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. ; The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007–2013) under Grant Agreement No. 603521 (PREFACE Project), and by the Spanish Project CGL2012-38923-C02-01. ; Peer reviewed
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 Niño influences rainfall variability at the beginning of the XX century and after 1970, while the Pacific Niño 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. ; The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement n° 603521 (PREFACE project) and the Spanish project CGL2012-38923-C02-01. ; Peer reviewed
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 Niño influences rainfall variability at the beginning of the XX century and after 1970, while the Pacific Niño 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. ; The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement n° 603521 (PREFACE project) and the Spanish project CGL2012-38923-C02-01. ; Peer reviewed ; Peer Reviewed
The thermocline is defined as the ocean layer for which the vertical thermal gradient is maximum. In the equatorial ocean, observations led to the use of the 20 °C isotherm depth (z20) as an estimate of the thermocline. This study compares z20 against the physical thermocline in the equatorial Atlantic and Pacific Oceans, using Simple Ocean Data Assimilation reanalysis and fifth phase of the Coupled Model Intercomparison Project preindustrial control simulations. Our results show that z20 is systematically deeper and flatter than the thermocline and does not respond correctly to surface wind stress variations. It is also shown that the annual cycle of z20 is much weaker than that of the physical thermocline. This happens in both equatorial basins and indicates that z20 does not react to the same mechanisms as the thermocline. This could have important consequences in the assessment of air‐sea coupling in current general circulation models and bias reduction strategies. ; The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement 603521 (PREFACE project) and from the Spanish Ministry of Science (Project PRE4CAST, CGL2017‐86415‐R). ; Peer reviewed
Atmosphere-ocean general circulation models (CGCMs) show important systematic errors. Simulated precipitation in the tropics is generally overestimated over the oceans south of the equator, and stratocumulus (SCu) clouds are underestimated above too warm sea surface temperatures (SSTs). In the extratropics, SSTs are also too warm over the Southern Ocean. We argue that ameliorating these extratropical errors in a CGCM can result in an improved model's performance in the tropics depending upon the success in simulating the sensitivity of SCu to underlying SST. Our arguments are supported by the very different response obtained with two CGCMs to an idealized reduction of solar radiation flux incident at the top of the atmosphere over the Southern Ocean. It is shown that local perturbation impacts are very similar in the two models but that SST reductions in the SCu regions of the southern subtropics are stronger in the model with the stronger SCu-SST feedbacks. ; NOAA's Climate Program Office, Climate Variability and Predictability Program Award. Grant Number: NA14OAR4310278. European Union Seventh Framework Programme. Grant Numbers: FP7/2007–2013, 60352 ; Peer reviewed
Review ; The atmospheric seasonal cycle of the North Atlantic region is dominated by meridional movements of the circulation systems: from the tropics, where the West African Monsoon and extreme tropical weather events take place, to the extratropics, where the circulation is dominated by seasonal changes in the jetstream and extratropical cyclones. Climate variability over the North Atlantic is controlled by various mechanisms. Atmospheric internal variability plays a crucial role in the mid-latitudes. However, El Niño-Southern Oscillation (ENSO) is still the main source of predictability in this region situated far away from the Pacific. Although the ENSO influence over tropical and extra-tropical areas is related to different physical mechanisms, in both regions this teleconnection seems to be non-stationary in time and modulated by multidecadal changes of the mean flow. Nowadays, long observational records (greater than 100 years) and modeling projects (e.g., CMIP) permit detecting non-stationarities in the influence of ENSO over the Atlantic basin, and further analyzing its potential mechanisms. The present article reviews the ENSO influence over the Atlantic region, paying special attention to the stability of this teleconnection over time and the possible modulators. Evidence is given that the ENSO–Atlantic teleconnection is weak over the North Atlantic. In this regard, the multidecadal ocean variability seems to modulate the presence of teleconnections, which can lead to important impacts of ENSO and to open windows of opportunity for seasonal predictability. ; We thank the Climatic Research Unit (CRU), the National Centers for Environmental Prediction (NCEP), the Met Office Hadley Centre and the US National Hurricane Center (NHC) for the Land Precipitation, reanalysis, SST and HURDAT2 datasets, respectively. Belen Rodríguez-Fonseca, Roberto Suárez-Moreno, Jorge López-Parages, Iñigo Gómara, Elsa Mohino, Teresa Losada and Antonio Castaño-Tierno are supported by the research projects PREFACE (EUFP7/2007-2013 Grant Agreement 603521) and MULCLIVAR (CGL2012-38923-C02-01-Spanish Ministry of Economy and Competitiveness). Blanca Ayarzagüena is supported by the Natural Environment Research Council (grant number NE/M006123/1). Julián Villamayor is granted through a scholarship from the MICINN—Spanish government (BES-2013-063821)
36 pages, 12 figures, 1 table ; he tropical Atlantic is home to multiple coupled climate variations covering a wide range of timescales and impacting societally relevant phenomena such as continental rainfall, Atlantic hurricane activity, oceanic biological productivity, and atmospheric circulation in the equatorial Pacific. The tropical Atlantic also connects the southern and northern branches of the Atlantic meridional overturning circulation and receives freshwater input from some of the world's largest rivers. To address these diverse, unique, and interconnected research challenges, a rich network of ocean observations has developed, building on the backbone of the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA). This network has evolved naturally over time and out of necessity in order to address the most important outstanding scientific questions and to improve predictions of tropical Atlantic severe weather and global climate variability and change. The tropical Atlantic observing system is motivated by goals to understand and better predict phenomena such as tropical Atlantic interannual to decadal variability and climate change; multidecadal variability and its links to the meridional overturning circulation; air-sea fluxes of CO2 and their implications for the fate of anthropogenic CO2; the Amazon River plume and its interactions with biogeochemistry, vertical mixing, and hurricanes; the highly productive eastern boundary and equatorial upwelling systems; and oceanic oxygen minimum zones, their impacts on biogeochemical cycles and marine ecosystems, and their feedbacks to climate. Past success of the tropical Atlantic observing system is the result of an international commitment to sustained observations and scientific cooperation, a willingness to evolve with changing research and monitoring needs, and a desire to share data openly with the scientific community and operational centers. The observing system must continue to evolve in order to meet an expanding set of research priorities and operational challenges. This paper discusses the tropical Atlantic observing system, including emerging scientific questions that demand sustained ocean observations, the potential for further integration of the observing system, and the requirements for sustaining and enhancing the tropical Atlantic observing system ; MM-R received funding from the MORDICUS grant under contract ANR-13-SENV-0002-01 and the MSCA-IF-EF-ST FESTIVAL (H2020-EU project 797236). GF, MG, RLu, RP, RW, and CS were supported by NOAA/OAR through base funds to AOML and the Ocean Observing and Monitoring Division (OOMD; fund reference 100007298). This is NOAA/PMEL contribution #4918. PB, MDe, JH, RH, and JL are grateful for continuing support from the GEOMAR Helmholtz Centre for Ocean Research Kiel. German participation is further supported by different programs funded by the Deutsche Forschungsgemeinschaft, the Deutsche Bundesministerium für Bildung und Forschung (BMBF), and the European Union. The EU-PREFACE project funded by the EU FP7/2007–2013 programme (Grant No. 603521) contributed to results synthesized here. LCC was supported by the UERJ/Prociencia-2018 research grant. JOS received funding from the Cluster of Excellence Future Ocean (EXC80-DFG), the EU-PREFACE project (Grant No. 603521) and the BMBF-AWA project (Grant No. 01DG12073C)
