Wolski et al. 2020), developing distributed hydrological models (Nezlin and Stein 2005;Gu et al. 2020), managing agriculture, and protecting eco-environment . In arid and semiarid regions, where water resources are scarce and highly sensitive to climate change, precipitation represents a primary factor influencing hydrological cycle processes and economic development (Ruiz-Sinoga et al. 2012;Lewandrowski et al. 2017). ...
Precipitation patterns are vital to water resource management and hydrological research, especially in the upper reaches of inland rivers in arid and semiarid areas. However, estimating spatiotemporal precipitation patterns at a basin scale is challenging due to limited observations. In this study, spatiotemporal patterns of precipitation amount, frequency, duration, and intensity at different time scales from 2014 to 2019 are estimated using the Bayesian maximum entropy method in the Tianlaochi catchment of the Heihe River watershed, northwest China. The study's results show that the annual average precipitation amount was 535.9 mm from 2014 to 2019, with precipitation amount between May and September accounting for 85.9% of the annual precipitation amount. For daily precipitation, the average frequency rate of light precipitation is highest at 59.55%, however, the average contribution rate of moderate precipitation is highest at 50.33%. The spatial distribution of precipitation is characterized by high-value areas concentrated in the central valley and low-value areas located at the catchment's outlet. The most important driving factors of precipitation patterns are elevation, relative humidity, and wind direction. These outcomes can be used to establish accurate hydrological models in the catchment and provide support for water resource management in the Heihe River watershed.
... Further, sufficient information about the prevalence of drought at the regional scale is required to establish effective drought mitigation policies. In recent research, several studies investigated drought spatial and spatiotemporal patterns using geostatistical methods (Wolski et al. 2021;Jiang et al. 2022;Ji et al. 2021). However, the difference between the frequencies of extreme climatic conditions, such as the dynamic structure of the rainfall pattern, is the most challenging issue around the world. ...
Drought is recurrently occurring in many parts of the globe. In contrast to other natural hazards, drought has complex climatic characteristics. Several environmental factors are involved in the occurrence of drought hazards. However, the selection of an appropriate drought index may incorporate to make efficient drought mitigation policies. Moreover, the spatial distributions of extreme weather conditions (Dry/Wet) are necessary to avoid the consequences of future drought hazards. In this study, we aimed to explore and compare the regional profile of Dry/Wet episodes under various drought measures. Consequently, this research proposes a new spatial comparative procedure to assess and evaluate the spatial predictive distributions of Dry/Wet episodes under various drought measures. The study incorporates three Standardized Drought Indices (SDIs) and a spatial Poisson log-normal model to assess and evaluate the spatial predictive distributions of Dry/Wet episodes in Pakistan. Results of this study show that the segregated patterns of Dry/Wet counts are moderately consistent with the climatology of the region. However, the spatial patterns of Wet/Dry counts under various drought measures are significantly different under each index. Therefore, this research suggests the simultaneous use of multiple drought measures for accurate and precise drought monitoring at the regional level.
... Recent prolonged droughts in the SWC have had significant negative effects on the local economy (Shepherd 2019). However, droughts are a relatively common occurrence in the SWC, but what makes them particularly damaging is when they last for consecutive years like the 2003-2004 extreme drought (Muller 2018) or the more recent "Day Zero" drought from 2015 to 2018 (Sousa et al. 2018;Wolski 2018;Wolski et al. 2021;Archer et al. 2019;Mahlalela et al. 2019). In the 2003-2004 case, the extended 2003-04 summer received below average rainfall while this was true for all the extended summers in 2014(De Kock et al. 2021. ...
Although on average, the southwestern Cape (SWC) of South Africa is a winter rainfall dominated region, almost 30% of the total rainfall occurs during the extended summer (October-March). A previous study showed that anomalously wet summers may help mitigate the effects of severe winter drought. Apart from that study, very little work has been done on summer rainfall variability over the SWC or the mechanisms associated with it. Here, station data and ERA5 reanalyses are used to investigated summer rainfall day variability and associated mechanisms. Interannual variability in summer rainfall day frequencies appears related to that in the South Atlantic High Pressure (SAHP) and westerly moisture fluxes across the midlatitude South Atlantic. Increased rainfall days are associated with cyclonic anomalies over the region and enhanced westerly moisture fluxes. Some of these circulation changes are related to the Southern Annular Mode, and in late summer, also to ENSO and changes in the zonal wavenumber 3 pattern. Significant decreasing trends in rainfall days were found in the mid- and late summer for the southern part of the region where most of the population lives and the main water supply dams are located. These trends seem associated with significant trends found in the southern boundary of the SAHP and in decreasing (increasing) South Atlantic storm counts in the 35°–45° S (50°–60° S) latitude bands.
... Understanding climate variability, climate change, and global warming is essential for understanding world's water cycle. Understanding the various water cycle's components has been the aim of a large body of studies on rainfall, flow, temperature, evaporation, and infiltration (Mengistu et al. 2021;Dang et al. 2021;Pokharel et al. 2020;Nistor et al. 2020;Li et al. 2020;Bouabdelli et al. 2020;Zerouali et al. 2020;Machiwal et al. 2019;Pathak et al. 2017;Santos et al. 2018;Gocic and Trajkovic 2014;Duncan et al. 2013;Meddi et al. 2010) to be the key elements in designing and managing civil engineering structures, particularly studies on the precipitation component (Wolski et al. 2021;Ghorbani et al. 2021a;Martinez-Artigas et al. 2020;Zerouali et al. 2021a;Machiwal et al. 2019;Merabti et al. 2018a, b;Lazri and Ameur 2018;Teodoro et al. 2016;Conway et al. 2009;Massei et al. 2011). Studies on rainfall are the most common compared to the other components; thus, the results of these works can help managers in decision-making processes in the field of water resources. ...
Due to its geographical location, Algeria is characterized by high spatiotemporal rainfall variability. In this study, data from 69 rain gauges located in representative humid, semiarid, and arid Mediterranean basins in northeastern Algeria were analyzed from 1970–2007 on a monthly scale using continuous wavelet analysis and hierarchical cluster analysis with the aim of regionalizing the rainfall patterns. The analysis shows that northern Algeria (Cluster 1), which has a humid climate, is dominated by periodic annual fluctuations in the 8–16-month band. This mode explains most of the total variance, with a contribution between 25 and 60%. In the Cluster 2 and Cluster 3 regions, the climate varies towards aridity (humid to arid from north to south), and the climate is dominated by long-term periodic phenomena characterizing multiannual fluctuations of 64–128 months to decadal periods greater than 128 months, which explains why the total cumulative contribution exceeds 50% of the total variance. In addition, the regional analysis of the isolated spectral bands of the 3–6-month (3 clusters), 8–16-month (3 clusters), and 1–3-year (4 clusters) scale-average variance revealed, globally and for the different regions, a long period of drought that was most pronounced during the 1970s, 1980s, and 1990s, whereas the wet years were marked by fluctuations that exceeded the 95% confidence level during the study period, with a very remarkable tendency towards wet conditions, particularly since the late 1990s. The obtained results can assist decision-makers in better sustainable development practices, especially in the fields of water resources, agriculture, and energy.
... Rain gauges have been the mainstay of global rainfall measurements for a very long time. Inadequate rain gauge density portends that the distribution of rainfalls cannot be well captured considering its high variability in time and space (Ouma et al., 2012;Ayanlade et al., 2018;Chen et al., 2018;Wolski et al., 2020). In Sub-Saharan Africa, the rain gauge density for a majority of the regions is often poor thereby limiting hydrological impact studies that require rainfall as a key forcing (Onyando et al., 2005;Liechti et al., 2012;Kimani et al., 2017). ...
Study region
Sio Malaba Malakisi river basin, East Africa.
Study focus
Poor rain-gauge density is a limitation to comprehensive hydrological studies in Sub-Saharan Africa. Consequently, Satellite precipitation products (SPPs) provide an alternative source of data for possible use in hydrological modeling. However, there is need to test their reliabilities across varied hydro-climatic and physiographic conditions to understand their applicability. Using two approaches, we evaluated the performance of six SPPs against gauge observations for possible water allocation studies in the SMMRB: (i) a point to pixel comparison using different statistical measures; (ii) hydrological evaluation of simulated discharge using the Continuous SEmi-distributed Runoff (COSERO) model approach.
New hydrological insights for the region
Our results indicate that CHIRPSv2 product performed best followed by MSWEPv2.2 as they suitably detected seasonal and annual rainfall amounts throughout the basin. However, at lower altitudes, most of the products overestimated rainfall as indicated by the performance measures. In some parts of the basin, the COSERO output signify an underperformance by PERSIANN-CDR and a good performance by GPM-3IMERG6. This could be attributed to differences in temporal dynamics of the products. In overall, seasonal trends captured by the SPPs can be used to support catchment management efforts in data scarce regions.
