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A decadal climate variation in the tropical Atlantic Ocean from thermodynamic air-sea interactions
Abstract
Rainfall variability in northeast South America1 and the Sahel region of Africa2-4 is profoundly influenced by the sea surface temperature (SST) of the tropical Atlantic Ocean. Of particular importance are relative changes in SST between the hemispheres on decadal timescales, a phenomenon often called the Atlantic SST dipole1,5. Here we propose that the decadal variation in the tropical SST dipole may be attributed to an unstable thermodynamic ocean-atmosphere interaction between wind-induced heat fluxes and SST. Using coupled ocean-atmosphere models, we show that the coupled dipole mode has a typical oscillation period of about a decade. The notion that the Atlantic dipole-like SST variability may be related to an oscillatory coupled mode might assist attempts to predict decadal climate variability in the tropical Atlantic region.
... Air-sea processes involved in the generation of TAV modes have been widely investigated. Wind-forced latent heat fluxes are the dominant physical processes responsible for the generation and development of the AMM, through the wind-evaporation-SST (WES) feedback (Amaya et al. 2017;Chang et al. 1997;Martín-Rey and Lazar 2019). Strong consensus for the thermodynamic nature of the AMM contrasts with the active debate on the mechanisms underlying the AZM generation. ...
... The development of the AMM starts with an anomalous reduction of climatological northeasterlies in late winter (Fig. 5e, along with Figs. S4e,q and S5e,q in the online supplemental material), favoring the warming of the surface ocean via reduced latent heat fluxes (Amaya et al. 2017;Chang et al. 1997;Martín-Rey and Lazar 2019). The interhemispheric SST difference produces a seesaw sea level pressure anomaly at both sides of the equator (not shown), giving rise to cross-equatorial winds blowing northward and reinforcing the initial SST pattern in boreal spring . ...
... A pronounced anomalous SSH elevation is restricted to the western equatorial Atlantic . Our results confirm the thermodynamic nature of the AMM and weak contribution of ocean wave propagation (Amaya et al. 2017;Chang et al. 1997). ...
The impact of tropical Atlantic Ocean variability modes in the variability of the upper-ocean circulation has been investigated. For this purpose, we use three oceanic reanalyses, an interannual forced-ocean simulation, and satellite data for the period 1982–2018. We have explored the changes in the main surface and subsurface ocean currents during the emergence of Atlantic meridional mode (AMM), Atlantic zonal mode (AZM), and AMM–AZM connection. The developing phase of the AMM is associated with a boreal spring intensification of North Equatorial Countercurrent (NECC) and a reinforced summer Eastern Equatorial Undercurrent (EEUC) and north South Equatorial Current (nSEC). During the decaying phase, the reduction of the wind forcing and zonal sea surface height gradient produces a weakening of surface circulation. For the connected AMM–AZM, in addition to the intensified NECC, EEUC, and nSEC in spring, an anomalous north-equatorial wind curl excites an oceanic Rossby wave (RW) that is boundary-reflected into an equatorial Kelvin wave (KW). The KW reverses the thermocline slope, weakening the nSEC and EUC in boreal summer and autumn, respectively. During the developing spring phase of the AZM, the nSEC is considerably reduced with no consistent impact at subsurface levels. During the autumn decaying phase, the upwelling RW-reflected mechanism is activated, modifying the zonal pressure gradient that intensifies the nSEC. The NECC is reduced in boreal spring–summer. Our results reveal a robust alteration of the upper-ocean circulation during AMM, AZM, and AMM–AZM, highlighting the decisive role of ocean waves in connecting the tropical and equatorial ocean transport.
... Similar to our ocean-circulation index, patterns of air-sea interaction with periodicity of 10-15 years have been observed in different parts of the Atlantic. Examples are the tripole pattern of SST in the North Atlantic linked to the NAO 32 , a cross-equatorial SST gradient in tropical Atlantic associated with changes in trade winds in both hemispheres 33 , and a dipole structure in the South Atlantic linked to the southeasterly trade winds and subtropical anticyclone 34 . Indeed, using composite analysis, it has been suggested 23 that these patterns constitute regional expressions of a pan-Atlantic pattern, the ADO. ...
... The associated windstress anomalies are indicative of weaker midlatitude westerlies over the North Atlantic 32 , stronger (weaker) southeasterly (northeasterly) trade winds to the south (north) of the equator, and cross-equatorial flow 17,23,33 . The SST and wind-stress anomalies over the tropical North Atlantic can be linked to a northerly displacement of the Inter-Tropical Convergence Zone (ITCZ) 17,33 . An index of the ITCZ is closely related to an ADOtype cross-equatorial anomalies in vertical gradient of diabatic heating (Supplementary. ...
... Thermodynamic air-sea interactions represented by the windevaporation-SST mechanism may amplify the ADO-related patterns through turbulent surface-heat fluxes 17,23,32,33 . This mechanism operates even under motionless ocean conditions in simplified climate models 23,33 , and is associated with variability of the subtropical anticyclones and meridional displacements of the ITCZ 33 . Turbulent fluxes drive phase change in ocean circulation 41 (Supplementary Fig. 5) and northward heat transport from the subtropical gyre towards the subpolar gyre of North Atlantic 21 . ...
Atlantic climate displays an oscillatory mode at a period of 10–15 years described as pan-Atlantic decadal oscillation. Prevailing theories on the mode are based on thermodynamic air-sea interactions and the role of ocean circulation remains uncertain. Here we uncover ocean circulation variability associated with the pan-Atlantic decadal oscillation using observational datasets from 1900–2009. Specifically, a sea level-derived index of ocean circulation also exhibits 10-15 year periodicity and leads the surface climate oscillation. The underlying ocean circulation links the extratropical and tropical Atlantic, where the maximum variance in surface-ocean temperature feeds back on the North Atlantic Oscillation (the leading mode of atmospheric variability over the North Atlantic region). Our findings imply that, rather than a passive role postulated by the thermodynamic paradigm, ocean circulation across the Atlantic plays an active role for the pan-Atlantic decadal climate oscillation.
... Its strongest amplitude occurs during the equatorial warm season (March, April and May, MAM) with SST anomalies of 0.3-0.5 • C in the subtropics (Carton et al. 1996;Xie and Carton 2004;Deser et al. 2010;Cabos et al. 2019). This mode shows variability from interannual to decadal scales (Carton et al. 1996;Chang et al. 1997;Amaya et al. 2017). The meridional SST gradient is also connected to the ITCZ displacement into the warmer hemisphere, therefore affecting the precipitation pattern over South America, in particular in NEB (Nobre and Shukla 1996;Ruiz-Barradas et al. 2000;Xie and Carton 2004;Chang et al. 2006;Deser et al. 2010). ...
... An anomalous cross-equatorial SST gradient pointing north drives a southwesterly surface wind anomaly, reducing the predominant trade winds over the warmer region (the northern Tropical Atlantic). This decreases the evaporative cooling at the surface, causing it to warmer further (Chang et al. 1997;Chiang and Vimont 2004;Mahajan et al. 2010;Amaya et al. 2017). In contrast, the other pole shows surface cooling, related to trade winds intensification. ...
... In contrast, the other pole shows surface cooling, related to trade winds intensification. These are linked to changes in surface heat fluxes, characterizing a positive wind-evaporation-SST (WES) feedback that acts to reinforce the anomalous cross-equatorial SST gradient (Xie and Philander 1994;Chang et al. 1997Chang et al. , 2006Amaya et al. 2017). In addition, results from satellite analysis during a strong AMM event in 2009 point to the importance of the wind-induced upwelling in the equatorial North Atlantic that could also act as positive feedback by further increasing the SST meridional gradient (Foltz et al. 2012). ...
Climate variability in the Tropical Atlantic is complex with strong ocean–atmosphere coupling, where the sea surface temperature variability impacts the hydroclimate of the surrounding continents. Most of the Tropical Atlantic interannual variability is explained by its equatorial (Atlantic Zonal Mode, AZM) and meridional (Atlantic Meridional Mode, AMM) modes of variability. Our results using an ensemble of 37 models from the Coupled Model Intercomparison Project Phase 6 historical simulations show multidecadal changes in the Tropical Atlantic variability from 1900 to 2014. Within it, most models (at least 60%) show a decrease in variability after the 1970 s, in accordance with observational data sets. The agreement among simulations points out the role of the external forcing. After 1970, an anthropogenically induced warming trend is observed in the equatorial Atlantic accompanied by a weakening of the winds. This drives a weakening in the Bjerknes Feedback by deepening the thermocline in the eastern equatorial Atlantic, reduced SST sensitivity to thermocline anomalies, thus decreasing AZM variability. The interplay of the meridional asymmetric warming in the Tropical Atlantic related to the anthropogenic forcing, the relaxed northeast trade winds, and the background state of the negative Atlantic Multidecadal Variability, decreases AMM variability despite the individual increase in variability of the North and South Tropical Atlantic. Associated with these factors the African Sahel shows a positive precipitation trend and the Intertropical Convergence Zone tends to shift northward, which reinforces the increased precipitation.
... Climate variability in the subtropical North Pacific has been widely studied [1][2][3][4][5][6][7] . While the variability of Pacific sea-surface temperatures (SSTs) in the low latitudes is dominated by El Niño-Southern Oscillation 8-10 (ENSO), numerous studies have suggested that the subtropical North Pacific may be home to an independent variability pattern that is maintained by the two-way coupling (hereafter just "coupling") between surface winds, evaporation, and SST (the socalled wind-evaporation-SST or WES feedback 11,12 ; see "Methods" for a short description), and forms a central part of the North Pacific Meridional Mode 1 (PMM hereafter). In its positive phase, the PMM pattern consists of warm SST anomalies that extend southwestward from the California coast toward the western equatorial Pacific, accompanied by a weakening of the northeast trade winds (Fig. 1a). ...
... It has been shown that atmospheric variability in the extratropical North Pacific, which is largely stochastic in nature 4 , can seed the development of the PMM during northern hemisphere winter 19 (December-January-February or DJF). As the PMM matures in spring (March-April-May or MAM), it propagates toward the equator as a coupled wind-SST mode, an intrinsic property of the WES feedback 11,12 . When the SST pattern approaches the equator, the accompanying weakening of the trade winds can initiate the development of an El Niño event that typically matures in the following winter (DJF) and couples tropical winds, SST and ocean circulation. ...
... The Wind-Evaporation-SST (WES) feedback A coupled air-sea feedback loop, in which a weakening of the subtropical trade winds causes a reduction of sea-surface evaporation 11,12 . This reduces the cooling by latent heat flux and results in a warming of the underlying SSTs. ...
Variations of sea-surface temperature (SST) in the subtropical North Pacific have received considerable attention due to their potential role as a precursor of El Niño-Southern Oscillation (ENSO) events in the tropical Pacific as well as their role in regional climate impacts. These subtropical SST variations, known as the North Pacific Meridional Mode (PMM), are thought to be triggered by extratropical atmospheric forcing and amplified by air-sea coupling involving surface winds, evaporation, and SST. The PMM is often defined through a statistical technique called maximum covariance analysis (MCA) that identifies patterns of maximum covariability between SST and surface winds. Here we show that SST alone is sufficient to reproduce the MCA-based PMM index with near-perfect correlation. This dominance of the SST suggests that the MCA-based definition of the PMM may not be ideally suited for capturing two-way wind-SST interaction or, alternatively, that this interaction is relatively weak. We further show that the MCA-based PMM definition conflates intrinsic subtropical and remote ENSO variability, thereby undermining its interpretation as an ENSO precursor. Our findings indicate that, while air-sea coupling may be important for variability in the subtropical North Pacific, it cannot be reliably identified by the MCA-based definition of the PMM. This highlights the need for refined tools to diagnose variability in the subtropical North Pacific.
... For the cold tongue region of the PO, the PO-IO coupling has the largest impact on forecast skills (with maximum differences in ACC of 0.3-0.4 at a lead time of 12 months) (Izumo et al. 2010;Cai et al. 2019), although the PO-AO and AO-IO coupling also have non-negligible contributions. The PO-IO coupling also exerts considerable impacts on SST forecast skill in the tropical IO, consistent with the fact that the ENSO-related changes in atmospheric circulation work to produce significant SST anomalies over the IO Dipole Mode Index (DMI) for the IOD (Fig. 3c) (Saji et al. 1999), the ATL3 index (Fig. 3b) for the Atlantic Niño (Zebiak 1993), and the NTA (Fig. 3d) and STA indices for the Atlantic meridional mode (Chang et al. 1997;Giannini et al. 2004) (See the figure captions for their definitions) and STA indices for the Atlantic meridional mode (Chang et al. 1997;Giannini et al. 2004; see the figure captions for their definitions). As in the Niño 3.4 index, interannual-to-decadal peaks of these indices are broadly captured in SF_CTL, while their magnitudes tend to be underestimated partially due to EOF truncation, especially for the ATL3 and STA indices. ...
