Teresa Losada

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...

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...

Can reducing the incoming energy flux over the Southern Ocean in a CGCM improve its simulation of tropical climate

NOAAs Climate Program Office, Climate Variability and Predictability Program Award. Grant Number: NA14OAR4310278. European Union Seventh Framework Programme. Grant Numbers: FP7/2007–2013, 60352

The non-stationary influence of the Atlantic and Pacific Niños on North Eastern South American rainfall

Rainfall variability over the tropical Atlantic region is dominated by changes in the surface temperature of the surrounding oceans. In particular, the oceanic forcing over Northeast of South America is dominated by the Atlantic interhemispheric temperature gradient, which leads its predictability. Nevertheless, in recent decades, the SST influence on rainfall variability in some tropical Atlantic regions has been found to be non-stationary, with important changes of the Atlantic and Pacific influence on Sahelian rainfall which appear to be modulated at multidecadal timescales. In this work, we revisit the SST influence over Northeast of South America including the analysis of the stationarity of this relationship at interannual timescales. Principal Component Analysis has been applied to the interannual component of rainfall during the March-April-May season. Results show how the SST forcing on the first mode of rainfall variability, which is a dipole-like pattern generated by the changes in the seasonal migration of the Intertropical Convergence Zone, is different depending of the considered period. The response to the SST anomalies in the Pacific basin is opposite to the Atlantic one and affects different areas. The Atlantic Nino influences rainfall variability at the beginning of the XX century and after 1970, while the Pacific Nino plays a major role in the variability of the rainfall in the Northeast of South America from 1970 onwards. The combined effect of both basins after the 1970s amplifies the anomalous rainfall response.

Extratropical Atmospheric Response to the Atlantic Niño Decaying Phase

Abstract The Atlantic Nino or Atlantic Equatorial Mode (EM) is the dominant coupled variability phenomenon in the tropical Atlantic basin during boreal summer. From the 1970s, the mode has changed, evolving in time from east to west and without persisting until the following winter. In a previous observational work, the authors have studied the atmospheric response to the EM during the 1979–2005 period, proposing three main issues along the decaying phase of this mode: 1) the continuous confinement of the anomalous deep convection over northeastern Brazil following the thermal-forcing decay; 2) an increasing dipole-like precipitation anomaly with dry conditions in the Florida–Gulf of Mexico region; and 3) the excitation of Rossby waves forced by the remaining upper-tropospheric divergence that are trapped into the subtropical jet but do not show a robust impact on the European sector. In this work, a 10-member ensemble simulation for the recent EM with the University of California, Los Angeles AGCM model ...

Tropical atmospheric response to decadal changes in the Atlantic Equatorial Mode

It has been shown that the atmospheric response to the Atlantic Equatorial Mode is non-stationary. After the 1970s, Sea Surface Temperature (SST) anomalies in the tropical Atlantic are able to alter the atmosphere in the tropical Pacific via modifications of the Walker circulation. Such changes could be related to the differences in the background state of the global SSTs before and after the 1970s, but also to changes in the interannual Equatorial Mode itself. In this work we first describe the differences in the interannual Equatorial Mode before and after the 1970s. Then we use two AGCMs to perform different sensitivity experiments changing the spatial structure of the Equatorial Mode, and we explore the differences in the atmospheric response over the tropical Pacific region to each of the SST patterns considered. It is shown that the changes in the Walker Atlantic–Pacific cell produced by the EM are stronger after the 1970s, and are reinforced by the change in the impact of the EM over the Indian Ocean and the Maritime Continent. It is also shown that, although the Atlantic–Pacific connection is established by the aforementioned changes in the Walker circulation between the two basins, the modulation of the Indian sector is crucial for a realistic simulation of such connection by climate models.

Impacts of the Atlantic Equatorial Mode in a warmer climate

The main source of sea surface temperature (SST) variability in the Tropical Atlantic at interannual time scales is the Equatorial Mode or Atlantic El Niño. It has been shown to affect the adjacent continents and also remote regions, leading to a weakened Indian Monsoon and promoting La Niña-type anomalies over the Pacific. However, its effects in a warmer climate are unknown. This work analyses the impact of the Equatorial Mode at the end of the twenty first century by means of sensitivity experiments with an atmosphere general circulation model. The prescribed boundary conditions for the future climate are based on the outputs from models participating in the coupled model intercomparison project—phase V. Our results suggest that even if the characteristics of the Equatorial Mode at the end of the twenty first century remained equal to those of the twentieth century, there will be an eastward shift of the main rainfall positive anomalies in the Tropical Atlantic and a weakening of the negative rainfall anomalies over the Asian monsoon due to the change in climatological SSTs. We also show that extratropical surface temperature anomalies over land related to the mode will change in regions like Southwestern Europe, East Australia, Asia or North America due to the eastward shift of the sea level pressure systems and related surface winds.

Large-scale atmospheric response to eastern Mediterranean summer-autumn SST anomalies and the associated regional impact

Since the Mediterranean Sea is halfway between subtropical and middle latitudes, and it represents a marginal oceanic region, research has tended to focus on how large-scale modes of atmospheric variability modulate its surface temperature. Conversely, the present study examines the potential influence of the Mediterranean Sea surface temperature (SST) anomalies on the Northern Hemisphere atmospheric circulation. In particular, this work explores the large-scale changes in the global circulation forced/influenced by the eastern Mediterranean summer-autumn SST pattern. To isolate the atmospheric response, AGCM sensitivity experiments with prescribed SST over the Mediterranean Sea and climatology elsewhere are analysed. Observational diagnostics upon the period used to define the boundary conditions (1979–2002) are also interpreted. Our results support the hypothesis of an atmospheric pattern initiated in the Mediterranean basin, pointing out both a local baroclinic response and a barotropic circumglobal anomaly. This atmospheric teleconnection pattern projects onto a hemispheric wave-like structure, reflecting the waveguide effect of the westerly jets. Results suggest, thereby, that the recurrent summer-autumn circumglobal teleconnection pattern can be excited locally by changes in the atmosphere over the Mediterranean region. A linear behaviour is found upon a regional impact over northeastern Africa. The remote impacts present however a nonlinear signature: anomalous warm conditions influencing on northern Europe and Euro–Asia, whereas anomalous cold conditions impacting more on the North Pacific basin. Limitations in our model setup are also discussed.

Multidecadal Modulation of ENSO Teleconnection with Europe in Late Winter: Analysis of CMIP5 Models

AbstractMany studies point to a robust ENSO signature on the North Atlantic–European (NAE) sector associated with a downstream effect of Rossby wave trains. Some of these works also address a nonstationary behavior of the aforementioned link, but only few have explored the possible modulating factors. In this study the internal causes within the ocean–atmosphere coupled system influencing the tropospheric ENSO–Euro-Mediterranean rainfall teleconnection have been analyzed. To this aim, unforced long-term preindustrial control simulations from 18 different CMIP5 models have been used. A nonstationary impact of ENSO on Euro-Mediterranean rainfall, being spatially consistent with the observational one, is found. This variable feature is explained by a changing ENSO-related Rossby wave propagation from the tropical Pacific to the NAE sector, which, in turn, is modulated by multidecadal variability of the climatological jet streams associated with the underlying sea surface temperature (SST). The results, there...