... The mid-1970s TWS change corresponds to a climate regime shift that has been linked to Interdecadal Pacific Oscillation (IPO) in previous studies [61,62]. Although the Cape Town region in the southwest portion of South Africa also experienced severe drought in 2015/2017, the drivers were different in this region [63]. Low-frequency teleconnections (e.g. ...
Water resources management is a critical issue in Africa where many regions are subjected to sequential droughts and floods. The objective of our work was to assess spatiotemporal variability in water storage and related controls (climate, human intervention) in major African aquifers and consider approaches toward more sustainable development. Different approaches were used to track water storage, including GRACE/GRACE Follow On satellites for Total Water Storage (TWS); satellite altimetry for reservoir storage, MODIS satellites for vegetation indices, and limited ground-based monitoring. Results show that declining trends in TWS (60–73 km3 over the 18 yr GRACE record) were restricted to aquifers in northern Africa, controlled primarily by irrigation water use in the Nubian and NW Saharan aquifers. Rising TWS trends were found in aquifers in western Africa (23–49 km3), attributed to increased recharge from land use change and cropland expansion. Interannual variability dominated TWS variability in eastern and southern Africa, controlled primarily by climate extremes. Climate teleconnections, particularly El Nino Southern Oscillation and Indian Ocean Dipole, strongly controlled droughts and floods in eastern and southern Africa. Huge aquifer storage in northern Africa suggests that the recent decadal storage declines should not impact the regional aquifers but may affect local conditions. Increasing groundwater levels in western Africa will need to be managed because of locally rising groundwater flooding. More climate-resilient water management can be accomplished in eastern and southern Africa by storing water from wet to dry climate cycles. Accessing the natural water storage provided by aquifers in Africa is the obvious way to manage the variability between droughts and floods.
... The project focus was driven in part by the City of Cape Town's increasing engagement and policy alignment with WSD that spans back to 2009, as well as the recent drought and associated "Day Zero" crisis that was largely as a result of significantly below average rainfall during the period 2015 to 2017 (Otto et al., 2018;Wolski et al., 2020). Cape Town relies on conventional centralised water management and planning approaches, with a water supply highly dependent (95%) on six large rainfed dams located outside of the city boundaries. ...
As South African cities urbanise alongside climate change, resource constraints, and socio-economic challenges, water sensitive (urban) design (WSD) is slowly gaining traction as a framework to address water security goals and entrench resilience. This article reflects on the progression of WSD in South Africa and discusses the broadening of its initial association with stormwater and physical infrastructure to include critical governance and institutional arrangements and social engagements at the core of a water sensitive transition. The approach is being adapted for the socio-economic challenges particular to South Africa, including basic urban water and sanitation service provision, WSD related skills shortages, a lack of spatial planning support for WSD, and the need for enabling policy. Since 2014, a national WSD Community of Practice (CoP) has been a key driver in entrenching and advancing this approach and ensuring that the necessary stakeholders are involved and sufficiently skilled. The WSD CoP is aimed at promoting an integrative approach to planning water sensitive cities, bridging the gaps between theory and practice and blending the social and physical sciences and silo divisions within local municipalities. Three South African examples are presented to illustrate the role of a CoP approach with social learning aspects that support WSD : (1) the “Pathways to water resilient South African cities” interdisciplinary project which shows the institutional (policy) foundation for the integration of WSD into city water planning and management processes; (2) the Sustainable Drainage Systems training programme in the province of Gauteng which demonstrates a skills audit and training initiative as part of an intergovernmental skills development programme with academic partners; and (3) a working group that is being established between the Institute for Landscape Architecture in South Africa and the South African Institution of Civil Engineering which illustrates the challenges and efforts of key professions working together to build WSD capacity.
... This indicates that besides poor technology and lack of other resources except finances, motivation towards conducting drought studies was low in the first study decade especially, but researchers became motivated as drought incidences became a topical issue due to increasing climate change. However drought socio-economic impact analysis dominates methods of drought reporting in Zimbabwe which actually is not supposed to be the case as drought is defined through scientific methods [39,98]. This calls for attention in the field of drought studies in the country. ...
Drought is ranked one of most devastating natural hazards in the world and this is also true in the context of Zimbabwe due to its constant recurrence. The article assesses spatial and temporal dynamics of drought in Zimbabwe between 1990 and 2020. Literature on spatial and temporal occurrence of drought in Zimbabwe highlights that drought occurrence was temporally and spatially heterogeneous from 1990 to 2020. Drought has been occurring since 1990s but its frequency was low during the 1990–2000 decade and high between 2000 and 2020. The most acknowledged drought was 1991/1992 followed by 1994/1995, 2000/2001, 2001/2002, 2004/2005, 2006/2007 and 2011/2012.However the current decade (2010–2020) is not yet well researched as shown by very low confirmation of drought years. Most of the studied literature confirmed drought conditions in Matebeleland and Masvingo followed by Midlands, Mashonaland East Mashonaland West and Mashonaland Central. Literature showed that the majority of studies on spatial and temporal distribution of drought in Zimbabwe were based on crop failure and livestock death assessments. Few studies used scientific meteorological and remote sensing based techniques to study drought in Zimbabwe and those few studies were conducted during the 2011–2020 decade. Therefore there is need to invest more in drought studies, especially use of remote sensing techniques to ensure effective monitoring of the spatio-temporal occurrence of drought in the country for strategic planning and implementation of drought risk reduction initiatives.
... Our multi-tree-ring parameter approach is aimed at investigating the potential of tree-ring data to 110 reconstruct climate in the Mediterranean region of South Africa, which recently has experienced 111 a strong multi-year rainfall deficit (Wolski et al., 2021). 112 ...
Clanwilliam cedar (Widdringtonia cedarbergensis; WICE), a long-lived conifer with distinct tree rings inCape Province, South Africa, has potential to provide a uniquely high-resolution climate proxy for southern Africa. However, the climate signal in WICE tree-ring width (TRW) is weak and the dendroclimatic potential of other WICE tree-ring parameters therefore needs to be explored. Here, we investigate the climatic signal in various tree-ring parameters, including TRW, Minimum Density (MND), Maximum Latewood Density (MXD), Maximum Latewood Blue Intensity (MXBI), and stable carbon and oxygen isotopes (δ¹⁸O and δ¹³C) measured in WICE samples collected in 1978. MND was negatively influenced by early spring (Oct-Nov) precipitation whereas TRW was positively influenced by spring November-December precipitation. MXD was negatively influenced by autumn (April-May) temperature whereas MXBI was not influenced by temperature. Both MXD and MXBI were negatively influenced by Jan-March and Jan-May precipitation respectively. We did not find a significant climate signal in either of the stable isotope time series, which were measured on a limited number of samples. WICE can live to be at least 356 years old and the current TRW chronology extends back to 1564 CE. The development of full-length chronologies of alternative tree-ring parameters, particularly MND, would allow for an annually resolved, multi-century spring precipitation reconstruction for this region in southern Africa, where vulnerability to future climate change is high.
... Understanding climate variability, climate change and global warming is essential for understanding world's water cycle. Understanding the various components water cycle's components has been the aim of a large body of studies on rainfall, flow, temperature, evaporation, infiltration (Mengistu et al., 2021;Dang et al., 2021;Pokharel et al., 2020;Nistor et al., 2020;Li et al, 2020;Bouabdelli et al, 2020;Zerouali et al., 2020;Machiwal et al., 2019;Pathak et al., 2017;Santos et al., 2018;Gocic and Trajkovic 2014;Duncan et al., 2013;Meddi et al., 2010) to be the key elements in designing and managing civil engineering structures, particularly studies on the precipitation component (Wolski et al., 2021;Martinez-Artigas et al., 2020;Zerouali et al., 2021;Machiwal et al., 2019;Lazri 3 and Ameur, 2018;Teodoro et al., 2015;Conway et al., 2010;Massei et al., 2011). Studies on rainfall are the most common compared to the other components; thus, the results of these works can help managers in decision-making processes in the field of water resources. ...