... For the cold tongue region of the PO, the PO-IO coupling has the largest impact on forecast skills (with maximum differences in ACC of 0.3-0.4 at a lead time of 12 months) (Izumo et al. 2010;Cai et al. 2019), although the PO-AO and AO-IO coupling also have non-negligible contributions. The PO-IO coupling also exerts considerable impacts on SST forecast skill in the tropical IO, consistent with the fact that the ENSO-related changes in atmospheric circulation work to produce significant SST anomalies over the IO Dipole Mode Index (DMI) for the IOD (Fig. 3c) (Saji et al. 1999), the ATL3 index (Fig. 3b) for the Atlantic Niño (Zebiak 1993), and the NTA (Fig. 3d) and STA indices for the Atlantic meridional mode (Chang et al. 1997;Giannini et al. 2004) (See the figure captions for their definitions) and STA indices for the Atlantic meridional mode (Chang et al. 1997;Giannini et al. 2004; see the figure captions for their definitions). As in the Niño 3.4 index, interannual-to-decadal peaks of these indices are broadly captured in SF_CTL, while their magnitudes tend to be underestimated partially due to EOF truncation, especially for the ATL3 and STA indices. ...
The impacts of tropical interbasin interaction (TBI) on the characteristics and predictability of sea surface temperature (SST) in the tropics are assessed with a linear inverse modelling (LIM) framework that uses SST and sea surface height anomalies in the tropical Pacific (PO), Atlantic (AO), and Indian Ocean (IO). The TBI pathways are shown to be successfully isolated in stochastically-forced simulations that modify off-diagonal elements of the linear operators. The removal of TBI leads to a substantial increase in the amplitude of El Niño-Southern Oscillation (ENSO) and related variability. Partial decoupling experiments that eliminate specific coupling components reveal that PO-IO interaction is the dominant contributor, whereas PO-AO and AO-IO interactions play a minor role. A series of retrospective forecast experiments with different operators shows that decoupling leads to a substantial decrease in ENSO prediction skill especially at longer lead times. The relative contributions of individual pathways to forecast skill are generally consistent with the results from the stochastically-forced experiments. Qualitatively similar results are obtained from an additional set of forecast experiments that partially apply initial conditions over specific basins, but several important differences were also found due to differences in the representations of each TBI pathway. Finally, the cause of contrasting SST anomalies over the AO after the extreme 1982/83 and 1997/98 El Niño events is explored using LIM forecast experiments to demonstrate the strength and flexibility of our LIM-based approach.
... The ENSO-related atmospheric teleconnection over the TNA changes the climatological wind speed and further induces surface heat flux anomalies. This surface heat flux change is reported to be the main factor in leading to the TNA SST variation (Chang et al., 1997(Chang et al., , 2006Enfield & Mayer, 1997;Klein et al., 1999;Wu & Zhang, 2010). In addition, Atlantic internal variability, especially the North Atlantic Oscillation (NAO), makes a great contribution to the TNA SST variation (Czaja et al., 2002;Huang & Shukla, 2005;Lee et al., 2008). ...
... Previous studies have demonstrated that the NAO, which is the dominant atmospheric variability over the North Atlantic (e.g., Hurrell, 1995), can affect the TNA SST variations (Chang et al., 1997(Chang et al., , 2006Chen et al., 2015;Czaja et al., 2002;Wu & Zhang, 2010;Yang et al., 2018;Yin & Zhou, 2019). Figures 12a and 12b show the composite differences of sea level pressure (SLP) anomalies and surface wind anomalies from winter to spring for anomalous TNA SST cases during LCP. ...
The summertime North Indian Ocean (NIO) and tropical North Atlantic (TNA) sea surface temperature (SST) anomalies exert important impacts on atmospheric circulation and climate variation. Little attention is paid on the interdecadal change of NIO–TNA SST connection. This study reveals that the NIO–TNA SST relationship experiences an obvious interdecadal change around the early 2000s. The NIO and TNA SST correlation is positive and significant before the early 2000s, while this connection becomes weak and insignificant after that. This interdecadal change is closely associated with the changes in the El Niño‐Southern Oscillation (ENSO) intensity. During 1980–2001, strong ENSO forces a sustained SST warming in the southwest Indian Ocean, which further warms the summertime NIO via inducing antisymmetric circulation and reducing the upward surface latent heat flux. Meanwhile, strong ENSO forces a summertime TNA warming via arousing the Pacific North American (PNA) teleconnection. The PNA weakens the TNA trade winds, then reduces the upward surface heat flux and consequently warms the TNA. Thus, NIO and TNA SST connection is established via ENSO events during 1980–2001. In contrast, during 2002–2020, the summertime NIO SST anomaly is still related to ENSO. However, the connection between the TNA SST anomaly and ENSO is interrupted due to the weak ENSO intensity. Such weak ENSO yields a much weaker PNA teleconnection, which is inefficient to generate the TNA warming. Further analysis shows that the TNA warming during 2002–2020 is generated mainly by the negative North Atlantic Oscillation. Therefore, the connection between NIO and TNA SST anomalies collapses under the weak ENSO condition.
... The weakening of vertical wind shear in NA could be partially the result of local ocean warming by the decreased anthropogenic aerosols through the wind-evaporation-SST feedback (15)(16) as an analogy of Atlantic Meridional Mode (AMM) (17)(18)(19). The surface ocean warming might have caused a northward shift of the Intertropical Convergence Zone that, in turn, leads to a northward shift in ascending branch of the Hadley circulation that reduces upper-level westerlies around the main development region of Atlantic TCs. ...
... The surface ocean warming might have caused a northward shift of the Intertropical Convergence Zone that, in turn, leads to a northward shift in ascending branch of the Hadley circulation that reduces upper-level westerlies around the main development region of Atlantic TCs. Meanwhile, it is argued that AMM is an intrinsic atmosphere-ocean coupled internal mode, and its decadal variation might have caused decadal variations in hydroclimate including TCs in the NA over the past 40 years (16,19). Because the SPEAR (Seamless System for Prediction and Earth System Research) model reasonably simulates AMM in terms of the amplitude and power spectrum as observed ( fig. ...
Over the past 40 years, anthropogenic aerosols have been substantially decreasing over Europe and the United States owing to pollution control measures, whereas they have increased in South and East Asia because of the economic and industrial growth in these regions. However, it is not yet clear how the changes in anthropogenic aerosols have altered global tropical cyclone (TC) activity. In this study, we reveal that the decreases in aerosols over Europe and the United States have contributed to significant decreases in TCs over the Southern Hemisphere as well as increases in TCs over the North Atlantic, whereas the increases in aerosols in South and East Asia have exerted substantial decreases in TCs over the western North Pacific. These results suggest that how society controls future emissions of anthropogenic aerosols will exert a substantial impact on the world's TC activity.
... In the NGoG, some strong warm or cold SST events are linked to the Atlantic Niño mode (e.g., Hardman-Mountford & McGlade, 2003). Apart from the equatorial mode, the Atlantic meridional mode or Atlantic dipole, may also be involved in the Gulf of Guinea SST anomalies (Burmeister et al., 2016;Chang et al., 1997;Foltz & McPhaden, 2010). The Atlantic meridional mode is defined by an anomalous meridional SST gradient with opposite anomalies north and south of the thermal equator (∼5°N), and appears in boreal spring (March-April-May). ...
... The Atlantic meridional mode is defined by an anomalous meridional SST gradient with opposite anomalies north and south of the thermal equator (∼5°N), and appears in boreal spring (March-April-May). This mode is thought to be largely controlled by thermodynamics through wind-induced evaporation and positive wind-evaporation-SST feedback (Chang et al., 1997(Chang et al., , 2001Foltz & McPhaden, 2010). ...
Particularly cool sea surface temperatures (SSTs) were observed in 2012 along the Northern Gulf of Guinea (NGoG) coast. This strong cooling event was seen from February to June and reached maxima in the coastal upwelling areas: SST anomalies of -1°C were observed in Sassandra Upwelling area in Côte d’Ivoire (SUC, situated east of Cape Palmas) and SST anomalies of -0.5°C were observed in Takoradi Upwelling area in Ghana (TUG, located east of Cape Three Points). In SUC and TUG regions, the 2012 decrease in SST was the coldest event recorded over the 1990-2018 period (29 years). From the analysis of regional simulations, we show that the mechanisms behind this SST decrease differ in the two regions. In the SUC region, we identify changes in both zonal advection (related to zonal SST gradient changes) and increased vertical mixing as the main drivers of the anomalous cooling. The anomalous vertical mixing is linked to increased vertical shear of the zonal current in response to the Guinea Current strengthening. In the TUG region, acceleration of the southward advection of the surface water, due to the intensification of the meridional Ekman current generated by the strengthening of the zonal wind stress, was identified as the major cause of the SST anomalous cooling.
... Significant negative teleconnection patterns were observed between the mode and the equatorial Pacific, most of the South Pacific and in the northwestern region of the Indian Ocean (Figure 5b). A positive teleconnection pattern was present along the equatorial Atlantic, with higher significance and prevalence of correlation coefficients higher than 0.45 around the South Atlantic tropical (SAT) region, outlined by [7] as one of the two regions chosen by them to propose their SST Atlantic dipole index. This teleconnection is coherent with results found in literature that show the influence of the Atlantic Ocean in the rainfall regime of the NEB [73][74][75]. ...
... Almost throughout the eastern area, the EOF values remained above 0.05. From the corresponding global SST field, it was possible to identify a significant negative teleconnection between the precipitation over the eastern NEB and the SST anomalies in the Caribbean Sea (between −0.4 and −0.6), as well as with the eastern tropical North Atlantic (Figure 5f), with the latter corresponding to the NAT index domain, and the other region defined by [7] for their aforementioned Atlantic dipole. It is useful to recall that the North Atlantic may, by itself, determine the latitudinal migration of the ITCZ, by leading the AMM [91,94]. ...
The Northeast region of Brazil (NEB) is characterized by large climate variability that causes extreme and long unseasonal wet and dry periods. Despite significant model developments to improve seasonal forecasting for the NEB, the achievement of a satisfactory accuracy often remains a challenge, and forecasting methods aimed at reducing uncertainties regarding future climate are needed. In this work, we implement and assess the performance of an empirical model (EmpM) based on a decomposition of historical data into dominant modes of precipitation and seasonal forecast applied to the NEB domain. We analyzed the model's performance for the February-March-April quarter and compared its results with forecasts based on data from the North American Multi-model Ensemble (NMME) project for the same period. We found that the first three leading precipitation modes obtained by empirical orthogonal functions (EOF) explained most of the rainfall variability for the season of interest. Thereby, this study focuses on them for the forecast evaluations. A teleconnection analysis shows that most of the variability in precipitation comes from sea surface temperature (SST) anomalies in various areas of the Pacific and the tropical Atlantic. The modes exhibit different spatial patterns across the NEB, with the first being concentrated in the northern half of the region and presenting remarkable associations with the El Niño-Southern Oscillation (ENSO) and the Atlantic Meridional Mode (AMM), both linked to the latitudinal migration of the intertropical convergence zone (ITCZ). As for the second mode, the correlations with oceanic regions and its loading pattern point to the influence of the incursion of frontal systems in the southern NEB. The time series of the third mode implies the influence of a lower frequency mode of variability, probably related to the Interdecadal Pacific Oscillation (IPO). The teleconnection patterns found in the analysis allowed for a reliable forecast of the time series of each mode, which, combined, result in the final rainfall prediction outputted by the model. Overall, the EmpM outperformed the post-processed NMME for most of the NEB, except for some areas along the northern region, where the NMME showed superiority.
... Alguns estudos têm mostrado o papel do SAM na influência e impactos sobre o clima de diversas regiões do Hemisfério Sul (Silvestri e Vera, 2003;Gillett et al., 2006;Pezza et al., 2008;Oliva e Justi da Silva, 2011;Vasconcellos et al., 2019;Fogt e Marshall, 2020). Outros trabalhos (e.g., Pezza et al., 2012) (Nobre e Shukla, 1996;Chang et al., 1997). Alguns trabalhos (Hounsou-gbo et al., 2015;Bittencourt, 2016) fizeram associações do padrão AMM com as anomalias de temperatura do oceano Atlântico Tropical e chuvas no nordeste brasileiro. ...
