Belén Rodríguez-Fonseca

Tropical Atlantic Variability Modes (1979-2002). Part I: Time-Evolving SST Modes Related to West African Rainfall

Abstract This work presents a description of the 1979–2002 tropical Atlantic (TA) SST variability modes coupled to the anomalous West African (WA) rainfall during the monsoon season. The time-evolving SST patterns, with an impact on WA rainfall variability, are analyzed using a new methodology based on maximum covariance analysis. The enhanced Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) dataset, which includes measures over the ocean, gives a complete picture of the interannual WA rainfall patterns for the Sahel dry period. The leading TA SST pattern, related to the Atlantic El Nino, is coupled to anomalous precipitation over the coast of the Gulf of Guinea, which corresponds to the second WA rainfall principal component. The thermodynamics and dynamics involved in the generation, development, and damping of this mode are studied and compared with previous works. The SST mode starts at the Angola/Benguela region and is caused by alongshore wind anomalies. It then propagates wes...

Variability and Predictability of West African Droughts: A Review on the Role of Sea Surface Temperature Anomalies

AbstractThe Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s and 1960s. Although rainfall partially recovered since the 1990s, the drought had devastating impacts on society. Most studies agree that this dry period resulted primarily from remote effects of sea surface temperature (SST) anomalies amplified by local land surface–atmosphere interactions. This paper reviews advances made during the last decade to better understand the impact of global SST variability on West African rainfall at interannual to decadal time scales. At interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel, and positive SST anomalies over the Mediterranean Sea tend to be associated with increased rainfall. At decadal time scales, warming over the tropics leads to drought over the Sahel, whereas warming over the North Atlantic promotes increased rainfall. Prediction systems have evolved from seasonal to decada...

Global Meteorological Drought: A Synthesis of Current Understanding with a Focus on SST Drivers of Precipitation Deficits

AbstractDrought affects virtually every region of the world, and potential shifts in its character in a changing climate are a major concern. This article presents a synthesis of current understanding of meteorological drought, with a focus on the large-scale controls on precipitation afforded by sea surface temperature (SST) anomalies, land surface feedbacks, and radiative forcings. The synthesis is primarily based on regionally focused articles submitted to the Global Drought Information System (GDIS) collection together with new results from a suite of atmospheric general circulation model experiments intended to integrate those studies into a coherent view of drought worldwide. On interannual time scales, the preeminence of ENSO as a driver of meteorological drought throughout much of the Americas, eastern Asia, Australia, and the Maritime Continent is now well established, whereas in other regions (e.g., Europe, Africa, and India), the response to ENSO is more ephemeral or nonexistent. Northern Euras...

Impacts of the Tropical Pacific/Indian Oceans on the Seasonal Cycle of the West African Monsoon

AbstractThe current consensus is that drought has developed in the Sahel during the second half of the twentieth century as a result of remote effects of oceanic anomalies amplified by local land–atmosphere interactions. This paper focuses on the impacts of oceanic anomalies upon West African climate and specifically aims to identify those from SST anomalies in the Pacific/Indian Oceans during spring and summer seasons, when they were significant. Idealized sensitivity experiments are performed with four atmospheric general circulation models (AGCMs). The prescribed SST patterns used in the AGCMs are based on the leading mode of covariability between SST anomalies over the Pacific/Indian Oceans and summer rainfall over West Africa. The results show that such oceanic anomalies in the Pacific/Indian Ocean lead to a northward shift of an anomalous dry belt from the Gulf of Guinea to the Sahel as the season advances. In the Sahel, the magnitude of rainfall anomalies is comparable to that obtained by other aut...

Processes in the Pacific La Niña onset triggered by the Atlantic Niño

Previous observational and model studies have shown that a warm (cold) event in the equatorial Atlantic during the boreal summer are related to the development of a Pacific La Niña (El Niño) event, that is fully developed in the following winter. Although the connection takes place via atmospheric bridge, the processes at work have not been clarified for such a remote and lagged relationship. The present paper uses a partially coupled atmosphere–ocean model to infer a mechanism by which a Pacific El Niño event can be developed. In this way, enhanced equatorial convection in the equatorial Atlantic during a warm event results in enhanced subsidence and surface wind divergence over the equatorial Pacific around the dateline. This wind anomaly contributes to pile up water in the western equatorial Pacific, triggering a perturbation in the depth of the oceanic thermocline, which propagates eastward as an equatorial Kelvin wave from autumn to winter. The thermocline shallowing as the wave propagates allows for cooling of the oceanic mixed layer through anomalous temperature advection by anomalous zonal currents and by mean vertical entrainment velocity. Zonal advective and thermocline feedbacks reinforce the surface winds anomalies over the central eastern equatorial Pacific setting up the conditions for the development of a cold event in this ocean. The sequence during an Atlantic cold event is similar with the appropriate change in signs. These findings are relevant to ENSO predictability at seasonal timescales.

Tropical Atlantic Variability Modes (1979–2002). Part II: Time-Evolving Atmospheric Circulation Related to SST-Forced Tropical Convection

Abstract The ways in which deep convection over the tropical Atlantic affects the midlatitude climate variability through meridional circulation, planetary wave teleconnection, and wave–mean flow interaction is examined for the 1979–2002 period, by following the North Atlantic anomalous rainfall evolution from summer to late winter. In this way, the first two covariability modes between anomalous summer tropical Atlantic sea surface temperature (SST) and anomalous summer–late-winter precipitation over the North Atlantic basin are analyzed using the same methodology of extended maximum covariance analysis developed for Part I. This work updates the results given by other authors, whose studies are based on different datasets dating back to the 1950s. To this end, the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) dataset, which includes measures over the ocean, is used to give a complete picture of the interannual rainfall patterns for the last decades. The first mode, which accoun...