Due to its geographical location, Algeria is characterized by high spatiotemporal rainfall variability. In this study, data from 69 rain gauges located in representative humid, semiarid and arid Mediterranean basins in northeastern Algeria were analyzed from 1970–2007 on a monthly scale using continuous wavelet analysis and hierarchical cluster analysis with the aim of regionalizing the rainfall patterns. The analysis shows that northern Algeria (cluster #1), which has a humid climate, is dominated by periodic annual fluctuations in the 8–16-month band. This mode explains most of the total variance, with a contribution between 25 and 60%. In the cluster #2 and cluster #3 regions, the climate varies towards aridity (humid to arid from north to south), and the climate is dominated by long-term periodic phenomena characterizing multiannual fluctuations of 64–128 months to decadal periods greater than 128 months, which explains why the total cumulative contribution exceeds 50% of the total variance. In addition, the regional analysis of the isolated spectral bands of the 3–6-month (3 clusters), 8–16-month (3 clusters), and 1–3-year (4 clusters) scale-average variance revealed, globally and for the different regions, a long period of drought that was most pronounced during the 1970s, 1980s, and 1990s, whereas the wet years were marked by fluctuations that exceeded the 95% confidence level during the study period, with a very remarkable tendency towards wet conditions, particularly since the late 1990s. The obtained results can assist decision-makers in better sustainable development practices, especially in the fields of water resources, agriculture, and energy.
... Over the past decades, all five Mediterranean-type climate areas of the globe have experienced severe droughts [1][2][3][4][5][6][7][8][9][10]. While dendrochronological studies show that droughts have long been a recurrent feature of Mediterranean-type climate ecosystems [3,11], the most recent episodes appear to have been exacerbated by increasing global temperatures and model-based projections of climate indicate that drying and warming will continue during the 21st century. ...
Rising temperatures and increasing drought in Mediterranean-type climate areas are expected to affect plant–pollinator interactions, especially in plant species with specialised pollination. Central Chile experienced a mega drought between 2010 and 2020 which reached an extreme in the austral summer of 2019–2020. Based on intensive pollinator sampling and floral studies we show that the subalpine form of Mutisia subulata (Asteraceae) is a specialised hummingbird-pollinated species. In a two-year study which included the severest drought year, we quantified visitation frequency, flower-head density, flower-head visitation rates, two measures of floral longevity, nectar characteristics and seed set and monitored climatic variables to detect direct and indirect climate-related effects on pollinator visitation. Flower-head density, nectar standing crop and seed set were significantly reduced in the severest drought year while nectar concentration increased. The best model to explain visitation frequency included flower-head density, relative humidity, temperature, and nectar standing crop with highly significant effects of the first three variables. Results for flower-head density suggest hummingbirds were able to associate visual signals with reduced resource availability and/or were less abundant. The negative effect of lower relative humidity suggests the birds were able to perceive differences in nectar concentration. Reduced seed set per flower-head together with the availability of far fewer ovules in the 2019–2020 austral summer would have resulted in a major reduction in seed set. Longer and more intense droughts in this century could threaten local population persistence in M. subulata.
Study region: Sio Malaba Malakisi river basin, East Africa. Study Focus: Poor rain-gauge density is a limitation to comprehensive hydrological studies in Sub-Saharan Africa. Consequently, Satellite precipitation products (SPPs) provide an alternative source of data for possible use in hydrological modelling. However, there is need to test their reliabilities across varied hydro-climatic and physiographic conditions to understand their applicability. In this study, we evaluated and compared the Tropical Rainfall Measuring Mission (TRMM-3B42 v7), Climate Hazards Group Infrared Precipitation (CHIRPS v2.0), Multi-Source Weighted-Ensemble Precipitation (MSWEP v2.2), Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Climate Data Record (PERSIANN-CDR) and Tropical Applications of Meteorology using Satellite (TAMSAT) against gauge observations, for possible use in water allocation studies. Furthermore, the Continuous Semi-distributed Runoff (COSERO) model was adapted using the SPPs and applied to generate discharges, which were cross-compared with observed discharges. New Hydrological Insights for the region: Our results indicate that the SPPs are able to detect seasonal rainfall patterns throughout the basin. At lower altitudes, the products overestimated rainfall events as indicated by the performance measures. The COSERO results indicate that PERSIANN-CDR and MSWEPv2.2 overcompensated and underestimated discharge throughout the basin. This could be attributed to differences in temporal dynamics of the products. In overall, seasonal trends captured by the SPPs can be used to support catchment management efforts in data scarce regions.
Southern African climate is strongly impacted by climate change. Precipitation is a key variable in this region as it is linked to agriculture and water supply. An analysis of simulations with a regional atmospheric model over the last decades and the 21st century shows that in the past precipitation has decreased in some coastal areas of South Africa and increased in the rest of southern Africa. However, it is projected to decrease over the whole southern part of the domain in the future. The
5 Agulhas Current System, including the current and the leakage, surrounds the continent in the east and south and it is found to impact precipitation. The reduction in the strength of the Agulhas Current is linked to a reduction in precipitation along the southeast coast. The Agulhas leakage, the part of the Agulhas Current that leaves the system and flows into the South Atlantic, impacts the precipitation in the southwest of the continent, located in the winter rainfall zone. A more intense Agulhas leakage is linked to a reduction in precipitation in this region.
This chapter underlines the need for interdisciplinary and transdisciplinary research to support drought risk monitoring for sustainable development using Earth observation (EO)‐based information products and socioeconomic statistics. While a large number of drought indicators already exists, clear guidance on how to embed these into policy advice is still needed. EO‐based information enables the monitoring over large areas and for various time periods at a pixel level. This information helps to analyze and understand drought characteristics across scales. At the same time, socioeconomic variables are difficult to monitor only with sensor data. Here, research relies on household survey data and census information, often only linked to administrative levels. To understand the different determinants of drought risk among the three pillars of sustainable development (environment, society, and economy), a comprehensive drought monitoring system can be built, which fulfils the need to provide an understanding of the actual drought risk and to align decisions at the policy level.
‘Climate’ has always been variable, as it refers to the long-term prevailing weather conditions while ‘weather’ refers to the place-specific atmospheric state at a given point in time. A change in climate typically takes many years to become apparent. Climate change introduces a host of new unknowns in water security, such as the potential rate of change in rainfall and temperature. While it is highly unlikely that weather patterns change completely and permanently over a couple of seasons, the possibility cannot be entirely ruled out. Rainfed dams provide not only the least costly water supply source, but also one in which the Western Cape’s water authorities have decades of management experience. A plethora of information is available on water supply in the region, including extensive rainfall records. The system design historically provided for sufficient security and a 98% level of assurance of supply, all other things being equal. But the uncertainties introduced by climate change have resulted in reduced trust in reconciling demand and supply. Suppressed demand also limits the potential to impose restrictions in future droughts. As long as carbon emissions continue to increase unabated, the impact of climate change on water security will grow and this risk will need to be actively managed.
Although climate change is a global phenomenon, its manifestations and consequences are different in different regions, and therefore climate information on spatial scales ranging from sub-continental to local is used for impact and risk assessments. Chapter 10 assesses the foundations of how regional climate information is distilled from multiple, sometimes contrasting, lines of evidence. Starting from the assessment of global-scale observations in Chapter 2, Chapter 10 assesses the challenges and requirements associated with observations relevant at the regional scale. Chapter 10 also assesses the fitness of modelling tools available for attributing and projecting anthropogenic climate change in a regional context starting from the methodologies assessed in Chapters 3 and 4. Regional climate change is the result of the interplay between regional responses to both natural forcings and human influence (considered in Chapters 2, 5, 6 and 7), responses to large-scale climate phenomena characterizing internal variability (considered in Chapters 1–9), and processes and feedbacks of a regional nature.
This paper assesses the options that developing countries have in ensuring food security in an environment where key climate parameters are changing rapidly. Based on a case study of Lesotho, the paper utilizes the Global Climate Model ensemble to determine future precipitation and temperature projections using data from Climate Explorer. The results indicate that in Lesotho, maximum temperature is likely to continue to increase. Coupled with a significant increase in precipitation under both Representative Concentration Pathway (RCP) 4.5 (p = 0.0008) and RCP 8.5 (p = 0.0001) scenarios and a significant increase of evaporation under the two scenarios for RCP 4.5 (p = 0.0008) and RCP 8.5 (p = 0.0103), the country’s preparedness for hazards arising from climate change is rendered uncertain. Despite this reality, we suggest that uncertainty could be reduced by reinforcing existing innovative measures that could improve the productive capacity of subsistence farmers, so that they meet their own food requirements, while preventing further environmental deterioration. While some measures will be based on the intensification of government-led social support mechanisms, others will depend on the support rendered to “tried and tested” traditional practices such as machobane and fato-fato, which have a long tradition in the country. However, on their own, these measures are insufficient to cope with rapidly changing climatic conditions, unless they are coupled with national research development initiatives, improved early warning systems, and enhancement of environmental monitoring capabilities, the implementation of which requires careful land use planning.