Este artigo tem como objetivo analisar relações entre as fases de padrões de teleconexões tropicais, subtropicais e extratropicais e as extensões de gelo marinho antártico no Mar de Weddell (MW). Busca-se melhor compreensão acerca da variabilidade do gelo marinho e das interações entre a criosfera e a atmosfera, além de prover dados para estudos sobre previsões e variabilidade climáticas. Este artigo analisa setembro, o mês com a maior cobertura de gelo marinho no Mar de Weddell. Utilizou-se os índices mensais dos padrões Modo Anular do Sul (Southern Annular Mode – SAM), Modo Meridional do Atlântico (Atlantic Meridional Mode – AMM) e Gradiente Subtropical do Atlântico Sul (GSA). O índice SAM foi calculado através da Função Ortogonal Empírica nas anomalias de altura geopotencial em 700 hPa na região ao sul de 30ºS, o índice AMM foi extraído do National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) e o índice GSA foi criado a partir dos dados de temperatura da superfície do mar (TSM) do ERSSTv5 do National Oceanic and Atmospheric Administration (NOAA). Nas avaliações conjuntas, as combinações de fases que se relacionaram às maiores extensões de gelo foram SAM negativo/AMM positivo e SAM negativo/GSA positivo enquanto as que se associaram às menores extensões foram as de sinais opostos. As correlações do SAM com o gelo marinho são negativas e dos índices AMM e GSA com o gelo marinho são positivas, mostrando coerência com os resultados apresentados nos compostos e nos gráficos boxplots.Extreme Antarctic Sea Ice In The Weddell Sea And Links With Teleconnection Patterns Of Climate VariabilityABSTRACTThis article examines the links between the phases of teleconnection patterns of climate variability and the Weddell’s Sea ice extent. The goal is to understand better the sea ice variability and connections between the cryosphere and the atmosphere. This paper focuses on September, the month with the largest sea ice cover in the Weddell Sea. The Southern Annular Mode (SAM), the Atlantic Meridional Mode (AMM) and the Atlantic Subtropical Gradient (ASG) monthly indices were used. The SAM index was calculated through Empirical Orthogonal Function in the geopotential height anomalies of 700 hPa. The AMM index was obtained by the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). The ASG index was created using sea surface temperature (SST) data obtained by the National Oceanic and Atmospheric Administration (ERSSTv5/NOAA) through the difference of SST anomalies averaged at two distinct areas situated in the Subtropical Atlantic Ocean. Considering the effect of these indices combined, the negative SAM/positive AMM phase and the negative SAM/positive ASG phase were associated with maximum Antarctic sea ice extents. The opposite combinations of indices were related with minimum Antarctic sea ice extents. The correlations between the SAM and the sea ice extent are negative and between AMM and ASG with the sea ice extent are positive. These results are coherent with the results presented in the composite maps and the boxplots charts.Keywords: Teleconnection patterns, climate variability, antarctic sea ice, Weddell Sea.
... The NTA has also been suggested to influence ENSO. Variability in this region has been associated with the Atlantic meridional mode (AMM; Hastenrath and Heller 1977;Xie 1996;Chang et al. 1997;Amaya et al. 2017), which tends to be most pronounced in spring. Ham et al. (2013a, b) suggest that a warm AMM event in spring excites atmospheric Rossby waves that induce cyclonic circulation anomalies over the northeastern subtropical Pacific, with cool SST anomalies on its western flank. ...
The influence of the tropical Atlantic on El Niño-Southern Oscillation (ENSO) is examined using dedicated climate model experiments with sea-surface temperature (SST) restoring. Partial SST restoring to climatology in the tropical Atlantic leads to slower decay of ENSO events and to a shift of the power spectrum to longer periods. Perfect model hindcast experiments with and without restoring tropical Atlantic SST to climatology indicate that both the northern tropical and equatorial Atlantic have a very small influence on ENSO development. During decaying ENSO events, on the other hand, northern tropical Atlantic SST anomalies strongly accelerate the decay. Key to the Atlantic influence on ENSO decay are Atlantic SST anomalies just north of the equator (~ 5°N). These lead to local convection anomalies that change the Walker circulation so as to accelerate ENSO decay. Importantly, anomalous events in either the northern tropical or equatorial Atlantic fail to develop in the hindcast ensemble mean, when tropical Pacific SSTs are restored to climatology. This indicates that anomalous tropical Atlantic events in boreal spring and summer are strongly dependent on preceding ENSO events in boreal winter. Thus, the role of the tropical Atlantic is to mediate a negative feedback of ENSO on itself. Despite this passive role of the tropical Atlantic in the Pacific-Atlantic interaction, accurate simulation of the Atlantic feedback should play some role in ENSO prediction. Further model experiments will be required to evaluate model dependence of these findings and to quantify the impact of the Atlantic on ENSO prediction skill.
... The mesoscale 'weather' components that we parameterize in this study have been recognized to be critical for both the mean state and the variability of the general ocean circulation (Condron and Renfrew 2013, Jung et al. 2014, Roberts et al. 2019, so are key for simulating changes in ocean heat transport convergence and the resultant regional ocean heat content (Danabasoglu et al. 2016, Grist et al. 2018; for basin-scale atmosphere-ocean feedbacks (Chang et al. 1997); and for inter-basin teleconnections and broader scale climate variability (Frankignoul et al. 2017, Zhang et al. 2019. Recent convection-permitting regional atmospheric climate simulations (Kendon et al 2014, Finney et al. 2020 ...
The ocean is forced by the atmosphere on a range of spatial and temporal scales. In numerical models the atmospheric resolution sets a limit on these scales and for typical climate models mesoscale (<500 km) atmospheric forcing is absent or misrepresented. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre (SPG) and the Atlantic Meridional Overturning Circulation (AMOC). Here we present ocean model simulations which demonstrate that the addition of realistic mesoscale atmospheric forcing leads to coherent patterns of change: a cooler sea surface in the tropical and subtropical Atlantic and deeper mixed layers in the subpolar North Atlantic in autumn, winter, and spring. These lead to robust statistically significant increases in the volume transport of the North Atlantic SPG by 10% and the AMOC by up to 10%. Our simulations use a novel stochastic parameterization – based on a cellular automaton algorithm – to represent spatially coherent weather systems realistically over a range of scales, including down to the smallest resolvable by the ocean grid (∼10 km). Convection-permitting atmospheric models predict changes in the intensity and frequency of mesoscale weather systems due to climate change, so representing them in coupled climate models would bring higher fidelity to future climate projections.
... The ITCZ positions were estimated considering the outgoing longwave radiation (OLR) minima. These minimum OLR values (i.e., < 240 W.m − 2 ) showed significant convective activity and may represent an average position of convective cloud cores (Chang et al., 1997;Wang and Fu, 2007). ...
A numerical investigation of the interannual Southwestern and Equatorial Atlantic warm pools (WPs) (SST>28°C) was conducted for the 2000-2014 period, focusing on two major opposite events over the past 39 years (1982-2020): an intense warming event in 2010 (sizeable warm pool) and a moderate-to-low event in 2012 (small warm pool). Some modes of climate variability (remote and local) were confronted with the WP area. Significant remote connections were found for the North Atlantic Oscillation (NAO) and the Atlantic Meridional Mode (AMM). Whereas, the Tropical Northern Atlantic (TNA) and Tropical Southern Atlantic (TSA) indices were locally related to anomalies over the WP area. On the months that preceded the 2010 WP event, an anomalous atmospheric circulation in the tropical North Atlantic basin occurred (extremely negative NAO and positive AMM), causing a major weakening of winds, shallower mixed layer, and higher SST. Consequently, the WP boundaries, which usually developed southward along the equator in the austral fall, were pushed northward as warm waters spread out into the north tropical basin. Locally, both basins (i.e., tropical north and south) were warmer than climatologically expected (positive TNA and TSA), leading to a significant increase in the WP area recorded in this period. On the other hand, during the months that preceded the 2012 WP, the trade winds were faster (positive NAO and positive AMM), leading to a condition close to the climatologically expected. The approximation of the center of the subtropical high in the southern ocean basin practically extinguished the portion of warm waters that usually spreads southwest along the basin. While the TNA was weakly positive, the TSA presented negative anomalies that ensured a significant reduction in the WP area in the southern basin during this time. Numerical results from the 2000-2014 run were contrasted with PIRATA buoys and satellite data to assess the model's performance. Investigations of the mixed layer temperature terms disclosed the predominance of the atmosphere over the oceanic terms during the warm pool area increase/decrease, showing a significantly different composition within the oceanic terms. The persistence of the same sign anomalies during the three months preceding the WP's seasonal development appeared to be a pivotal factor in understanding how different the events in 2010 and 2012 were. Net heat flux anomalies, mainly controlled by anomalous wind patterns, were primarily responsible for the extreme events observed in 2010 and 2012.
... Sea surface temperatures (SSTs) over the tropical Atlantic exhibit interannual to decadal variability characterized by the interhemispheric gradient (hereafter, referred to as the Atlantic meridional mode, or the AMM; Nobre and Shukla, 1996;Chang et al. 1997;Xie 1999;Chiang and Vimont 2004). These SST anomalies are accompanied by a displacement of the intertropical convergence zone (ITCZ) toward the warmer hemisphere, affecting the precipitation over northeast Brazil and West Africa (Nobre and Shukla 1996;Chiang et al. 2002;Foltz et al. 2012;Kushnir et al. 2006;Paccini et al. 2021). ...
A simple air–sea coupled model for wind–evaporation–sea surface temperature (SST), wind-induced turbulence–mixed layer (ML)–SST, and wind–evaporation–ML–SST feedback is extended to unitedly represent the precipitation anomaly associated with moisture convergence and the ML depth (MLD) anomaly due to freshwater-induced buoyancy flux. An eigenanalysis reveals the presence of yet another feedback accompanying a cross-equatorial SST gradient. The feedback operates as follows: A cross-equatorial SST gradient anomaly forces surface wind anomalies to blow toward the warmer hemisphere, causing low-level convergence (divergence) and hence a positive (negative) precipitation anomaly in the warmer (cooler) hemisphere. The positive (negative) precipitation anomaly stratifies (destabilizes) the near-surface ocean and results in a shallower (deeper) ML, which enhances (reduces) the warming by climatological shortwave radiation, and thus provides positive feedback to the initial SST gradient anomaly. The strength of this feedback is similar to the three known feedbacks in terms of stability. Sensitivity experiments with the coupled general circulation model MIROC6 reveal that the precipitation-induced buoyancy flux anomaly accounts for up to ∼14% of the Atlantic meridional mode (AMM) amplitude in boreal spring through affecting the MLD anomaly in the deep tropics, which is consistent with the simple model results, supporting the existence of the feedback. In contrast, the evaporation-induced buoyancy anomaly contributes only marginally to the MLD and thus the SST anomalies. The ML temperature budget from MIROC6 confirms that sensitivity changes of the ML to the warming by climatological shortwave radiation due to the MLD anomaly are important in generating the SST anomalies associated with the AMM, which is consistent with previous observational studies.
Significance Statement
It is known that year-to-year variations in the sea surface north–south temperature gradient in the tropical Atlantic can affect the climate in both surrounding and remote regions. In this study, we used theoretical and state-of-the-art climate models to investigate a tropical air–sea coupled process and establish its contribution to the Atlantic climate variability. As a result, we identified a previously unknown process contributing to north–south gradient variations in which the atmosphere and ocean interact to enhance the initial variation, thus forming a positive feedback loop. In particular, precipitation and near-surface ocean state changes were determined to be essential. This feedback process accounts for up to 14% of the north–south temperature gradient variations in the tropical Atlantic during the boreal spring.
... Gischler and Oschmann (2005) observed a decadal cyclicity of approximately 10 to 15 years, based on δ 18 O values within this colony and suggested a likely connection to the Atlantic SST dipole variation. Chang et al. (1997) modelled the variation of the Atlantic SST dipole with a quasi-decadal periodicity (on average around 12 to 13 years) and proposed a thermodynamic interaction between the Atlantic Ocean and the atmosphere, leading to reciprocal influences of SST variations and windinduced heat entry. In the present study, time series analysis of intra-annual density datasets of the studied O. faveolata colony reveals a periodicity of approximately 9 to > 10 years, which might be a supporting evidence for an influence of the Atlantic SST dipole. ...
The quantification of skeletal density in massive scleractinians is necessary for a better understanding of skeletal growth in reef-forming corals. However, skeletal density is difficult to quantify and requires sophisticated analytical techniques. In this study, two-dimensional grid-scanning gamma densitometry is used for the first time, to quantify skeletal density fluctuations at higher temporal (intra-annual) resolution as compared to previous annual bulk densities determined with this approach. For testing this application and to evaluate its use for being a tool in coral sclerochronology, a colony of the widespread Atlantic massive coral Orbicella faveolata from the central Belize Barrier Reef (Central America) is herein investigated. In the studied coral, temporal resolution of individual density values corresponds to an approximately biweekly resolution. A long-term decline in (intra-)annual skeletal density is observed combined with reduced calcification rates. This indicates a limitation in the capability for skeletal formation in O. faveolata corals within the central Belize Barrier Reef, expressed in reduced skeletal carbonate accretion. In general, time series analyses and statistical correlations of the obtained high-resolution density datasets with skeletal growth patterns (linear extension rates, calcification rates) and geochemical (δ 13 C, δ 18 O) data reveal a complex interplay of environmental parameters, which might have controlled the skeletal density in the studied coral.
... The NTA has also been suggested to in uence ENSO. Variability in this region has been associated with the Atlantic meridional mode (AMM; Hastenrath and Heller 1977;Xie 1996;Chang et al. 1997; Amaya et al. 2017), which tends to be most pronounced in spring. Ham et al. (2013ab) suggest that a warm AMM event in spring excites atmospheric Rossby waves that induce cyclonic circulation anomalies over the northeastern subtropical Paci c, with cool SST anomalies on its western ank. ...
The influence of the tropical Atlantic on El Niño-Southern Oscillation (ENSO) is examined using dedicated climate model experiments with sea-surface temperature (SST) restoring. Partial SST restoring to climatology in the tropical Atlantic leads to slower decay of ENSO events and to a shift of the power spectrum to longer periods. Perfect model hindcast experiments with and without restoring tropical Atlantic SST to climatology indicate that both the northern tropical and equatorial Atlantic have a very small influence on ENSO development. During decaying ENSO events, on the other hand, northern tropical Atlantic SST anomalies strongly accelerate the decay. Key to the Atlantic influence on ENSO decay are Atlantic SST anomalies just north of the equator (~ 5N). These lead to local convection anomalies that change the Walker circulation so as to accelerate ENSO decay. Importantly, anomalous events in either the northern tropical or equatorial Atlantic fail to develop in the hindcast ensemble mean, when tropical Pacific SSTs are restored to climatology. This indicates that anomalous tropical Atlantic events in boreal spring and summer are strongly dependent on preceding ENSO events in boreal winter. Thus, the role of the tropical Atlantic is to mediate a negative feedback of ENSO on itself. Despite this passive role of the tropical Atlantic in the Pacific-Atlantic interaction, accurate simulation of the Atlantic feedback should play some role in ENSO prediction. Further model experiments will be required to evaluate model dependence of these findings and to quantify the impact of the Atlantic on ENSO prediction skill.