On the Atlantic–Pacific Niños connection: a multidecadal modulated mode

Atlantic and Pacific El Niño are the leading tropical oceanic variability phenomena at interannual timescales. Recent studies have demonstrated how the Atlantic Niño is able to influence on the dynamical processes triggering the development of the Pacific La Niña and vice versa. However, the stationarity of this interbasin connection is still controversial. Here we show for the first time that the Atlantic–Pacific Niños connection takes place at particular decades, coinciding with negative phases of the Atlantic Multidecadal Oscillation (AMO). During these decades, the Atlantic–Pacific connection appears as the leading coupled covariability mode between Tropical Atlantic and Pacific interannual variability. The mode is defined by a predictor field, the summer Atlantic Sea Surface Temperature (SST), and a set of predictand fields which represent a chain of atmospheric and oceanic mechanisms to generate the Pacific El Niño phenomenon: alteration of the Walker circulation, surface winds in western Pacific, oceanic Kelvin wave propagating eastward and impacting on the eastern thermocline and changes in the Pacific SST by internal Bjerknes feedback. We suggest that the multidecadal component of the Atlantic acts as a switch for El Niño prediction during certain decades, putting forward the AMO as the modulator, acting through changes in the equatorial Atlantic convection and the equatorial Pacific SST variability. These results could have a major relevance for the decadal prediction systems.

ENSO influence on the North Atlantic European climate: a non-linear and non-stationary approach

AbstractEl Niño Southern Oscillation (ENSO) impact on the North Atlantic European sector (NAE) is still under discussion. Recent studies have found a non stationary feature of this teleconnection, suggesting an effective modulating role of the ocean mean state. Nevertheless, physical explanations about the underlying mechanisms have been little studied in the available literature. In addition, ENSO events show different SST spatial patterns, phases, and amplitudes, which can also influence on the related remote impacts. In view of all this, in the present study a set of partially coupled experiments have been performed with a global atmospheric general circulation model in which different SST ENSO patterns are superimposed over distinct Pacific and Atlantic SST mean states. These SST background conditions are constructed according to the observational difference between periods with a distinct impact of ENSO on the leading Euro-Mediterranean rainfall mode in late winter-early spring. Our results point to two distinct mechanisms associated with ENSO that can be modulated by the SST mean state: (1) the thermally driven direct circulation (Walker and Hadley cells) connecting the Atlantic and Pacific basins, and (2) the Rossby wave propagation from the tropical Pacific to the North Atlantic. The former elucidates that the positive NAO-like pattern usually related to La Niña events could be only valid for selected decades. The latter explains a reinforced signature of Eastern Pacific Niños on the Euro-Mediterranean rainfall when the tropical Pacific is warmer than usual and the North Atlantic is colder than usual. This feature is consistent with the changing ENSO impact identified in previous studies and demonstrates how the ENSO teleconnection with the NAE climate at interannual timecales could be modulated by multidecadal changes in the SST. According to our results, the assumption of stationarity which is still common to many studies of ENSO teleconnections clearly has to be questioned.

Atlantic opportunities for ENSO prediction

El Nino–Southern Oscillation (ENSO) is the dominant mode of interannual climate variability with worldwide impacts. The knowledge of ENSO drivers and the underlying mechanisms is crucial to improve ENSO prediction, which still remains a challenge. The recently discovered connection between an Atlantic Nino (Nina) and a Pacific Nina (Nino), through an air-sea coupled mechanism during the first and last decades of the twentieth century, highlights an opportunity for ENSO prediction. Here a statistical cross-validated hindcast of ENSO along the twentieth century is presented, considering the Atlantic sea surface temperatures as the unique predictor field, and a set of atmospheric and oceanic variables related to the Atlantic-Pacific connection as the predictand field. The observed ENSO phase is well reproduced, and the skill is enhanced at the beginning and the end of the twentieth century. Understanding this multidecadal modulation of the Atlantic-Pacific connection could help to improve seasonal-to-decadal forecasts of ENSO and its associated impacts.

Winter 10-Day Coupled Patterns between Geopotential Height and Iberian Peninsula Rainfall Using the ECMWF Precipitation Reanalysis

Abstract Ten-day winter anomalous precipitation variability in the Iberian Peninsula (IP), related to the North Atlantic atmospheric general circulation, is analyzed using 24-h forecast ECMWF precipitation data over 1979–96. The three main modes that explain the 10-day winter precipitation variability are described using the empirical orthogonal function (EOF) analysis. Singular value decomposition (SVD) analysis between the anomalous geopotential height for different levels and the IP precipitation is used as a tool to establish the relation between the precipitation and the North Atlantic atmospheric general circulation, showing how the first three SVD modes collect practically all possible precipitation–atmosphere links in these timescales. These resultant SVD atmospheric patterns are related to other known teleconnection patterns, such as the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the Scandinavian pattern (SCA), and the east Atlantic pattern (EA). At the same time, the main res...