Anthropogenic forcing of the climate is estimated to have increased the likelihood of the 2015–2017 Western Cape drought, also called ‘Day Zero’ drought, by a factor of three, with a projected additional threefold increase of risk in a world with 2 °C warming. Here, we assess the potential for geoengineering using stratospheric aerosols injection (SAI) to offset the risk of ‘Day Zero’ level droughts in a high emission future climate using climate model simulations from the Stratospheric Aerosol Geoengineering Large Ensemble Project. Our findings suggest that keeping the global mean temperature at 2020 levels through SAI would offset the projected end century risk of ‘Day Zero’ level droughts by approximately 90%, keeping the risk of such droughts similar to today’s level. Precipitation is maintained at present-day levels in the simulations analysed here, because SAI (i) keeps westerlies near the South Western
Cape in the future, as in the present-day, and (ii) induces the reduction or reversal
of the upward trend in southern annular mode. These results are, however,
specific to the SAI design considered here because using different model,
different SAI deployment experiments, or analysing a different location
might lead to different conclusions.
This policy brief highlights the adverse economic consequences of the current drought in the Western Cape on the agricultural sector in order to provide guidance to decision makers.
In early 2018 Cape Town’s water supply dropped so severely that the city ran the risk of running out of water, precipitating what is now known as the “Day Zero” crisis. Reservoir storage simulations point towards anomalously low rainfall as the driver of the water crisis, but evaluation of the observational record is still needed. Here, Natalie Burls of George Mason University, USA and colleagues evaluate historical rainfall variability using station data from the South African Weather Service. They find that a combination of low rainfall intensity for the 2015–2017 period and long-term declines in the number of rainfall days precipitated the crisis. Notably, the numbers of mid-latitude frontal systems which bring the majority of winter rainfall to the region show no significant downward trend. The declining rainfall characteristics are consistent with changes expected from the poleward expansion of the Hadley Circulation Cell and the associated southward shift in regional storm tracks.
Mediterranean-type climates are defined by temperate, wet winters, and hot or warm dry summers and exist at the western edges of five continents in locations determined by the geography of winter storm tracks and summer subtropical anticyclones. The climatology, variability, and long-term changes in winter precipitation in Mediterranean-type climates, and the mechanisms for model-projected near-term future change, are analyzed. Despite commonalities in terms of location in the context of planetary-scale dynamics, the causes of variability are distinct across the regions. Internal atmospheric variability is the dominant source of winter precipitation variability in all Mediterranean-type climate regions, but only in the Mediterranean is this clearly related to annular mode variability. Ocean forcing of variability is a notable influence only for California and Chile. As a consequence, potential predictability of winter precipitation variability in the regions is low. In all regions, the trend in winter precipitation since 1901 is similar to that which arises as a response to changes in external forcing in the models participating in phase 5 of the Coupled Model Intercomparison Project. All Mediterranean-type climate regions, except in North America, have dried and the models project further drying over coming decades. In the Northern Hemisphere, dynamical processes are responsible: development of a winter ridge over the Mediterranean that suppresses precipitation and of a trough west of the North American west coast that shifts the Pacific storm track equatorward. In the Southern Hemisphere, mixed dynamic-thermodynamic changes are important that place a minimum in vertically integrated water vapor change at the coast and enhance zonal dry advection into Mediterranean-type climate regions inland.
The southwest region of South Africa is the only part of southern Africa that predominantly receives its total annual rainfall during the austral winter months (April–September). In 2015–2017, this part of the country experienced extreme dry conditions which led to the severe water shortages experienced in the city of Cape Town. In this study, focused is placed on understanding the contribution of the early winter period (April–May) to wet and dry years in the southwestern part of South Africa. This period is of particular interest given its key role in the recent drought, the lack of previous work on this season, and climate change projections that the winter rainy season may shorten in duration. The early winter is found to be prone to dry conditions in recent decades, such that five of the six driest April–May in recent record have occurred after the year 2000. The dry early winters in particular tend to be associated with a weaker subtropical jet, less moisture flowing into the domain and a more stable atmosphere. It is found that although there is a moderate relationship between the Southern Annular Mode and early winter rainfall, it is not as strong as that compared to the full winter period. An analysis of CMIP5 models find that the projections portray the winter rainfall region in South Africa as being exposed to an increased likelihood of early winter dry conditions into the future (2040–2060). However, it remains a challenge for these models to reasonably capture the onset of winter rainfall in South Africa.
In the period 2015-2017, the Western Cape region has suffered from three consecutive years of below average rainfall - leading to a prolonged drought and acute water shortages, most prominently in the city of Cape Town. After testing that the precipitation deficit is the primary driver behind the reduced surface water availability, we undertake a multi-method attribution analysis for the meteorological drought, defined in terms of a deficit in the 3 years running mean precipitation averaged over the Western Cape area. The exact estimate of the return time of the event is sensitive to the number of stations whose data is incorporated in the analysis but the rarity of the event is unquestionable, with a return time of more than a hundred years. Synthesising the results from five different large model ensembles as well as observed data gives a significant increase by a factor of three (95% confidence interval 1.5-6) of such a drought to occur because of anthropogenic climate change. All the model results further suggest that this trend will continue with future global warming. These results are in line with physical understanding of the effect of climate change at these latitudes and highlights that measures to improve Cape Town's resilience to future droughts are an adaptation priority.
Since 2015 the greater Cape Town area (∼3.7 million people) has been experiencing the worst drought of the last century. The combined effect of this prolonged dry period with an ever-growing demand for water culminated in the widely publicized 'Day Zero' water crisis. Here we show how: (i) consecutive significant decreases in rainfall during the last three winters led to the current water crisis; (ii) the 2015-2017 record breaking drought was driven by a poleward shift of the Southern Hemisphere moisture corridor; (iii) a displacement of the jet-stream and South Atlantic storm-track has imposed significantly drier conditions to this region. Decreasing local rainfall trends are consistent with an expansion of the semi-permanent South Atlantic high pressure, and reflected in the prevalence of the positive phase of the Southern Annular Mode. Large-scale forcing mechanisms reveal the intensification and migration of subtropical anticyclones towards the mid-latitudes, highlighting the link between these circulation responses and the record warm years during 2015-2017 at the global scale.
Purpose of Review
Subtropical highs are an important component of the climate system with clear implications on the local climate regimes of the subtropical regions. In a climate change perspective, understanding and predicting subtropical highs and related climate is crucial to local societies for climate mitigation and adaptation strategies. We review the current understanding of the subtropical highs in the framework of climate change.
Recent Findings
Projected changes of subtropical highs are not uniform. Intensification, weakening, and shifts may largely differ in the two hemispheres but may also change across different ocean basins. For some regions, large inter-model spread representation of subtropical highs and related dynamics is largely responsible for the uncertainties in the projections. The understanding and evaluation of the projected changes may also depend on the metrics considered and may require investigations separating thermodynamical and dynamical processes.
Summary
The dynamics of subtropical highs has a well-established theoretical background but the understanding of its variability and change is still affected by large uncertainties. Climate model systematic errors, low-frequency chaotic variability, coupled ocean-atmosphere processes, and sensitivity to climate forcing are all sources of uncertainty that reduce the confidence in atmospheric circulation aspects of climate change, including the subtropical highs. Compensating signals, coming from a tug-of-war between components associated with direct carbon dioxide radiative forcing and indirect sea surface temperature warming, impose limits that must be considered.
Drought is a creeping phenomenon whose effects evolve with time, yet the start and end is often only clear in the hindsight. The present study assessed drought conditions using two categories of drought indicators computed from precipitation data sets measured by weather stations across the Western Cape Province, South Africa for the period 1985 to 2016. The first category was the Standardized Precipitation Index (SPI) accumulated over 3-, 6- and 12-months (hereafter of SPI-3, SPI-6 and SPI-12 respectively). The second category consists of the four Drought Monitoring Indicators (DMI) i.e., Drought Duration (DD), Severity (DS), Intensity (DI) and Frequency (DF). Firstly, analysis of SPI-3, SPI-6 and SPI-12 illustrate that between 1985 and 2016, the Western Cape Province experienced recurrent mild drought conditions. This suggests that the drought conditions experienced during 2015/2016 hydrological year (hereafter current) in the Western Cape Province is a manifestation of past drought conditions. Secondly, analysis of trends in DMI series depict a noticeable spatial-temporal dependence wherein the southern and western regions experienced more severe droughts compared to the eastern and northern regions of the Western Cape Province. Results also show that the DMI trends exhibit up to ~8% variability over the past decade. Overall, the current drought conditions in the Western Cape Province continues to adversely affect agricultural production while the water reservoirs are at below 30% capacity implying that the socio-economic impacts of these droughts will continue to reverberate for many months to come. Though the on-going drought conditions in the Western Cape Province is a regular part of nature’s cycle, analysis of historical drought characteristics based on drought indicators is an important first step towards placing the current drought conditions into perspective, and contribute to triggering action and response thereof. All these lay the foundation for drought monitoring and contribute towards the development of drought early warning.