... The AMM is defined as the leading MCA mode of the SST and the zonal and meridional components of the 10 m winds over the tropical Atlantic (32 • N-22 • S, 74 • W to the African coast; Chiang and Vimont 2004). The dynamic mechanisms of the AMM have been extensively studied, including the trade wind variations (Nobre and Shukla 1996) and wind-evaporation-SST (WES) feedback (Chang et al 1997). The AMM modulates the interannual variability of the coupled ocean-atmosphere system over the tropical Atlantic, and is often considered independent of the ENSO (Chiang and Vimont 2004). ...
Antarctic sea ice plays an important role in polar ecosystems and global climate, while its variability is affected by many factors. Teleconnections between the tropical and high latitudes have profound impacts on Antarctic climate changes through the stationary Rossby wave mechanism. Recent studies have connected long-term Antarctic sea ice changes to multidecadal variabilities of the tropical ocean, including the Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation. On interannual timescales, whether an impact exists from teleconnection of the tropical Atlantic is not clear. Here we find an impact of sea surface temperature (SST) variability of the tropical Atlantic meridional dipole mode on Antarctic sea ice that is most prominent in austral autumn. The meridional dipole SST anomalies in the tropical Atlantic force deep convection anomalies locally and over the tropical Pacific, generating stationary Rossby wave trains propagating eastward and poleward, which induce atmospheric circulation anomalies affecting sea ice. Specifically, convective anomalies over the equatorial Atlantic and Pacific are opposite-signed, accompanied by anomalous wave sources over the subtropical Southern Hemisphere. The planetary-scale atmospheric response has significant impacts on sea ice concentration anomalies in the Ross Sea, near the Antarctic Peninsula, and east of the Weddell Sea.
... The spring climate mode in the Atlantic and Pacific, known as the meridional mode, is closely connected with the equatorial Niño (Nobre and Shukla 1996;Chang et al. 1997Chang et al. , 2000Servain et al. 1999;Chang et al. 2007;Foltz and McPhaden 2010). The Atlantic meridional mode characterizes an anomalous interhemispheric SST gradient, coherent with cross-equatorial wind anomalies in the western Atlantic, following WES feedback (Nobre and Shukla 1996). ...
This study analyzed the downwelling Rossby waves in the south Indian Ocean (IO)-induced spring asymmetric mode and the relationship with the Indian Ocean dipole (IOD) event based on observations and reanalysis datasets. The westward downwelling Rossby waves favor significant sea surface temperature (SST) warming in the Seychelles thermocline dome that triggers atmosphere response and the asymmetric mode in spring. The zonal sea level pressure gradient causes anomalous easterly winds in the central and eastern equatorial IO, cooling the SST off Sumatra–Java. Meanwhile, the remainder of the downwelling Rossby waves reach the west coast, transform to northward coastal-trapped waves, and then reflect as eastward downwelling Kelvin waves along the equator. The downwelling Kelvin waves reach the Sumatra–Java coast during late spring to early summer, favoring SST warming in the southeastern tropical Indian Ocean. Thus, there are two types of ocean–atmosphere response almost at the same time along the equator. The final SST status depends on which process is stronger and, as a consequence, triggers a negative or a positive phase of the IOD event in the fall season. The results show four positive and three negative IOD events related to the above processes from 1960 to 2019. The strong downwelling Rossby waves are easier to induce an intense asymmetric mode and negative IOD event, usually associated with preceding strong El Niño in the Pacific. In contrast, the weak downwelling Rossby waves tend to induce a weak asymmetric mode and positive IOD event, usually associated with preceding weak El Niño or anomalous anticyclonic atmospheric circulation in the southeastern IO.
... It is now considered to be the major mechanism driving the Atlantic Meridional Mode (AMM) (e.g. Carton et al., 1996;Chang et al., 1997;Xie, 1999;Mahajan et al., 2010). The AMM is one of the main modes of variability of the tropical Atlantic Ocean. ...
The tropical Atlantic Ocean has a highly contrasted surface salinity, with low surface salinity in the western and central parts of the basin. This low salinity is due to an important freshwater supply from large rivers such as the Amazon, Orinoco and Congo, and from heavy precipitation in the intertropical convergence zone. This results in a strong salinity stratification that may influence the vertical mixing, and thus the sea surface temperature (SST) and air-sea fluxes. The aim of this thesis is to investigate the influence of salinity - and especially the strong salinity stratification - on the tropical Atlantic Ocean climate. To do so, a 1/4° coupled ocean–atmosphere model of the region is developed, using NEMO as the ocean component, WRF as the atmospheric component and OASIS as the coupler. The use of a coupled ocean-atmosphere model allows to take into account all the air-sea interactions and feedback processes, which have been shown to impact the regional climate and are at the heart of this study. A series of sensitivity experiments is then conducted with this model, in order to assess the impact of increasingly detailed processes. First, the impact of the total salinity stratification on SST and air-sea fluxes is assessed by removing it from the model. Then, the Amazon and Orinoco rivers, major contributors to salinity stratification in the tropical Atlantic, are removed from the model. Interannual variability of river discharge is then studied to quantify the impact of Amazon extreme floods. Finally, the experiment without salinity stratification is conducted in a future climate, where several of the key variables identified in the present climate are very distinct from their present state, allowing a deeper understanding of the processes at stake. From these sensitivity tests, a consistent mechanism emerges in the northwestern tropical Atlantic in summer. The presence of salinity stratification decreases the cooling by vertical mixing, which leads to an increase in SST. This warming is then damped by a negative feedback from the atmosphere: the oceanic response is mitigated by a decrease in net heat flux. This decrease in net heat flux is primarily due to an increase in latent heat loss, but also to a reduction in shortwave radiation reaching the ocean surface, related to an increase in deep convection and associated cloud cover and precipitation. The final change in SST is determined by the balance between the warming due to the vertical mixing and the cooling due to the atmospheric feedback, both depending on the sensitivity test. However, the resulting SST change is always relatively small (0.5°C maximum). In winter, the impacts of salinity stratification are much weaker, most probably because of a deeper mixed layer at this time. SST changes are finally observed in the cold tongue region, related to changes in the thermocline depth. Subsurface temperature changes are present throughout the year, but the seasonality of upwelling occurrence dictates the timing of SST changes. In this thesis, the impact of salinity stratification on the mean tropical Atlantic climate is studied, but the model developed here could be adapted to study the impact of salinity stratification on the cyclones. Indeed, the Amazon plume is crossed by numerous tropical cyclones, and the impact of its associated salinity stratification remains controversial.
... However, it is evident that the previous winter NAO induces a SST signal in the Caribbean, which persists during the ERS. This is similarly suggested in previous studies (Giannini et al. 2000(Giannini et al. , 2001aHu et al. 2011), and can be explained by the wind, evaporation, and SST feedback (WES; Xie and Philander 1994;Chang et al. 1997). Under a positive NAO phase, the SST gradient between the cold SST signal in the Caribbean Sea and the warm SST signal across the Northwestern Caribbean and central TNA produces a localized meridional SLP gradient that intensifies the trades across the Caribbean Sea, enhancing the release of latent heat thus continuing the cold SST signal. ...
The Caribbean is a complex region that heavily relies on its seasonal rainfall cycle for its economic and societal needs. This makes the Caribbean especially susceptible to hydro-meteorological disasters (e.g., droughts and floods), and other weather/climate risks. Therefore, effectively predicting the Caribbean rainfall cycle is valuable for the region. The efficacy of predicting the Caribbean rainfall cycle is largely dependent on effectively characterizing the climate dynamics of the region. However, the dynamical processes and climate drivers that shape the seasonal cycle are not fully understood, as previous observational studies show inconsistent findings as to what mechanisms influence the mean state and variability of the cycle. These inconsistencies can be attributed to the limitations previous studies have when investigating the Caribbean rainfall cycle, such as using monthly or longer resolutions in the data or analysis that often mask the seasonal transitions and regional differences of rainfall, and investigating the Caribbean under a basin-wide lens rather than a sub-regional lens. This inhibits the ability to accurately calculate and predict subseasonal-to-seasonal (S2S) rainfall characteristics in the region. To address these limitations and inconsistencies, the research in this thesis examines the seasonal climatology, variability, and characteristics of the Caribbean rainfall cycle under a sub-regional and temporally fine lens in order to investigate the prediction of the cycle.
Regional variations and dynamical processes of the Caribbean annual rainfall cycle are assessed using (1) a principal component analysis across Caribbean stations using daily observed precipitation data; and, (2) a moisture budget analysis. The results show that the seasonal cycle of rainfall in the Caribbean hinges on three main facilitators of moisture convergence: the Atlantic Intertropical Convergence Zone (ITCZ), the Eastern Pacific ITCZ, and the North Atlantic Subtropical High (NASH). A warm body of sea-surface temperatures (SSTs) in the Caribbean basin known as the Atlantic Warm Pool (AWP) and a low-level jet centered at 925hPa over the Caribbean Sea known as the Caribbean Low-Level Jet (CLLJ) modify the extent of moisture provided by these main facilitators. The interactions of these dynamical processes are responsible for shaping the seasonal components of the annual rainfall cycle: The Winter Dry Season (WDS; mid-November to April); the Early-Rainy Season (ERS; mid-April to mid-June); an intermittent relatively dry period known as the mid-summer drought, (MSD; mid-June to late August), and the Late-Rainy Season (LRS; late August to late November). Five geographical sub-regions are identified in the Caribbean Islands, each with its unique set of dynamical processes, and consequently, its unique pattern of rainfall distribution throughout the rainy season: Northwestern Caribbean, the Western Caribbean, the Central Caribbean, the Central and Southern Lesser Antilles, and Trinidad and Tobago and Guianas. Convergence by sub-monthly transients contributes little to Caribbean rainfall.
The wettest and driest Caribbean ERS and LRS years’ are then explored by conducting the following: (1) a spatial composite of rainfall using the daily rainfall data; and, (2) spatial composites of SSTs, sea-level pressure (SLP), and mean flow moisture convergence and transports using monthly data. The ERS and LRS are impacted in distinctly different ways by two different, and largely independent, large-scale phenomena, external to the region: a SLP dipole mode of variability in the North Atlantic known as the North Atlantic Oscillation (NAO), and the El Nino Southern Oscillation (ENSO). Dry ERS years are associated with a persistent dipole of cold and warm SSTs over the Caribbean Sea and Gulf of Mexico, respectively, that are caused by a preceding positive NAO state. This setting involves a wind-evaporation-SST (WES) feedback expressed in enhanced trade winds and consequently, moisture transport divergence over all of the Caribbean, except in portions of the Northwestern Caribbean in May. A contribution from the preceding winter cold ENSO event is also discernible during dry ERS years. Dry LRS years are due to the summertime onset of an El Niño event, developing an inter-basin SLP pattern that moves moisture out of the Caribbean, except in portions of the Northwestern Caribbean in November. Both large-scale climate drivers would have the opposite effect during their opposite phases leading to wet years in both seasons.
Existing methodologies that calculate S2S rainfall characteristics were not found to be suitable for a region like the Caribbean, given its complex rainfall pattern; therefore, a novel and comprehensive method is devised and utilized to calculate onset, demise, and MSD characteristics in the Caribbean. When applying the method to calculate S2S characteristics in the Caribbean, meteorological onsets and demises, which are calculated via each year’s ERS and LRS mean thresholds, effectively characterize the seasonal evolution of mean onsets and demises in the Caribbean. The year-to-year variability of MSD characteristics, and onsets and demises that are calculated by climatological ERS and LRS mean thresholds resemble the variability of seasonal rainfall totals in the Caribbean and are statistically significantly correlated with the identified dynamical processes that impact each seasonal component of the rainfall cycle.
Finally, the seasonal prediction of the Caribbean rainfall cycle is assessed using the identified variables that could provide predictive skill of S2S rainfall characteristics in the region. Canonical correlation analysis is used to predict seasonal rainfall characteristics of station-averaged sub-regional frequency and intensity of the ERS and LRS wet days, and magnitude of the MSD. Predictor fields are based on observations from the ERA-Interim reanalysis and GCM output from the North America Multi-Model Ensemble (NMME). Spearman Correlation and Relative Operating Characteristics are applied to assess the forecast skill. The use of SLP, 850-hPa zonal winds (u850), vertically integrated zonal (UQ), and meridional (VQ) moisture fluxes show comparable, if not better, forecast skill than SSTs, which is the most common predictor field for regional statistical prediction. Generally, the highest ERS predictive skill is found for the frequency of wet days, and the highest LRS predictive skill is found for the intensity of wet days. Rainfall characteristics in the Central and Eastern Caribbean have statistically significant predictive skill. Forecast skill of rainfall characteristics in the Northwestern and Western Caribbean are lower and less consistent. The sub-regional differences and consistently significant skill across lead times up to at least two months can be attributed to persistent SST/SLP anomalies during the ERS that resemble the North Atlantic Oscillation pattern, and the summer-time onset of the El Niño-Southern Oscillation during the LRS. The spatial pattern of anomalies during the MSD bears resemblance to both the ERS and LRS spatial patterns.
The findings from this thesis provide a more comprehensive and complete understanding of the climate dynamics, variability, and annual mean state of the Caribbean rainfall cycle. These results have important implications for prediction, decision-making, modeling capabilities, understanding the genesis of hydro-meteorological disasters, investigating rainfall under other modes of variability, and Caribbean impact studies regarding weather risks and future climate.
... Although anomalous upwelling largely controls the SST variability over the North-West African upwelling region, Wind-Evaporation-SST (WES) feedback (Xie and Philander 1994) may also contribute, since it is one of the major drivers of SST variability in the Tropical North Atlantic Ocean (Polo et al. 2005;Chang et al. 1997;Amaya et al. 2017). The boundary layer WES feedback is a purely thermodynamic mechanism which, in the deep tropics, works as follows: consider an anomalous interhemispheric northward positive (negative) SST gradient. ...
Sea surface temperature (SST) variability in the North Eastern Tropical Atlantic has its center of action in the Senegalese–Mauritanian upwelling system, where its drivers are wind-induced ocean dynamics and air–sea thermodynamic processes. Thus, a better understanding of the local wind variations, together with their predictability, contributes to a more comprehensive assessment of the SST variability in that region. In this study, we use monthly data from two ocean reanalyses, SODA and ORAS-5, and a regional forced-ocean simulation to characterize the interannual SST variability off the Senegalese Coast in the common period 1960–2008. Local indices of the mixed layer heat budget during the major upwelling season (February–March–April) exhibit pronounced interannual to decadal variability. We demonstrate that the local interannual SST variability undergoes inter-decadal fluctuation and concomitant changes in its local and remote drivers. Off-Senegal SST variability was largely controlled by wind-induced Ekman transport during the 1960s–1970s, acting under favorable thermocline and mixed layer conditions. However, from 1980s onwards, the drastically reduced Ekman impact observed on local SSTs is associated with a deeper thermocline. This shift in the effectiveness of the dynamic mechanisms coincides with a more active ENSO teleconnection with upwelling before the 1980s. An extended SODA record reveals that the multidecadal modulator of the ENSO impact on the North-eastern Tropical Atlantic resembles the negative phase of the Atlantic Multidecadal Variability. Our results bring to light the fundamental role played by the global decadal background state in the activation of the drivers and air-sea mechanisms responsible for generating the interannual off-Senegal SST variability.
... Estimated rock class by using input parameters from various rock units is from poor to fair Type of rock mass support to be used has been roughly calculated by using Q classification. Rock mass along different chainages has been identified as poor to good quality rock mass [9]. By Grimstad & Barton (1993) chart, rock support has been proposed for different tunnel chainages based on the ratio of span and diameter of the tunnel to excavation support ratio (ESR) versus Q-value as provided in the (Table 3). ...
Most of hydropower tunnels have squeezing problems due to weak rock mass quality and high overburden. The induced stress level exceeds the strength of rock mass tunnel fails. Stress plays a crucial role in developing brittle fractures, rock strength reduction and rock mass instabilities the critical stress is an indicator for the support design of tunnel. To quantify stress state and deformation of circular tunnel, an empirical, semi analytical and a two-dimensional boundary element numerical analysis have been made in this paper. Initially, rock mass characterization is carried out by using Q and RMR methods which defined rock mass as poor to good and fair respectively. Then for identification of squeezing empirical approaches are used, results shown that there is no squeezing in the tunnel. However, 200 m stretch of tunnel undergoes severe squeezing as identified by Hoek and Marinos approach which is further refined by using Numerical program phase2 which yields displacement values quiet nearer to values obtained by semi-analytical approach by considering rock mass as plastic material. Tunnel support is estimated by using Q and RMR systems respectively which is verified by Numerical analysis.
... This system further develops toward the south and south-west regions through the Low-Level Jet (Garreaud et al., 2009) and to the southeast through the SACZ (Carvalho et al., 2004). Unlike the SAMS, the ITCZ is a tropical belt of maximum precipitation (Haug et al., 2001) formed by the convergence of trade winds, which carry humidity from the Atlantic Ocean, driven to the warmer hemisphere as a consequence of insolation (Chang et al., 1997;Nobre and Srukla, 1996). At a seasonal scale, the ITCZ shifts to the south during the austral autumn, extending the SAMS wet season, while a northern shift of the ITCZ is observed during the austral winter, with most of the continental rainfall located basically north of the equator, driving the southern Amazon region to its dry season (Garreaud et al., 2009;Maksic et al., 2019). ...
The Amazon Basin is one of the most productive regions in the world and an important carbon sink. However, lake productivity has varied throughout the Holocene, as preserved in lacustrine sedimentary records. Concentrations of chlorophyll pigmented derivatives that are mainly derived from phytoplankton and macrophyte populations can be used to infer lake production levels. Here we use the chlorophyll derivatives concentrations analyzed by spectrophotometer in sediment cores from nine lakes distributed throughout the Brazilian Amazon Basin to document the continental-scale changes in lake production during the Holocene. Chlorophyll derivatives have varied with changes in precipitation rate throughout the last 10,000 years, similar to other climate records in tropical South America, including Ti concentration from the Cariaco Basin, δ13C from Lake Titicaca, and refractory black carbon in Nevado Illimani. Increasing precipitation is responsible for increasing the nutrient supply into the lake, which stimulates primary production. Our analysis was compared to climate-related parameters, suggesting an increasing trend of lake production rates during the wetter Late and Early Holocene, while lower production rates characterized the dry phase of the Middle Holocene. Therefore, the chlorophyll derivatives concentrations generally follow precipitation changes in the Amazon Basin during the Holocene.
... Tropical oceans exhibit strong interactions between the atmosphere and the ocean (e.g., Bjerknes 1969;Chang et al. 1997;Wang and Enfield 2003). Sea surface temperature anomalies (SSTA) over tropical oceans induce atmospheric circulation response by deep atmospheric convection, and the atmospheric circulation in turn impacts the SST variability. ...
This study identifies that the spring sea surface temperature anomalies (SSTA) over the western tropical Atlantic (WTA) has a pronounced negative correlation with the following winter El Niño–Southern Oscillation (ENSO) variability. This negative correlation is stronger than that between the SSTA over each of the Atlantic Niño region, the north tropical Atlantic or the Atlantic warm pool, and the succeeding winter ENSO. Different SST datasets can recognize the strongest correlation, suggesting that the modulation of the WTA SSTA to the following ENSO variability is robust. Moreover, this modulation has a decadal shift, being weak before the mid-1980s but significantly enhanced thereafter. The intensified modulation of the WTA SSTA to the following ENSO after the mid-1980s displays as that the positive WTA SSTA during spring leads to a significant negative SSTA over the central and eastern tropical Pacific during the subsequent summer, which further develops into La Niña events during the following winter. This significant negative SSTA is induced by the strengthened anomalous easterly over the central equatorial Pacific associated with the enhanced WTA precipitation anomalies in response to the WTA SSTA. Further analysis suggests that this intensified modulation tends to be attributed to the increase in climatological mean SST over the WTA after the mid-1980s. The warmer WTA SST mean states is likely to enhance the convective activity and relevant atmospheric circulation in response to the WTA SSTA forcing, resulting in the intensified modulation of the WTA SSTA to ENSO variability after the mid-1980s.
... During boreal spring, conditions in the tropical Atlantic are favorable for the development of an anomalous interhemispheric SST gradient, giving rise to the meridional mode. The wind-evaporation-SST feedback mechanism appears to be an important driver of this thermodynamic mode (e.g., [33]). Positive feedback is strongest in the western Atlantic warm pool region where the NECC originates. ...
In this paper, the role of oceanic Rossby waves in climate variability is reviewed, as well as their dynamics in tropical oceans and at mid-latitudes. For tropical oceans, both the interactions between equatorial Rossby and Kelvin waves, and off-equatorial Rossby waves are privileged. The difference in the size of the basins induces disparities both in the forcing modes and in the dynamics of the tropical waves, which form a single quasi-stationary wave system. For Rossby waves at mid-latitudes, a wide range of periods is considered, varying from a few days to several million years when very-long-period Rossby waves winding around the subtropical gyres are hypothesized. This review focuses on the resonant forcing of Rossby waves that seems ubiquitous: the quasi-geostrophic adjustment of the oceans favors natural periods close to the forcing period, while those far from it are damped because of friction. Prospective work concentrates on the resonant forcing of dynamical systems in subharmonic modes. According to this new concept, the development of ENSO depends on its date of occurrence. Opportunities arise to shed new light on open issues such as the Middle Pleistocene transition.
... One of the impacts of their positive (negative) phase is the intensification (weakening) of the northeast trade winds in the tropical North Atlantic due to the intensification (weakening) of the North Atlantic Subtropical high-pressure system. Consequently, there is a cooling (warming) of the ocean surface through the increase (decrease) of the latent heat flux loss to the atmosphere, according to wind-evaporation-SST feedback (Chang et al., 1997;Czaja et al., 2002;Xie & Philander, 1994). Therefore, the negative correlations of the NAO and AO with SST' on the north coast of Brazil can be explained by the changes in the northeast trade winds. ...
We investigate the spatial and temporal patterns of satellite‐derived sea surface temperature, salinity and chlorophyll‐a concentration along the eastern South American coast. Two decade‐long time series (2002–2019; except for the salinity) allowed us to investigate changes from seasonal to interannual time scales on an array that stretches from the south (42°S) to the north (10°N) of the continent and from isobaths ranging from −50 to −1000 m away from the coast. The novelty of our approach is to assess comparatively the magnitude, variability, and spectrum of these variables following the same isobaths and using the same methodology. This allowed us to examine the influence of large scale ocean circulation patterns entangled with the local forcing systems such as river discharge and coastal currents, and to quantify to what extent these patterns are spatially and temporally consistent. The seasonal cycle of the temperature explains more variance than that of salinity and chlorophyll on average. Comparatively, salinity has a weak seasonal signal, except near major rivers. Significant long‐term trends were observed in specific regions in the salinity time series. Our study revealed distinct interannual changes at 2‐ and 4‐year period in the whole array with the largest spectral peaks near the La Plata and the Amazon Rivers. Within this period band, thermal signals propagate northward along the whole array. Statistical correlations between satellite‐derived variables and several climate indices suggest remote forcing.
... Variability on these time scales is characterized by two primary modes, a meridional and zonal mode. The Atlantic meridional mode (hereafter abbreviated as AMM) features an anomalous cross-equatorial SST gradient that peaks in boreal spring (March-April-May; MAM) and is sustained by a positive atmosphere-ocean feedback between wind, evaporation and SST (WES feedback; Chang et al. 1997;Xie 1999;Amaya et al. 2017). Associated with the AMM is an anomalous displacement of the intertropical convergence zone (ITCZ) toward the warmer hemisphere (Chiang and Vimont 2004). ...
Interannual sea surface temperature (SST) variations in the tropical Atlantic Ocean lead to anomalous atmospheric circulation and precipitation patterns with important ecological and socioeconomic consequences for the semiarid regions of sub-Saharan Africa and northeast Brazil. This interannual SST variability is characterized by three modes: an Atlantic meridional mode featuring an anomalous cross-equatorial SST gradient that peaks in boreal spring; an Atlantic zonal mode (Atlantic Niño mode) with SST anomalies in the eastern equatorial Atlantic cold tongue region that peaks in boreal summer; and a second zonal mode of variability with eastern equatorial SST anomalies peaking in boreal winter. Here we investigate the extent to which there is any seasonality in the relationship between equatorial warm water recharge and the development of eastern equatorial Atlantic SST anomalies. Seasonally stratified cross-correlation analysis between eastern equatorial Atlantic SST anomalies and equatorial heat content anomalies (evaluated using warm water volume and sea surface height) indicate that while equatorial heat content changes do occasionally play a role in the development of boreal summer Atlantic zonal mode events, they contribute more consistently to Atlantic Niño II, boreal winter events. Event and composite analysis of ocean adjustment with a shallow water model suggest that the warm water volume anomalies originate mainly from the off-equatorial northwestern Atlantic, in agreement with previous studies linking them to anomalous wind stress curl associated with the Atlantic meridional mode.
... DMI has large influence on the climate of the tropical Indian ocean, as it is accompanied by anomalous low-level winds, and a significant contributor to tropical rainfall variability [14][15][16] . Lastly, TASI has been linked with the northward and southward shift of the inter-tropical convergence zone, influencing strong climate anomalies in the surrounding region 17 . Unlike Nino3.4 which is SST averaged over a certain ocean region (5° N-5° S, 170W-120W), DMI and TASI are calculated as the SST differences between two regions. ...