In most Mediterranean climate (MedClim) regions around the world, global climate models (GCMs) consistently project drier futures. In California, however, projections of changes in annual precipitation are inconsistent. Analysis of daily precipitation in 30 GCMs reveals patterns in projected hydrometeorology over each of the five MedClm regions globally and helps disentangle their causes. MedClim regions, except California, are expected to dry via decreased frequency of winter precipitation. Frequencies of extreme precipitation, however, are projected to increase over the two MedClim regions of the Northern Hemisphere where projected warming is strongest. The increase in heavy and extreme precipitation is particularly robust over California, where it is only partially offset by projected decreases in low-medium intensity precipitation. Over the Mediterranean Basin, however, losses from decreasing frequency of low-medium-intensity precipitation are projected to dominate gains from intensifying projected extreme precipitation. MedClim regions are projected to become more sub-tropical, i.e. made dryer via pole-ward expanding subtropical subsidence. California’s more nuanced hydrological future reflects a precarious balance between the expanding subtropical high from the south and the south-eastward extending Aleutian low from the north-west. These dynamical mechanisms and thermodynamic moistening of the warming atmosphere result in increased horizontal water vapor transport, bolstering extreme precipitation events.
The aim of this study is to update the analysis of historical rainfall trends with reference to the work from previous studies, through optimizing the highest spatial resolution with the longest possible period of analysis, i.e., 1921–2015. Two interlinked datasets, namely the district rainfall and individual rainfall stations datasets were used for the trend analyses, namely, daily time series of 60 individual rainfall stations and the daily district rainfall of 88 of 94 rainfall districts in South Africa. The extreme precipitation indices defined by the World Meteorological Organization Expert Team on Climate Change Detection and Indices were applied. In general, the results show an increase in rainfall for most rainfall stations in the southern interior of South Africa, and indications of decreases in rainfall in the far northern and north-eastern parts. The increase in the annual rainfall in the south is reflected in the seasonal trends, where summer rainfall shows a similar increase, but also extends into the central interior. For other seasons, most of the country shows no significant historical trends in annual total rainfall. From the extreme rainfall analyses, an increase in daily rainfall extremes in the southern to western interior is apparent. Also, most of the country experienced increases in the intensity of daily rainfall, which confirms global results in general. Decreases in rainfall from wet spells were noted in most places over the east and north-east, while the southern and eastern parts along the escarpment experienced shorter annual dry spells. This study improves on previous studies in the region by more than doubling the analysis period, largely eliminating the influence that decadal-scale cycles might have on analyses over shorter periods. However, some differences in the trend results compared to previous studies are apparent, e.g., less pronounced drying in the east and the previously observed increase in rainfall in the western and southern interior not extending as far as the south-western Cape. The observed trends broadly confirm those of projected changes in summer rainfall, i.e., an increase in the west and decrease in the east. © 2017, South African Water Research Commission. All rights reserved.
A robust positive trend in the Southern Annular Mode (SAM) is projected for the end of the 21st century under the Representative Concentration Pathway 8.5 scenario, which results in rainfall decreases in the midlatitudes and increases in the high latitudes in the Southern Hemisphere (SH). We find that this SAM trend also increases rainfall over the SH subtropics in austral summer but not in winter, leading to a pronounced wintertime poleward expansion of the subtropical dry zone. These dynamically driven rainfall changes by the SAM appear to oppose the thermodynamically driven projected rainfall changes in the SH subtropics and midlatitudes, whereas the two components reinforce each other in the high latitudes. However, we show that most climate models fall short in capturing the observed SAM component driven by the El Niño–Southern Oscillation and associated rainfall in the austral warm seasons, which limits our confidence in quantifying the contribution of the SAM to projected rainfall changes.
Decadal to multidecadal 'ENSO-like' SST and circulation patterns in joint EOFs of global SST and MSLP, may be linked to South African rainfall variability. For eastern/northern South Africa (summer rainfall), the average of the 9-13 year and 18-39 year bandpass filtered EOFs tracks the rainfall well for 1903-1998. Higher (lower) pressure, warmer (cooler) SST in the Pacific and Indian Oceans and offshore (onshore) winds characterise the positive (negative) EOF signal and decreased (increased) rain. For southwestern South Africa (winter rainfall), the average of the 13-18 and 18-39 year filtered EOFs tracks the rainfall well from 1903-1982 with the 9-13 year EOF working better from 1983-1998. The suggested mechanism involves shifts in the upper level jet and in the strength of the subtropical high pressure belt and circumpolar trough.
The link between interannual variability of seasonal rainfall over the Cape south coast of South Africa and different synoptic types as well as selected teleconnections is explored. Synoptic circulation over the region is classified into different synoptic types by employing a clustering technique, the self-organizing map (SOM), on daily circulation data for the 33-year period from 1979 to 2011. Daily rainfall data are used to investigate interannual variability of seasonal rainfall within the context of the identified synoptic types. The anomalous frequency of occurrence of the different synoptic types for wet and for dry seasons differs significantly within the SOM space, except for austral spring. The main rainfall-producing synoptic types are to a large extent consistent for wet and dry seasons. The main rainfall-producing synoptic types have a notable larger contribution to seasonal rainfall totals during wet seasons than during dry seasons, consistent with a higher frequency of occurrence of the main rainfall-producing synoptic types during wet seasons compared to dry seasons. Dry seasons are characterized by a smaller contribution to seasonal rainfall totals by all the different synoptic types, but with the largest negative anomalies associated with low frequencies of the main rainfall-producing synoptic types. The frequencies of occurrence of specific configurations of ridging high pressure systems, cut-off lows and tropical-temperate troughs associated with rainfall are positively linked to interannual variability of seasonal rainfall. It is also shown that the distribution of synoptic types within the SOM space is linked to the Southern Annular Mode and El Niño Southern Oscillation, implying some predictability of intraseasonal variability at the seasonal time scale.
High-resolution observations along with atmospheric and oceanic reanalyses are diagnosed to understand how and why southeastern Atlantic SSTs have changed over the 1982–2013 period. Multiple datasets are used to evaluate confidence. Results indicate significant SST warming trends (0.5–1.5 K per 32-years) along the Guinean and Angolan/Namibian Coasts, and a cooling trend (−0.10 to −0.60 K per 32-years) over the subtropical South Atlantic between 18°S and 28°S. SST trends are shown to vary over the annual cycle with the greatest changes occurring during November–January. Analysis of the ocean surface heat balance reveals that the austral summer SST warming trend along the Angolan/Namibian Coast is associated with an increase in the net downward atmospheric heat flux. In addition, there is a decrease in coastal upwelling due to circulation changes related to a poleward shift of the South Atlantic subtropical anticyclone and an intensification of the southwestern African thermal low. The cooling trend over the subtropical South Atlantic is also associated with the poleward shift of the South Atlantic anticyclone, as stronger surface winds enhance latent heat loss from the ocean over this region. Positive SST trends along the Guinean coast are found to be primarily associated with changes internal to the ocean, specifically, reduced coastal upwelling, diffusion, and enhanced horizontal transport of warmer water. These results highlight the need to better understand South Atlantic subtropical anticyclone and the continental thermal low interactions and their implications for present day climate variability and future climate change.
Presented here is a global analysis of frontal activity variability derived from ERA-Interim data over the 34-yr period of January 1979–March 2013 using a state-of-the-art frontal tracking scheme. In December–February over that epoch, there is a northward shift of frontal activity in the Pacific in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), the largest trends are identified in the austral summer and are manifested by a southward shift of frontal activity over the Southern Ocean.
Variability of frontal behavior is found to be closely related to the main modes of atmospheric circulation, such as the North Atlantic Oscillation (NAO) for the Atlantic–European sector in the NH and the southern annular mode (SAM) in the middle and high latitudes of the SH. A signal associated with El Niño and hence emanating from the tropics is also apparent in the behavior of frontal systems over the Pacific by a reduction in the number of fronts in the middle South Pacific and intensification of frontal activity in high and low latitudes throughout the year. It is shown in general that the associations of the large-scale modes with frontal variability are much stronger than with cyclones. This indicates that the quantification of the behavior of fronts is an important component of understanding the climate system. At the very high latitudes, it is also shown here that, in the recent years of rapid sea ice reduction in the Arctic, there have been fewer summer fronts observed over the Canadian Arctic.
Observed trends in seasonal and annual total rainfall, number of rain days and daily maximum and minimum temperature were calculated for a number of stations in South Africa for the period 1960-2010. Statistically significant decreases in rainfall and the number of rain days are shown over the central and northeastern parts of the country in the autumn months and significant increases in the number of rain days around the southern Drakensberg are evident in spring and summer. Maximum temperatures have increased significantly throughout the country for all seasons and increases in minimum temperatures are shown for most of the country. A notable exception is the central interior, where minimum temperatures have decreased significantly. Regionally aggregated trends for six water management zones covering the entire country are not evident for total rainfall, but there are some significant trends for the number of rain days. Temperature in these zones has increased significantly for most seasons, with the exception of the central interior. Comparison of the observed trends with statistically downscaled global climate model simulations reveals that the models do not represent the observed rainfall changes nor the cooling trend of minimum temperature in the central interior. Although this result does not rule out the possibility of attributing observed local changes in rainfall to anthropogenically forced global change, it does have major implications for attribution studies. It also raises the question of whether an alternative statistical downscaling method or dynamical downscaling through the use of a regional climate model might better represent regional and local climatic processes and their links to global change.
This paper describes the construction of an updated gridded climate dataset (referred to as CRU TS3.10) from monthly observations at meteorological stations across the world's land areas. Station anomalies (from 1961 to 1990 means) were interpolated into 0.5° latitude/longitude grid cells covering the global land surface (excluding Antarctica), and combined with an existing climatology to obtain absolute monthly values. The dataset includes six mostly independent climate variables (mean temperature, diurnal temperature range, precipitation, wet-day frequency, vapour pressure and cloud cover). Maximum and minimum temperatures have been arithmetically derived from these. Secondary variables (frost day frequency and potential evapotranspiration) have been estimated from the six primary variables using well-known formulae. Time series for hemispheric averages and 20 large sub-continental scale regions were calculated (for mean, maximum and minimum temperature and precipitation totals) and compared to a number of similar gridded products. The new dataset compares very favourably, with the major deviations mostly in regions and/or time periods with sparser observational data. CRU TS3.10 includes diagnostics associated with each interpolated value that indicates the number of stations used in the interpolation, allowing determination of the reliability of values in an objective way. This gridded product will be publicly available, including the input station series (http://www.cru.uea.ac.uk/ and http://badc.nerc.ac.uk/data/cru/). © 2013 Royal Meteorological Society
The contribution of Cut-off Lows (CoLs) to precipitation and extreme rainfall frequency in South Africa has been quantified from 402 station records over the period 1979–2006. Firstly, 500 hPa CoL trajectories over Southern Africa and surrounding oceans were determined and their features thoroughly analyzed. In a second step, using daily precipitable water, outgoing long wave radiation data and station rainfall records, an area was defined where the occurrence of CoLs is associated with rainfall over South Africa. CoLs transiting in the 2.5°E–32.5°E/20°S–45°S are more likely to produce precipitation over the country. When 500 hPa CoLs are centered just off the west coast of the country (around 15°E/32.5°S) their impact is substantial in term of daily rainfall intensity and spatial coverage. CoL rainy days have been studied and it is shown that they significantly contribute to precipitation in South Africa, more strongly along the south and east coasts as well as inland, over the transition zone between the summer and winter rainfall domains where they contribute between 25 to more than 35 % of annual accumulation. At the country scale, CoL rainfall is more intense and widespread in spring than during other seasons. Over the analyzed period, a significant trend in annual CoLs’ frequency shows an increase of about 25 %. This increase is mainly realized in spring and in a lesser extent in summer. This trend is accompanied by a significant increase in the frequency of CoL rainy days specifically along the south coast and over the East of the country during the spring–summer period. In parallel, it is shown that from late spring until summer CoLs’ frequency varies significantly accordingly with large scale circulation modes of the Southern Hemisphere such as the Pacific South American pattern (PSA). This positive trend in CoLs’ frequency may be related with the positive trend in the PSA during the spring–summer period over the three last decades.
Decadal to multidecadal `ENSO-like' SST and circulation patterns in joint EOFs of global SST and MSLP, may be linked to South African rainfall variability. For eastern/northern South Africa (summer rainfall), the average of the 9-13 year and 18-39 year bandpass filtered EOFs tracks the rainfall well for 1903-1998. Higher (lower) pressure, warmer (cooler) SST in the Pacific and Indian Oceans and offshore (onshore) winds characterise the positive (negative) EOF signal and decreased (increased) rain. For southwestern South Africa (winter rainfall), the average of the 13-18 and 18-39 year filtered EOFs tracks the rainfall well from 1903-1982 with the 9-13 year EOF working better from 1983-1998. The suggested mechanism involves shifts in the upper level jet and in the strength of the subtropical high pressure belt and circumpolar trough.
Relationships between the Antarctic Oscillation (AAO) and winter rainfall over western South Africa are investigated. This region receives most of its annual rainfall during austral winter and often experiences severe drought. It is found that 6 (6) of the 7 (8) wettest (driest) winters (JJA) during 1948-2004 occur during negative (positive) AAO phase, i.e., positive (negative) pressure anomalies over Antarctica and negative (positive) anomalies over the midlatitudes. The mechanisms by which the AAO appears to influence winter rainfall involve shifts in the subtropical jet, and changes in the low-level moisture flux upstream over the South Atlantic and in the mid-level uplift, low-level convergence and relative vorticity over the region. The anomalous circulation patterns extend into spring; thus, the springs following the identified winters also show similar rainfall anomalies.
Whereas the impact of ENSO on the African summer rainfall regions is largely documented and still regularly investigated, little is known about its impact on the winter rainfall regions located at the southwestern and northwestern tips of Africa. Yet, these regions are densely inhabited and are net exporters of high-quality agricultural products. Here we analyze the relationship between El Niño Southern Oscillation (ENSO) and South Africa austral winter rainfall using a 682 raingauges daily rainfall database documenting the period 1950-1999. The May, June and July (MJJ) seasonal rainfall amount shows a positive correlation with the Niño3.4 index that becomes significant since the so-called 1976/1977 climate regime shift. Wet spells properties (length, frequency and intensity) at the raingauge scale indicate that high (low) MJJ seasonal rainfall amounts recorded during El Niño (La Niña) events are the result of longer (shorter) wet spells in the Cape Town area and more (less) frequent wet spells north of 33 °S. Wet spells with daily rainfall amounts ranging between 10 and 50 mm are also more (less) frequent. Atmospheric dynamics fields during wet spells feature lower (higher) pressure and northwesterly (southerly) wind anomalies in the troposphere over the region. This suggests that rain-bearing systems are deeper (thinner) and larger (smaller) in extent, and located farther north (south) during El Niño (La Niña) events.
Annual global surface temperature and global land surface temperature trends are calculated for all possible periods of the historical record between 1850 and 2009. Two-dimensional parameter diagrams show the critical influence of the choice of start and end years on the calculated trend and associated temperature changes and suggest time scales required to establish robust trends.
The largest trends and associated temperature changes are all positive and have occurred over periods ending in recent years. Substantial positive changes also occurred from the early twentieth century until the mid-1940s. The continents exhibit greater long-term warming than the global average overall, but less warming in the early part of the century (segments ending in the 1940s). The recent period of short-term cooling beginning in the late 1990s is neither statistically significant nor unusual in the context of trend variability in the full historical record.
Global-mean and land surface temperature changes for periods ending in recent years and longer than about 90 years are extremely unlikely to have occurred by chance. In contrast, short-term trends over less than a few decades are generally not statistically significant. This implies significant contributions of decadal variability to trends estimated over such short time periods.
The Southwestern Cape (SWC) region of South Africa is characterized by winter rainfall brought mainly via cold fronts and by substantial interannual variability. Previous work has found evidence that the interannual variability in SWC winter rainfall may be related to sea surface temperature (SST) in the South Atlantic Ocean and to large-scale ocean-atmosphere interaction in this region. During wet winters, SST tends to be anomalously warm (cool) in the southwest Atlantic and southeast Atlantic (central South Atlantic). Atmospheric general circulation model experiments with various idealized SST anomalies in the South Atlantic are used to explore mechanisms potentially associated with the rainfall variability. The model results suggest that the atmosphere is sensitive to subtropical-midlatitude SST anomalies in the South Atlantic during winter. Locally, there are changes to the jet position and strength, low-level relative vorticity, and convergence of moisture and latent heat flux that lead to changes in rainfall over the SWC. The model response to the SST forcing also shows large-scale anomalies in the midlatitude Southern Hemisphere circulation, namely, an Antarctic Oscillation-type mode and wavenumber-3 changes, similar to those observed during anomalous winters in the region.