This paper describes a global monthly gridded Sea Surface temperature (SST) and Sea Ice Concentration (SIC) dataset for the period 1000-1849, which can be used as boundary conditions for atmospheric model simulations. The reconstruction is based on existing coarse-resolution annual temperature ensemble reconstructions, which are then augmented with intra-annual and sub-grid scale variability. The intra-annual component of HadISST.2.0 and oceanic indices estimated from the reconstructed annual mean are used to develop grid-based linear regressions in a monthly stratified approach. Similarly, we reconstruct SIC using analog resampling of HadISST.2.0 SIC (1941-2000), for both hemispheres. Analogs are pooled in four seasons, comprising of 3-months each. The best analogs are selected based on the correlation between each member of the reconstructed SST and its target. For the period 1780 to 1849, We assimilate historical observations of SST and night-time marine air temperature from the ICOADS dataset into our reconstruction using an offline Ensemble Kalman Filter approach. The resulting dataset is physically consistent with information from models, proxies, and observations.
... Alguns estudos têm mostrado o papel do SAM na influência e impactos sobre o clima de diversas regiões do Hemisfério Sul (Silvestri e Vera, 2003;Gillett et al., 2006;Pezza et al., 2008;Oliva e Justi da Silva, 2011;Vasconcellos et al., 2019;Fogt e Marshall, 2020). Outros trabalhos (e.g., Pezza et al., 2012) (Nobre e Shukla, 1996;Chang et al., 1997). Alguns trabalhos (Hounsou-gbo et al., 2015;Bittencourt, 2016) fizeram associações do padrão AMM com as anomalias de temperatura do oceano Atlântico Tropical e chuvas no nordeste brasileiro. ...
This article examines the links between the phases of teleconnection patterns of climate variability and the Weddell’s Sea ice extent. The goal is to understand better the sea ice variability and connections between the cryosphere and the atmosphere. This paper focuses on September, the month with the largest sea ice cover in the Weddell Sea. The Southern Annular Mode (SAM), the Atlantic Meridional Mode (AMM) and the Atlantic Subtropical Gradient (ASG) monthly indices were used. The SAM index was calculated through Empirical Orthogonal Function in the geopotential height anomalies of 700 hPa. The AMM index was obtained by the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). The ASG index was created using sea surface temperature (SST) data obtained by the National Oceanic and Atmospheric Administration (ERSSTv5/NOAA) through the difference of SST anomalies averaged at two distinct areas situated in the Subtropical Atlantic Ocean. Considering the effect of these indices
combined, the negative SAM/positive AMM phase and the negative SAM/positive ASG phase were associated with maximum Antarctic sea ice extents. The opposite combinations of indices were related with minimum Antarctic sea ice extents. The correlations between the SAM and the sea ice extent are negative and between AMM and ASG with the sea ice extent are positive. These results are coherent with the results presented in the composite maps and the boxplots charts.
Keywords: Teleconnection patterns, climate variability, antarctic sea ice, Weddell Sea.
... 3 of 17 Chang et al., 1997;Hou, Bahr, Schmidt, et al., 2020;Seidov & Maslin, 2001). Such changes influence the large-scale atmospheric circulation across the South Atlantic Ocean and South America, which is, for example, documented by a southward shift and/or expansion of the Hadley Cell during high-northern-latitude cold phases (Asmerom et al., 2020;Vuille et al., 2012). ...
Historic droughts document the strong spatio‐temporal variability of the South American Monsoon System, which currently provides more than two thirds of the rainfall in tropical South America. The drivers of this variability have remained not well understood due to the lack of continuous, high‐resolution paleorecords, especially from the more arid regions of tropical South America. Here we present a novel record of moisture availability across eastern South America for the past ∼5,000 years from a sediment core retrieved off eastern Brazil. We document distinct decadal‐ to millennial‐scale spatial shifts of major atmospheric convection centers that caused increasingly pronounced droughts in eastern South America over the past ∼2,000 years. These fluctuations were triggered by climate anomalies in the high northern latitudes and propagated into equatorial latitudes via fluctuations in North Atlantic Overturning Circulation strength. As global warming is expected to decrease oceanic overturning due to enhanced meltwater input into the North Atlantic while at the same time reducing precipitation over eastern South America, an increasing risk for long‐lasting droughts can be expected for this region, posing severe socio‐economic challenges.
... The NTA mainly arises from latent heat flux anomalies associated with anomalous northeasterly trades (19)(20)(21). Specifically, the windevaporation-SST feedback (22), mainly confined to the deep tropics, contributes to the development of anomalous SST (23,24), but forcings outside of the tropical Atlantic are required to reinforce the NTA temperature anomaly (2,25,26). The North Atlantic Oscillation (NAO) is one such forcing (27,28). ...
Variability of North Tropical Atlantic (NTA) sea surface temperature (SST), characterized by a near-uniform warming at its positive phase, is a consequential mode of climate variability. Modulated by El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation, NTA warm anomalies tend to induce La Niña events, droughts in Northeast Brazil, increased frequency of extreme hurricanes, and phytoplankton blooms in the Guinea Dome. Future changes of NTA variability could have profound socioeconomic impacts yet remain unknown. Here, we reveal a robust intensification of NTA variability under greenhouse warming. This intensification mainly arises from strengthening of ENSO-forced Pacific-North American pattern and tropospheric temperature anomalies, as a consequence of an eastward shift of ENSO-induced equatorial Pacific convection and of increased ENSO variability, which enhances ENSO influence by reinforcing the associated wind and moist convection anomalies. The intensification of NTA SST variability suggests increased occurrences of extreme NTA events, with far-reaching ramifications.
... The northward shift of the HC shifts the center of the ascending motion northward to 10°N and substantially enhance the convection over the NAM region, and then, the NASM precipitation. Actually, the pattern of SAT response over the tropical Atlantic in CPL runs is not unlike the Atlantic meridional mode, which is characterized by meridional SST gradients over the tropical Atlantic, and is reported to regulate the position of the ITCZ and Hadley circulation (Chang et al. 1997;Chiang et al. 2002;Chiang and Vimont 2004). This study shows a robust relationship between the topography of the TP and NASM precipitation, complimentary to previous perspective that TP uplift can substantially change rainfall over the Asian monsoon region. ...
It has been well known that the uplift of the Tibetan Plateau (TP) can significantly enhance the Asian monsoon. Here, by comparing the sensitivity experiments with and without the TP, we find that the TP uplift can also increase the precipitation of the North American Summer Monsoon (NASM), with atmosphere teleconnection accounting for 6% and oceanic dynamical process accounting for another 6%. Physically, the TP uplift generates a stationary Rossby wave train traveling from the Asian continent to the North Atlantic region, resulting in an high-pressure anomaly over the tropical-subtropical North Atlantic. This high pressure system enhances the low-level easterly winds, forcing an enhanced upward motion over the North American monsoon (NAM) region and then an increase in summer precipitation there. In addition, the TP uplift enhances the Atlantic meridional overturning circulation, which reduces the meridional temperature gradient and leads to a northward shift of Hadley Cell over eastern Pacific-Atlantic section. The latter shifts the convection center northward to 10°N and further increases the NASM precipitation. The enhanced NASM precipitation can also be understood by the northward shift of Intertropical Convergence Zone. Our study implies that the changes of NAM climate can be affected by not only local process but also remote forcing, including those from Asian highland region.
Tropical Atlantic Ocean is the smallest of the three tropical ocean basins. However, it has many distinct types of variations that manifest beyond the tropical latitudes of the Atlantic. The chapter discusses some of these features including the phenomenon of the Atlantic Niño, Benguela Niño, and the Atlantic Meridional Mode.KeywordsBenguela NiñoAtlantic meridional modeAtlantic NiñoNordesteAngola-Benguela frontal zonestatic stabilityNorth-Atlantic Oscillation
This chapter presents a conceptual discussion on how ocean–atmosphere interactions are key to outstanding aspects of climate variability. The principal goal is to describe the mechanisms by which the atmosphere and ocean interact, and their perturbation feedback on each other, as well as how these interactions can lead to a new breed of modes in the coupled ocean–atmosphere system. The realization of such local interactions can project onto basin scales, and subsequently to the other basins.
It is well known that the El Niño–Southern Oscillation (ENSO) is the most prominent mode of interannual climate variability in the tropics. The ENSO is a coupled, tropical ocean–atmosphere system that fluctuates on a time scale of two to seven years in the Pacific (Philander, 1990). The ENSO extremes are labeled as either a warm or cold phase, yet its amplitude varies across a continuum with essentially Gaussian statistics (Trenberth, 1997). Characterizing the warm (cold) ENSO phase is the presence of the anomalously warm (cold) sea surface temperatures (SSTs) in the eastern and/or central equatorial Pacific known as the El Niño (La Niña) event.
This study reveals the role of the tropical Atlantic variability in modulating barrier layer thickness (BLT) in peak seasons. Based on reanalysis data during 1980–2016, statistical and dynamical analyses are performed to investigate the mechanism of BLT variability associated with the tropical Atlantic modes. The regions with significant correlation between BLT and tropical Atlantic modes are located on the northwest and southeast coasts of the tropical Atlantic, which are consistent with BLT maximum variability regions. In boreal spring, BLT decreases in the northwest because less latent heat release affected by weak trade wind related to the Atlantic meridional mode (AMM) shoals the isothermal layer depth (ITLD). In the south equatorial Atlantic, deepened mixed layer depth (MLD) is controlled by the decreasing freshwater input brought by a northward shift of the intertropical convergence zone (ITCZ) and further leads to a thinner barrier layer (BL). However, a shoaling MLD appears in the north equatorial Atlantic, which results from excessive freshwater input, causing a thick BL there. In boreal summer, positive runoff anomaly caused by the Atlantic equatorial mode (AEM) leads to upper warming of the tropical northwest Atlantic and a shallowing ITLD, favoring a thinner BL there. However, a southward shift of ITCZ brings more freshwater into the south equatorial Atlantic, inducing a shallowing MLD as well as a thicker BL. AEM-driven horizontal heat advection of the south equatorial current contributes to a thick ITLD in the central southern tropical Atlantic and thus increases BLT.
Significance Statement
This research aims to reveal how the tropical Atlantic meridional and equatorial interannual climatic modes affect barrier layer thickness (BLT). These two climate modes can affect the wind field, ocean current, and precipitation through air–sea interaction processes, and further affect mixing, heat–salt transport, and stratification in the upper ocean and thus BLT. This finding is important because the barrier layer restricts the exchange of heat, momentum, mass, and nutrients between the mixed layer and the thermocline, thereby impacting local and remote weather events, the ecological environment, and the climate. Our results provide guidance for interpreting the interannual variability of BLT in the tropical Atlantic.
North Atlantic tropical cyclones (TCs) have considerable interannual variability, with La Niña and the positive phase of the Atlantic Meridional Mode (AMM) tending to drive active hurricane seasons, and El Niño and the negative AMM often driving inactive seasons. Here, we analyze how active and inactive Atlantic hurricane seasons may change in the future using the high resolution Energy Exascale Earth System Model (E3SM). We performed atmosphere‐only simulations forced by sea‐surface temperature patterns characteristic of La Niña and the positive AMM jointly, and El Niño and the negative AMM jointly, in historical and future climates. Projected Atlantic TCs become more frequent in the future by approximately 34% during El Niño and negative AMM and by 66% during La Niña and positive AMM, with a significant increase in the portion of intense TCs. Warmer SSTs increase TC potential intensity, with reduced wind shear and increased mid‐tropospheric humidity further supporting TC activity.
Over the last 20 years, developments in climatology have provided an amazing array of explanations for the pattern of world climates. This textbook, first published in 2006, examines the earth's climate systems in light of this incredible growth in data availability, data retrieval systems, and satellite and computer applications. It considers regional climate anomalies, developments in teleconnections, unusual sequences of recent climate change, and human impacts upon the climate system. The physical climate forms the main part of the book, but it also considers social and economic aspects of the global climate system. This textbook has been derived from the authors' extensive experience of teaching climatology and atmospheric science. Each chapter contains an essay by a specialist in the field to enhance the understanding of selected topics. An extensive bibliography is included and lists of websites for further study. This textbook will be invaluable to advanced students of climatology and atmospheric science.
Over the last 20 years, developments in climatology have provided an amazing array of explanations for the pattern of world climates. This textbook, first published in 2006, examines the earth's climate systems in light of this incredible growth in data availability, data retrieval systems, and satellite and computer applications. It considers regional climate anomalies, developments in teleconnections, unusual sequences of recent climate change, and human impacts upon the climate system. The physical climate forms the main part of the book, but it also considers social and economic aspects of the global climate system. This textbook has been derived from the authors' extensive experience of teaching climatology and atmospheric science. Each chapter contains an essay by a specialist in the field to enhance the understanding of selected topics. An extensive bibliography is included and lists of websites for further study. This textbook will be invaluable to advanced students of climatology and atmospheric science.
Over the last 20 years, developments in climatology have provided an amazing array of explanations for the pattern of world climates. This textbook, first published in 2006, examines the earth's climate systems in light of this incredible growth in data availability, data retrieval systems, and satellite and computer applications. It considers regional climate anomalies, developments in teleconnections, unusual sequences of recent climate change, and human impacts upon the climate system. The physical climate forms the main part of the book, but it also considers social and economic aspects of the global climate system. This textbook has been derived from the authors' extensive experience of teaching climatology and atmospheric science. Each chapter contains an essay by a specialist in the field to enhance the understanding of selected topics. An extensive bibliography is included and lists of websites for further study. This textbook will be invaluable to advanced students of climatology and atmospheric science.
Over the last 20 years, developments in climatology have provided an amazing array of explanations for the pattern of world climates. This textbook, first published in 2006, examines the earth's climate systems in light of this incredible growth in data availability, data retrieval systems, and satellite and computer applications. It considers regional climate anomalies, developments in teleconnections, unusual sequences of recent climate change, and human impacts upon the climate system. The physical climate forms the main part of the book, but it also considers social and economic aspects of the global climate system. This textbook has been derived from the authors' extensive experience of teaching climatology and atmospheric science. Each chapter contains an essay by a specialist in the field to enhance the understanding of selected topics. An extensive bibliography is included and lists of websites for further study. This textbook will be invaluable to advanced students of climatology and atmospheric science.
Over the last 20 years, developments in climatology have provided an amazing array of explanations for the pattern of world climates. This textbook, first published in 2006, examines the earth's climate systems in light of this incredible growth in data availability, data retrieval systems, and satellite and computer applications. It considers regional climate anomalies, developments in teleconnections, unusual sequences of recent climate change, and human impacts upon the climate system. The physical climate forms the main part of the book, but it also considers social and economic aspects of the global climate system. This textbook has been derived from the authors' extensive experience of teaching climatology and atmospheric science. Each chapter contains an essay by a specialist in the field to enhance the understanding of selected topics. An extensive bibliography is included and lists of websites for further study. This textbook will be invaluable to advanced students of climatology and atmospheric science.
Over the last 20 years, developments in climatology have provided an amazing array of explanations for the pattern of world climates. This textbook, first published in 2006, examines the earth's climate systems in light of this incredible growth in data availability, data retrieval systems, and satellite and computer applications. It considers regional climate anomalies, developments in teleconnections, unusual sequences of recent climate change, and human impacts upon the climate system. The physical climate forms the main part of the book, but it also considers social and economic aspects of the global climate system. This textbook has been derived from the authors' extensive experience of teaching climatology and atmospheric science. Each chapter contains an essay by a specialist in the field to enhance the understanding of selected topics. An extensive bibliography is included and lists of websites for further study. This textbook will be invaluable to advanced students of climatology and atmospheric science.
Over the last 20 years, developments in climatology have provided an amazing array of explanations for the pattern of world climates. This textbook, first published in 2006, examines the earth's climate systems in light of this incredible growth in data availability, data retrieval systems, and satellite and computer applications. It considers regional climate anomalies, developments in teleconnections, unusual sequences of recent climate change, and human impacts upon the climate system. The physical climate forms the main part of the book, but it also considers social and economic aspects of the global climate system. This textbook has been derived from the authors' extensive experience of teaching climatology and atmospheric science. Each chapter contains an essay by a specialist in the field to enhance the understanding of selected topics. An extensive bibliography is included and lists of websites for further study. This textbook will be invaluable to advanced students of climatology and atmospheric science.
The wind–evaporation–SST (WES) feedback describes a coupled mechanism by which an anomalous meridional sea surface temperature (SST) gradient in the tropics evolves over time. As commonly posed, the (positive) WES feedback depends critically on the atmospheric response to SST anomalies being governed by a process akin to that argued by Lindzen and Nigam in 1987, and omits an alternative process by which SST anomalies modulate surface wind speed through vertical momentum mixing as proposed by Wallace et al. and Hayes et al. in 1989. A simple model is developed that captures the essential coupled dynamics of the WES feedback as commonly posed, while also allowing for momentum entrainment in response to evolving SST anomalies. The evolution of the coupled system depends strongly on which effects are enabled in the model. When both effects are accounted for in idealized cases near the equator, the initial anomalous meridional SST gradient grows over a time scale of a few months but is damped within one year. The sign and magnitude of the WES feedback depend on latitude within the tropics and exhibit hemispheric asymmetry. When constrained by realistic profiles of prevailing zonal wind, the model predicts that the WES feedback near the equator is stronger during boreal winter, while the domain over which it is positive is broader during boreal summer, and that low-frequency climate variability can also modulate the strength and structure of the WES feedback. These insights may aid in the interpretation of coupled climate behavior in observations and more complex models.
Significance Statement
Regional climate variability on time scales from months to decades, including El Niño, relies heavily on feedbacks between the atmosphere and the ocean in which some initial change in the environment is either amplified or damped over time. Several conceptual models for such feedbacks have been devised over the years to explain the coupled climate behavior seen in observations and computer simulations. A rather ubiquitous one is called the wind–evaporation–SST (WES) feedback, but the typical phrasing of it does not incorporate a potentially important influence of ocean temperature changes on the stability of the atmosphere above it. This study adds that effect to the WES feedback framework and examines climate variability through the lens of the augmented conceptual model.
Plain Language Summary
Although the number of global tropical cyclones (TCs) has been relatively constant from year‐to‐year in recent decades, the reason remains unknown. It is important to understand what can lead to global TC frequency variations because of its link with TC impacts. We investigated years in which observed global TC activity deviated from the 1980–2021 average. We found that global TC activity is significantly linked with ocean variability, most strongly with El Niño–Southern Oscillation (ENSO). La Niña, which is marked by cool eastern equatorial Pacific sea‐surface temperature (SST) anomalies, is associated with less global TC activity, and vice versa for El Niño. A new physically‐based index for ENSO, the ENSO Longitude Index (ELI), explains annual global named storm days and ACE as well as the SST anomaly‐based Niño 3.4 index. This is because the ELI accounts for the nonlinear response of thunderstorm activity to SST, accounts for changes in the background SST state associated with the seasonal cycle and/or climate change, and better captures ENSO's spatial diversity than Niño 3.4. This research reveals that reliable future projections of ENSO are necessary, but not sufficient, to understand whether global TC frequency may change in the future.
Livro oriundo do projeto de pesquisa entitulado IMPACTOS DAS MUDANÇAS CLIMÁTICAS EM EXTREMOS HIDROLÓGICOS (SECAS E CHEIAS), financiado pelas CAPES e Agência Nacional de Águas e Saneamento Básico por meio do Edital Mudanças do Clima e Recursos Hídricos n° 19/2015.
Instituições participantes do projeto de pesquisa
Programa de Pós-Graduação em Engenharia Civil (Recursos Hídricos) da Universidade Federal do Ceará
Programa de Pós-Graduação em Tecnologia Ambiental e Recursos Hídricos da Universidade de Brasília
Programas de Pós-Graduação em Engenharia Civil e Ambiental, em Gestão e Regulação de Recursos Hídricos, e em Sistemas Agroindustriais, da Universidade Federal de Campina Grande
Organizadores
Francisco de Assis de Souza Filho (UFC)
Carlos de Oliveira Galvão (UFCG)
Dirceu Silveira Reis Junior (UnB)
Revisores
Daniel Antônio Camelo Cid (UFC)
Maycon Breno Macena da Silva (UFCG)
Apresentação
As mudanças climáticas têm nos recursos hídricos uma de suas dimensões mais relevantes. Os impactos das mudanças climáticas nos extremos hidrológicos (secas e cheias) podem impor aumento significativo da vulnerabilidade das populações humanas e do desenvolvimento social. Avaliar os riscos de aumento da frequência destes eventos e as severidades dos mesmos é passo inicial e necessário para a proposição de estratégias de adaptação que possibilitem maior resiliência da sociedade à variabilidade e mudança climática.
Objetivando construir análise deste processo e propostas de mitigação, a Universidade Federal do Ceará (UFC), a Universidade de Brasília (UnB) e a Universidade Federal de Campina Grande (UFCG) decidiram constituir uma rede de colaboração com outras instituições internacionais e submeter proposta para o Edital Mudanças do Clima e Recursos Hídricos n° 19/2015 CAPES-ANA.
A proposta intitulada “Impactos das Mudanças Climáticas em Extremos Hidrológicos (Secas e Cheias)” recebeu financiamento deste edital e os resultados do trabalho de pesquisa financiados por este projeto constituem os capítulos do presente livro. Os grupos de pesquisa da UFCG, UFC e UnB possuem colaboração anterior a este projeto, notadamente na Rede Brasileira de Pesquisas sobre Mudanças Climáticas Globais – REDE CLIMA, e as atividades desenvolvidas neste projeto podem ser consideradas no contexto desta rede de colaboração. As pesquisas foram desenvolvidas nos Programas de Pós-Graduação em Engenharia Civil (Recursos Hídricos) da Universidade Federal do Ceará, em Tecnologia Ambiental e Recursos Hídricos da Universidade de Brasília, em Engenharia Civil e Ambiental, em Gestão e Regulaçã o de Recursos Hídricos, e em Sistemas Agroindustriais, da Universidade Federal de Campina Grande. Alunos de graduação também foram envolvidos no projeto.
Diversos pesquisadores deste projeto tiveram bolsas financiadas pelo CNPq, pela CAPES e pela Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico (FUNCAP), a quem agradecemos. O presente livro é dividido em três partes: (i) modelos climáticos e detecção de mudanças; (ii) impactos das mudanças climáticas e (iii) estratégias de adaptação à mudança climática.
Detailed knowledge of the long-term interface of climate and rainfall variability is essential for managing agricultural activities in Greater Horn of Africa countries.
Previous studies have demonstrated that North Tropical Atlantic (NTA) warming can be modulated by El Niño events through atmospheric teleconnections. Considering the diversity of El Niño events, this study focuses on different responses of the NTA sea surface temperature (SST) to eastern Pacific (EP), central Pacific type I and type II (CP‐I and CP‐II) El Niño events in their decaying springs. In EP and CP‐II El Niño events, the pattern of NTA warming is well established, while the NTA SST fails to warm in CP‐I El Niño events. In EP and CP‐II El Niño events, anomalous cyclones can be excited in the subtropical Atlantic. Consequently, anomalous southwesterly winds in the NTA weaken the background northeasterly winds and suppress oceanic latent heat loss, leading to NTA warming. However, in CP‐I El Niño events, anomalous anticyclones are excited in the subtropical Atlantic and anomalous northeasterly winds in the NTA strengthen the background northeasterly winds, which facilitate oceanic latent heat loss and lead to NTA cooling. The inconsistent remote atmospheric responses of the NTA are attributed to the different patterns of warm and/or cold SST anomalies in the tropical Pacific in the decaying springs of different types of El Niño events. The physical mechanism linking the SSTs in the tropical Pacific and the NTA has been verified in a slab ocean model.
The influence of the tropical Atlantic on El Niño-Southern Oscillation (ENSO) is examined using sensitivity experiments with the SINTEX-F general circulation model with prescribed sea surface temperature (SST) distributions based on observations for the period 1982-2018. In the control experiment (CTRL) observed SSTs are prescribed over the global oceans, whereas in the sensitivity experiment (OTA) observed SSTs are prescribed in the tropical Atlantic only, while in other regions the climatological annual cycle is prescribed.
A composite analysis of the model output suggests that cold SST events in the northern tropical Atlantic (NTA) during boreal spring are associated with near-surface wind changes over the equatorial and subtropical Pacific that are conducive to the development of El Niño, consistent with previous studies. The amplitude of these changes, however, is at most 20% of those observed during typical El Niño events. Likewise, warm events in the equatorial Atlantic produce only about 10% of the wind changes seen in the western equatorial Pacific during the developing phase of typical La Niña events. Similar results are obtained from a partial regression analysis performed on an ensemble of atmosphere-only simulations from the Atmospheric Model Intercomparison Project (AMIP) Phase 6 though the equatorial Atlantic influence is stronger in AMIP. Further analysis of the AMIP models indicates that model biases do not have a major impact on the Atlantic-to-Pacific influence. Overall, the results suggest that the tropical Atlantic has a rather weak influence on ENSO development and mostly acts to modulate ongoing events rather than initiate them.
We present global fields of decadal annual surface temperature anomalies, referred to the period 1951-1980, for each decade from 1881-1890 to 1981-1990 and for 1984-1993. In addition, we show decadal calendar-seasonal anomaly fields for the warm decades 1936-1945 and 1981-1990. The fields are based on sea surface temperature (SST) and land surface air temperature data. The SSTs are corrected for the pre-World War II use of uninsulated sea temperature buckets and incorporate adjusted satellite-based SSTs from 1982 onward. The generally cold end of the nineteenth century and start to the twentieth century are confirmed, toegether with the substantial warming between about 1920 and 1940. Slight cooling of the northern hemisphere took place between the 1950s and the mid-1970s, although slight warming continued south of the equator. Recent warmth has been most marked over the northern continents in winter and spring, but the 1980s were warm almost everywhere. -from Authors
Numerous analyses of relatively short (25-30 years in length) time series of the observed surface temperature of the tropical Atlantic Ocean have indicated the possible existence of decadal timescale variability. It was decided to search for such variability in 100-yr time series of sea surface temperature (SST) measured aboard ships and available in the recently published Global Ocean Surface Temperature Atlas (GOSTA). Fourier and singular spectrum analyses of the GOSTA SST time series averaged over 11 subregions, each approximately 1 x 10{sup 6}km{sup 2} in area, show that pronounced quasi-oscillatory decadal ({approximately}-20 yr) and multidecadal ({approximately}30-40 yr) timescale variability exists in the GOSTA dataset over the tropical Atlantic. Motivated by the above results, SST variability was investigated in a 200-yr integration of a global model of the coupled oceanic and atmospheric general circulations developed at the geophysical Fluid Dynamics Laboratory (GFDL). The second 100 yr of SST in the coupled model`s tropical Atlantic region were analyzed with a variety of techniques. Analyses of SST time series, averaged over approximately the same subregions as the GOSTA time series, showed that the GFDL SST anomalies also undergo pronounced quasi-oscillatory decadal and multidecadal variability but at somewhat shorter timescales than the GOSTA SST anomalies. Further analyses of the horizontal structures of the decadal timescale variability in the GFDL coupled model showed the existence of two types of variability in general agreement with results of the GOSTA SST time series analyses. One type, characterized by timescales between 8 and 11 yr, has high spatial coherence within each hemisphere but not between the two hemispheres of the tropical Atlantic. A second type, characterized by timescales between 12 and 20 yr, has high spatial coherence between the two hemispheres. 31 refs., 14 figs., 3 tabs.