Daily European station series (1901–99) of surface air temperature and precipitation from the European Climate Assessment dataset are statistically tested with respect to homogeneity. A two-step approach is followed. First, four homogeneity tests are applied to evaluate the daily series. The testing variables used are (1) the annual mean of the diurnal temperature range, (2) the annual mean of the absolute day-to-day differences of the diurnal temperature range and (3) the wet day count (threshold 1 mm). Second, the results of the different tests are condensed into three classes: ‘useful’, ‘doubtful’ and ‘suspect’. A qualitative interpretation of this classification is given, as well as recommendations for the use of these labelled series in trend analysis and variability analysis of weather extremes. In the period 1901–99, 94% of the temperature series and 25% of the precipitation series are labelled ‘doubtful’ or ‘suspect’. In the sub-period 1946–99, 61% of the temperature series and 13% of the precipitation series are assigned to these classes. The seemingly favourable scores for precipitation can be attributed to the high standard deviation of the testing variable, and hence the inherent restricted possibilities for detecting inhomogeneities. About 65% of the statistically detected inhomogeneities in the temperature series labelled ‘doubtful’ or ‘suspect’ in the period 1946–99 can be attributed to observational changes that are documented in the metadata. For precipitation this percentage is 90%. Copyright © 2003 Royal Meteorological Society
We analyze changes and fluctuations in sea surface temperature at the monthly scale around the South African coastline from 1982 to 2009.. There is a statistically significant negative trend of up to 0.55°C per decade in the Southern Benguela from January to August. Cooling trend of lesser magnitude is observed at the South Coast from May to August. This is due to an increase in upwelling favorable southeasterly and easterly wind. Positive trend in sea surface temperature of up to 0.55°C per decade occurred in most part of the Agulhas Current system at all months of the year except for KwaZuluNatal. El Nino Southern Oscillation is significantly positively correlated at the 95 % level with Southern Benguela and South Coast from February to May and negatively correlated with the Agulhas Current system South of 36°S. El Nino suppresses upwelling along the coast while La Nina increases it. Although, there does not seem to be a linear relationship between strength of ENSO and magnitude of coastal SST perturbation, El Nino and La Nina appear to be linked with major warm and cool events at the seasonal scale in late summer in the Southern Benguela and the South Coast. A word of caution is given on low resolution reanalyzed climate data (ERA40 and NCEP) and optimally interpolated Reynolds sea surface temperature used here.
Cut-Off Lows (COLs) are known for heavy rainfall in the Western Cape, but there is a dearth of information on COL rainfall characteristics over the Cape. To bridge this gap we analysed three types of datasets (observation, satellite and reanalysis) to study the characteristics of COLs that occurred in the vicinity of the Western Cape over 37 years (1981–2017) and applied a self-organising Map (SOM) to classify the COLs into groups based on their rainfall patterns. The results show about 10 COLs (per year) occur over the Western Cape and contribute about 11% of the annual rainfall in the cape but with a large inter-annual variability. In 2015 and 2016, the COLs occurred more frequently and contributed more rainfall than normal which reduced the drought severity. But, in 2017, the frequency and rainfall contribution of the COLs were below normal. Nevertheless, more COLs does not always mean higher COL rainfall as >45% of the COLs over the Western Cape produce little or no rainfall. The SOM results reveal that the spatial distribution of COL rainfall can be grouped into four major patterns. The first pattern indicates rainfall over the entire Western Cape, while the second shows little or no rainfall; the third and fourth patterns feature rainfall over the south-east coast and south-west coast, respectively. Wet COLs are associated with more southward transport of warm, moist tropical air into the Western Cape and greater contrasts between the warm and cold air masses compared to the dry COLs.
The rainfall series from the South African Astronomical Observatory in Cape Town, South Africa, is one of the longest known single site instrumental records in the southern hemisphere, spanning over 176 years. Rainfall data are analysed to determine trends and periodicity in the series for annual, seasonal and monthly time scales. Using the Mann Kendall test and Sen's slope, significant negative rainfall trends are recorded for the months of March and October, and for the spring season (from September to November). Using the Mann Kendall and its modified versions to account for serial correlation, as well as a multi‐temporal trend analysis, we demonstrate a positive rainfall trend during the first 60 years (i.e., 1841–1900), which thereafter changes to a long‐term (1900–2016) negative trend, but incorporating a shorter 40 year significant positive trend between 1930 and 1970. We identify cyclic patterns with recorded periods of 9–12 years, 16–30 years and 30–42 years for rainfall, the Southern Annular Mode (SAM) and Southern Oscillation Index (SOI). In addition to the notable 9–12 year rainfall cycle that is evidently associated with sunspot cycles, 20–30 year and longer 32–40 year rainfall, solar, SAM and SOI cycles are also identified. This article is protected by copyright. All rights reserved.
This study examines the relationship between midlatitude synoptic activity and variations in the width of the tropics in the Southern Hemisphere for the period 1979–2016. The edge of the tropical belt is defined here in terms of the latitude of the subtropical ridge (STR) of sea level pressure, and eddy activity in the midlatitudes is characterized by the behavior of atmospheric fronts. It is shown that the location and intensity of the STR are significantly correlated with the number of cold fronts between 20° and 40°S and that these relationships exhibit seasonal and zonal asymmetry. The link between the STR and the number of fronts is analyzed in five sectors of the Southern Hemisphere to reveal regional differences in their behavior and relationship with the southern annular mode. Some earlier studies on the widening of the tropics suggest that such changes may be caused by a shift in the location of midlatitude eddies. Our analysis explores the connection between these on a synoptic time scale. It shows that the variability of the width of the tropics is indeed strongly influenced by changes in the midlatitude synoptic activity, and that changes in synoptic activity lead those in the edge of the tropical belt by approximately one day.
The Western Cape region of South Africa is a key producing area for South African agriculture, with a strong dependence on austral winter rainfall. The past three years have, however, seen extensive drought impacting this region, with significant impacts on agriculture. In this article, we unpack how the drought unfolded, as well as possibilities in predicting winter rainfall. We consider how drought impacted agriculture, looking in depth at two commodities key to the winter rainfall region agricultural sector – namely, wheat and apples; concluding with a brief discussion of implications for the future. Keywords: Drought, Agriculture, Seasonal forecasting, Southern Africa
The Angola low is a key feature of the southern Africa wet season atmosphere that influences precipitation across the continent. This paper uses ERA-Interim to show that the synoptic expression of the Angola low is a combination of dry heat lows and moist tropical low pressure systems. The Angola heat low and Angola tropical low composites are contrasted against similar lows observed in other continental tropical regions and found to be broadly comparable. The implications that the distinction between dry and moist events has for the interannual relationship among the Angola low, precipitation, and ENSO are examined. The tropical lows exhibit unusual semistationary behavior by lingering in the Angola region rather than traveling offshore. This behavior is proposed to be caused by an integrated sea breeze–anabatic wind that enhances (inhibits) cyclonic vorticity stretching and convection inland (near the coast). The combined effect of the heat lows and the anchored tropical lows creates the Angola low in the climatological average. By elucidating the mechanisms of the Angola low, this research improves the foundation of process-based evaluation of southern Africa present and future climate in CMIP and AMIP models.
In recent decades, the subtropical edges of Earth's Hadley circulation have shifted poleward. Some studies have concluded that this observed tropical expansion is occurring more rapidly than predicted by global climate models. However, recent modeling studies have shown that internal variability can account for a large fraction of the observed circulation trends, at least in an annual-mean, zonal-mean framework. This study extends these previous results by examining the seasonal and regional characteristics of the recent poleward expansion of the Hadley circulation using seven reanalysis datasets, sea level pressure observations, and surface wind observations. The circulation has expanded the most poleward during summer and fall in both hemispheres, with more zonally asymmetric circulation trends occurring in the Northern Hemisphere (NH). The seasonal and regional characteristics of these observed trends generally fall within the range of trends predicted by climate models for the late twentieth and early twenty-first centuries, and in most cases, the magnitude of the observed trends does not exceed the range of interdecadal trends in the models' control runs, which arise exclusively from internal variability. One exception occurs during NH fall when large observed poleward shifts in the atmospheric circulation over the North Atlantic sector exceed nearly all trends projected by models. While most recent NH circulation trends are consistent with a change in phase of the Pacific decadal oscillation (PDO), the observed circulation trends over the North Atlantic instead reflect 1) large natural variability unrelated to the PDO and/or 2) a climate forcing (or the circulation response to that forcing) that is not properly captured by models.
During the austral summer season (November–February), southern African rainfall, south of 20°S, has been shown to vary over a range of timescales, from synoptic variability (3-7 days, mostly Tropical-Temperate Troughs) to interannual variability (2-8 years, reflecting the regional effects of El Niño Southern Oscillation). There is also evidence for variability at quasi-decadal (8-13 years) and interdecadal (15-28 years) timescales, linked to the Interdecadal Pacific Oscillation and the Pacific Decadal Oscillation, respectively. This study aims to provide an overview of these ranges of variability, their influence on regional climate and large-scale atmospheric convection, and quantify uncertainties associated with each timescale. We do this by applying k-means clustering onto long-term (1901–2011) daily Outgoing Longwave Radiation anomalies derived from the 56 individual members of the 20th Century Reanalysis. Eight large-scale convective regimes are identified.