Two indices related to the sea surface temperature (SST) variability in the tropical Atlantic are proposed. One index describes the SST averaged over the whole basin (30°N to 20°S, 60°W to 15°E), and the other illustrates a meridional dipole between the northern and southern hemispheres. The computational method for obtaining these indices is intentionally kept simple, the objective being to reproduce the signature of the main results previously provided from more complicated statistical analyses. Monthly time series for both indices are produced from 1964 up to the present time. The whole basin index exhibits principally a sustained warming which has intensified since about 1975, and it has a significant periodicity close to that of the quasi-biennial oscillation. The dipole index exhibits a decadal-scale variation, and its building up seems to be related to other worldwide climtic changes, as for instance El Niño/Southern Oscillation extreme episodes, rainfall variabilities over the Brazilian Nordeste and African Sahel.
Using the comprehensively quality-controlled Meteorological Office
Historical Sea Surface Temperature data set (MOHSST)1,2 we
show for the first time that persistently wet and dry periods in the
Sahel region of Africa are strongly related to contrasting patterns of
sea-surface temperature (SST) anomalies on a near-global scale. The
anomalies include relative changes in SST between the hemispheres, on
timescales of years to tens of years, which are most pronounced in the
Atlantic. Experiments with an 11-level global atmospheric general
circulation model (AGCM) support the idea that the worldwide SST
anomalies modulate summer Sahel rainfall through changes in tropical
atmospheric circulation3-6. El Niño events may also
play a part. We do not discount the effects of soil moisture and albedo
changes in the Sahel7,8, although Courel et al.9
have questioned the importance of albedo changes, but we do suggest that
worldwide SST anomalies may have a more fundamental influence on Sahel
rainfall.
Using 100 years of global temperature anomaly data, we have performed a singluar value decomposition of temperature variations in narrow frequency bands to isolate coherent spatio-temporal modes of global climate variability. Statistical significance is determined from confidence limits obtained by Monte Carlo simulations. Secular variance is dominated by a globally coherent trend; with nearly all grid points warming in phase at varying amplitude. A smaller, but significant, share of the secular variance corresponds to a pattern dominated by warming and subsequent cooling in the high latitude North Atlantic with a roughly centennial timescale. Spatial patterns associated with significant peaks in variance within a broad period range from 2.8 to 5.7 years exhibit characteristic El Nino-Southern Oscillation (ENSO) patterns. A recent transition to a regime of higher ENSO frequency is suggested by our analysis. An interdecadal mode in the 15-to-18 years period and a mode centered at 7-to-8 years period both exhibit predominantly a North Atlantic Oscillation (NAO) temperature pattern. A potentially significant decadal mode centered on 11-to-12 years period also exhibits an NAO temperature pattern and may be modulated by the century-scale North Atlantic variability.
The establishment of a thermally direct local circulation which has its ascending branch at about 10 deg N and its descending branch over northeast Brazil and the adjoining oceanic region is proposed as a possible mechanism for the occurrence of severe droughts over this Brazilian region. The driving for this anomalous circulation is provided by enhanced moist convection due to the effect of warmer sea surface anomalies over the northern tropical Atlantic and cooling associated with colder sea surface temperature anomalies in the southern tropical Atlantic. A simple primitive equation model is used to calculate the frictionally-controlled and thermally-driven circulation due to a prescribed heating function in a resting atmosphere, and a series of numerical experiments are carried out to test the sensitivity of the Goddard Laboratory's model to prescribed sea surface temperature anomalies over the tropical Atlantic.
Investigation is made of the interannual and intraseasonal variability of the southwest monsoon layer's thickness and moisture content over West Africa, and its water vapor supply to this region from the tropical Atlantic, for the Subsaharan (10°–20° N) rainy season of July–September. Results for the very deficient 1968. 1971. and 1972 Subsaharan rainy seasons. whose departures from the 1941–74 mean ranged between −0.78σ and − 1.39σ. are compared with those for the near-average rainy seasons of 1967, 1969, and 1975 (–0.27σ to + 0.06σ). The observational base consists of monthly mean rawinsonde data from West African stations and seasonal estimates of the tropical Atlantic near-surface specific humidity field. Subsaharan drought does not appear to be associated with the northward supply of unusually dry surface air to West Africa from the tropical Atlantic. Over West Africa, the magnitude of the vertically integrated monsoon layer advective water vapor flux tends to be proportional to the layer thickness. The extremely poor 1972 Subsaharan rainy season (–1.39σ) coincided with particularly shallow southwesterly flow across the Gulf of Guinea coast (–5°N); thicker monsoon layers are found characteristic of the less severe 1968 (–0.78°) and 1971 (–0.88°) droughts. However, the monsoon layer depth here during the nondrought study years does not always exceed that for drought months. The direction of the low-level water vapor flux above Dakar, a sensitive Subsaharan location (−15°N) on the West African coast, shows a stronger tendency to be from north of west during unproductive Subsaharan rainy seasons (− 0.78σ to − 1.39σ) than in near-average ones (–0.27σ to + 0.06σ). This suggests the southwest monsoon does not extend as far north along the West African coast during Subsaharan droughts as in more abundant rainy seasons. In contrast to the foregoing Gulf of Guinea results, the monsoon layer over the interior of the Subsaharan zone at ˜ 13°N exhibits minimal variability for the drought study years. The better developed nondrought southwesterly flows here tended to be thicker than those characteristic of most drought months. DOI: 10.1111/j.1600-0870.1983.tb00197.x
A simple model of the upper ocean is used to simulate the seasonal cycle
of sea surface temperature (SST) in the tropical Pacific Ocean. The
model is an extension of the conventional 1½-layer reduced
gravity system that includes the physics of the surface mixed layer and
allows prediction of the sea surface temperature. Two types of mixed
layer models, a constant depth mixed layer model of Cane and Zebiak and
a variable depth mixed layer model of Kraus and Turner, are examined.
The numerical simulations indicate that the simple models are capable of
capturing the essential seasonal SST variability in the tropical Pacific
when forced with climatological surface forcing. The sensitivity study
of model SST response to changes in the surface forcing demonstrates
that although the response to surface heat flux forcing dominates the
seasonal cycle of SST in most areas of the tropical Pacific, the
dynamical response of the ocean to changes in the wind also makes a
significant contribution to the seasonal variation of SST in the eastern
equatorial Pacific. The seasonal variation of the winds is particularly
important in maintaining the phase structure of the annual SST signal
along the equator. Moreover, the sensitivity of the SST response depends
on the form of vertical entrainment parameterization in the model. It is
shown that the Cane-Zebiak type of model has a bias toward the dynamical
response of the ocean to the winds, whereas the Kraus-Turner type of
model has bias toward the surface heat flux variations. The simple model
results are also compared with those from an ocean general circulation
model.
Approximations suggested by a scaling analysis are used to obtain analytic results for the eigenmodes of the system. A slow time scale, unstable eigenmode associated with the time derivative of the SST equation is suggested to be important in giving rise to interannual oscillations. This slow SST mode is not necessarily linked to conventional equatorial oceanic wave modes. A useful limit of this mode is explored in which the wave speed of uncoupled oceanic wave modes is fast compared to the time scales that arise from the coupling. This is referred to as the fast-wave limit. The dispersion relationship in this limit is used to present a number of coupled feedback mechanisms, which contribute simultaneously to the instability of the SST mode. -from Author
General circulation mechanisms instrumental in both annual cycle and interannual variability of rainfall are studied with reference to key regions of the tropical Americas and Africa, including the Central American-Caribbean area, northern Northeast Brazil, Subsaharan Africa, the Angola coast and the Zaire (Congo) and Amazon basins. For most of these regions, rainfall anomalies tend to be associated with departures in the large-scale atmospheric and oceanic fields that correspond to the pattern changes in the annual alternation of dry and rainy seasons. -from Author
A hybrid coupled model for the tropical Pacific ocean-atmosphere system is used to simulate El Nino-Southern Oscillation (ENSO) interannual variability and to investigate the role of coupling in the season cycle. An ocean GCM (OGCM) is coupled to an empirical atmospheric model that specifies a wind stress field from a given sea surface temperature (SST) field. The stress is estimated by singular value decomposition of the covariance between observed surface wind stress and SST fluctuations. Two versions of the atmospheric model are employed; one includes only spatial patterns of the atmospheric feedbacks associated with interannual variability, whereas the other also includes spatial patterns associated with the annual cycle. In the latter version wind stress coupling in the seasonal cycle is modeled on the same basis as in the interannual variability. The seasonal cycle enters through prescribed heat flux and is modified by momentum-flux feedbacks. In the OGCM, two vertical mixing schemes-Philander-Pacanowski (PP) and a modified scheme-are used. Simulated ENSO anomalies have a reasonable spatial structure compared to observation, and the form is not strongly sensitive to the atmospheric model or mixing scheme. SST anomalies evolve largely as a standing oscillation, though with some westward propagation; heat content evolution is characteristic of subsurface memory, consistent with a mixed SST-ocean dynamics mode regime. In the absence of the seasonal cycle, the ENSO period is affected by vertical mixing: about 2.3 years for the modified scheme and slightly less than 2 years for the PP scheme. Indications of irregular or multifrequency behavior are also found. Interaction with the seasonal cycle frequency locks the interannual signal to a quasi-biennial period. The seasonal cycle in the eastern Pacific is well simulated by the coupled model. 56 refs., 25 figs.
This paper introduces a conceptual framework for comparing methods that isolate important coupled modes of variability between time series of two fields. Four specific methods are compared: principal component analysis with the fields combined (CPCA), canonical correlation analysis (CCA) and a variant of CCA proposed by Barnett and Preisendorfer (BP), principal component analysis of one single field followed by correlation of its component amplitudes with the second field (SFPCA), and singular value decomposition of the covariance matrix between the two fields (SVD). SVD and CPCA are easier to implement than BP, and do not involve user-chosen parameters. All methods are applied to a simple but geophysically relevant model system and their ability to detect a coupled signal is compared as parameters such as the number of points in each field, the number of samples in the time domain, and the signal-to-noise ratio are varied. In datasets involving geophysical fields, the number of sampling times ma...
A coupled atmosphere-ocean model is developed and used to study the ENSO (El Niñ/Southern Oscillation) phenomenon. With no anomalous external forcing, the coupled model reproduces certain key features of the observed phenomenon. including the recurrence of warm events at irregular intervals with a preference for three to four years. It is shown that the mean sea surface temperature, wind and ocean current fields determine the characteristic spatial structure of ENSO anomalies. The tendency for phase-locking of anomalies is explained in terms of a variation in coupling strength associated with the annual cycle in the mean fields. Sensitivity studies reveal that both the amplitude and the time of scale of the oscillation are sensitive to several parameters that affect the strength of the atmosphere–ocean coupling. Stronger coupling implies larger oscillations with a longer time scale. A critical element of the model oscilliation is the variability in the equatorial heat content of the upper o...
Using a dynamically motivated analysis of observations, and an intermediate-level coupled model, the interannual variability within the equatorial Atlantic is studied. It is found that a significant part of the observed variability can be described by an equatorial coupled mode akin to ENSO (El Niño-Southern Oscillation). The Atlantic mode signature is even more tightly focused on the equator and is situated proportionally farther to the west within the basin than its Pacific counterpart.Model simulations capture the equatorial coupled mode in relatively pure form and, for what are thought to be the most realistic parameter choices, show interannual oscillations favoring a 4-year period, which are not self-sustaining. The simulated spatial patterns agree well with those extracted from observations, including those features that distinguish the Atlantic from the Pacific.Sensitivity experiments show that the Atlantic coupled-mode signal is less robust than the corresponding Pacific ENSO signal but is still well-defined qualitatively, within reasonable parameter ranges. The results demonstrate that the primary mechanisms of oscillation for the Atlantic and Pacific are the same but that differences in the zonal structure and strength of air-sea coupling and mean ocean stratification offset the large differences in basin size, allowing similar oscillation periods for the two basin modes. An explanation for the distinct spatial patterns of simulated Atlantic and Pacific anomalies is found in the differences in climatological mean fields and ocean basin configurations.Together, the observational and model results present a picture of equatorial Atlantic variability in which coupled equatorial dynamics play an important but not exclusive role. It appears that the coupling is sufficiently strong to leave its imprint on the total variability but too weak to dictate it entirely, even at the equator.