Results show that: (i) the seasonal occurrence of the regimes significantly varies at the low-frequency timescales mentioned above; (ii) these modulations account for a significant fraction of seasonal rainfall variability over the region; (iii) significant associations are found between some of the regimes and the aforementioned modes of climate variability; and (iv) associated uncertainties in the regime occurrence and convection anomalies strongly decrease with time, especially the phasing of transient variability. The short-lived synoptic anomalies and the low-frequency anomalies are shown to be approximately additive, but even if they combine their respective influence at both scales, the magnitude of short-lived perturbations remains much larger.
With reservoirs running dry, Capetonians are bracing themselves for the day when their water supply may be switched off. Piotr Wolski delves into the rainfall record to put this drought into historical context With reservoirs running dry, Capetonians are bracing themselves for the day when their water supply may be switched off. Piotr Wolski delves into the rainfall record to put this drought into historical context.
This study examines for the first time the changing characteristics of summer and winter southern African rainfall, and their teleconnections with large-scale climate through the dominant timescales of variability. As determined by wavelet analysis, the austral summer and winter rainfall indices exhibit three significant timescales of variability over the 20th century: interdecadal (15-28 year), quasi-decadal (8-13 year) and interannual (2-8 year). Teleconnections with global sea-surface temperature and atmospheric circulation anomalies are established here, but are different for each timescale. Tropical/subtropical teleconnections emerge as the main driver of austral summer rainfall variability. Thus, shifts in the Walker circulation are linked to the El Niño Southern Oscillation (ENSO) and, at decadal timescales, to decadal ENSO-like patterns related to the Pacific Decadal Oscillation and the Interdecadal Pacific Oscillation. These global changes in the upper-zonal circulation interact with asymmetric ocean-atmospheric conditions between the South Atlantic and South Indian Oceans; together these lead to shift in the South Indian Convergence Zone, and a modulation of the development of convective rain bearing systems over southern Africa in summer. Such regional changes, embedded in quasi-annular geopotential patterns, consist of easterly moisture fluxes from the South Indian High, which dominate southerly moisture fluxes from the South Atlantic High. Austral winter rainfall variability is more influenced by mid-latitude atmospheric variability, in particular the Southern Annular Mode. The rainfall changes in the southwestern regions of southern Africa are determined by asymmetrical changes in the mid-latitude westerlies between the Atlantic and Indian Oceans.
Several studies have documented the poleward shift of the midlatitude westerly jet of the Southern Hemisphere during the last decades of the twentieth century, mainly during the warm season. In this work the consistency between this change and the seasonal changes in frontal activity and precipitation are explored. The authors also attempt to identify the correlation between frontal activity and precipitation changes.
Frontal activity is defined using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) dataset for the period 1962–2001 as the temperature gradient times the relative vorticity at 850 hPa. Considering cyclonic systems only, an enhancement of the frontal activity at high latitudes in the last two decades is apparent. However, the pattern of frontal activity change is not zonally symmetric, with the zonal asymmetries consistent with the climate change signal of the zonal anomaly of the 300-hPa geopotential height.
The pattern of precipitation change, showing midlatitude drying and high-latitude moistening, is consistent with the pattern of the frontal activity change, explaining to a large extent both the zonal mean and asymmetric rainfall changes. This consistency is also found in terms of the year-to-year variability of the zonal mean at both mid- and high latitudes. However, the frontal activity has a complex relationship with rainfall (not every frontal system is associated with rainfall events), and this consistency is unclear over some specific regions.
Results presented here highlight the robust link between the change in the asymmetric component of the upper-level circulation, the frontal activity, and rainfall over the mid- to high latitudes of the Southern Hemisphere.
Evidence is presented of a statistical relationship between anomalies in winter sea-ice over the So Atlantic sector of the Antarctic and winter rainfall over western South Africa. A positive (negative) correlation exists between sea-ice concentration over the Weddell Sea/Drake Passage region (east of the Weddell Sea from 0 to 30°E) and rainfall between May and September. When broken down into early, mid and late winter, the relationships appear stronger for early (May to July) and mid (June to August) winter than for late (July to September) winter and the month of July. In all cases, the relationships occur at 1 to 2 mo lead, suggesting that some predictability of winter rainfall may exist based on sea-ice concentration found earlier in the season. Analysis of circulation patterns associated with anomalously wet and dry winters from 1982 to 2004 indicates that the former are characterised by a cyclonic anomaly over southern South Africa that stretches southwest over the mid-latitude South Atlantic. In addition, there are increases in low level westerly moisture flux, and enhanced uplift, relative cyclonic vorticity and convergence over and upstream of the region. These patterns, combined with evidence of a northward shifted and more intense subtropical jet, indicate that the mid-latitude storm track is located anomalously far north during wet winters and that the cold fronts, which climatologically bring most of the annual rainfall, are likely to intensify just upstream of southwestern South Africa.
This paper describes a comprehensive set of fully automated quality assurance (QA) procedures for observations of daily surface temperature, precipitation, snowfall, and snow depth. The QA procedures are being applied operationally to the Global Historical Climatology Network (GHCN)-Daily dataset. Since these data are used for analyzing and monitoring variations in extremes, the QA system is designed to detect as many errors as possible while maintaining a low probability of falsely identifying true meteorological events as erroneous. The system consists of 19 carefully evaluated tests that detect duplicate data, climato-logical outliers, and various inconsistencies (internal, temporal, and spatial). Manual review of random samples of the values flagged as errors is used to set the threshold for each procedure such that its false-positive rate, or fraction of valid values identified as errors, is minimized. In addition, the tests are arranged in a deliberate sequence in which the performance of the later checks is enhanced by the error detection capabilities of the earlier tests. Based on an assessment of each individual check and a final evaluation for each element, the system identifies 3.6 million (0.24%) of the more than 1.5 billion maximum/minimum temperature, precipitation, snowfall, and snow depth values in GHCN-Daily as errors, has a false-positive rate of 1%-22%, and is effective at detecting both the grossest errors as well as more subtle inconsistencies among elements.
The least squares estimator of a regression coefficient β is vulnerable to gross errors and the associated confidence interval is, in addition, sensitive to non-normality of the parent distribution. In this paper, a simple and robust (point as well as interval) estimator of β based on Kendall's [6] rank correlation tau is studied. The point estimator is the median of the set of slopes (Yj - Yi)/(tj-ti) joining pairs of points with ti ≠ ti, and is unbiased. The confidence interval is also determined by two order statistics of this set of slopes. Various properties of these estimators are studied and compared with those of the least squares and some other nonparametric estimators.
Cape Town Tourism Research Rep
(CTT (Cape Town Tourism)), C. T. T. (2018). Cape Town Tourism Research Rep. Retrieved from
http://www.capetown.travel/wp-content/uploads/2018/06/CTT-Research-Report-April-
Variability and change in occurrence frequency of large-scale weather patterns associated with the Day Zero Drought in South Africa's Winter Rainfall Zone
- S Conradie
- P Wolski
- B C Hewitson
Conradie, S., Wolski, P., Hewitson, B.C. (in prep). Variability and change in occurrence frequency
of large-scale weather patterns associated with the Day Zero Drought in South Africa's Winter
Rainfall Zone.
Climate trends in South Africa Observed and modelled trends in rainfall and temperature for South Africa
- N C Mackellar
- M G New
- C Jack
MacKellar, N. C., New, M. G., & Jack, C. (2014). Climate trends in South Africa Observed and
modelled trends in rainfall and temperature for South Africa : 1960 -2010. South African Journal
of Science, 110(7), 1-13.
Mainstreaming Climate Change in Urban Development
- D Scott
- H Davies
- M New
Scott, D., Davies, H., New, M. (2019). Mainstreaming Climate Change in Urban Development.
University Of Cape Town Press, Cape Town.
GPCC Full Data Monthly Product Version 2018 at 0.5°: Monthly Land-Surface Precipitation from Rain-Gauges built on GTS-based and Historical Data
- U Schneider
- A Becker
- P Finger
- A Meyer-Christoffer
- M Ziese
Schneider, U., Becker, A., Finger, P., Meyer-Christoffer, A., Ziese, M. (2018). GPCC Full Data
Monthly Product Version 2018 at 0.5°: Monthly Land-Surface Precipitation from Rain-Gauges
built on GTS-based and Historical Data. DOI: 10.5676/DWD_GPCC/FD_M_V2018_050