The South Atlantic Anticyclone as a key player for the representation of the tropical Atlantic climate in coupled climate models

The key role of the South Atlantic Anticyclone (SAA) on the seasonal cycle of the tropical Atlantic is investigated with a regionally coupled atmosphere–ocean model for two different coupled domains. Both domains include the equatorial Atlantic and a large portion of the northern tropical Atlantic, but one extends southward, and the other northwestward. The SAA is simulated as internal model variability in the former, and is prescribed as external forcing in the latter. In the first case, the model shows significant warm biases in sea surface temperature (SST) in the Angola-Benguela front zone. If the SAA is externally prescribed, these biases are substantially reduced. The biases are both of oceanic and atmospheric origin, and are influenced by ocean–atmosphere interactions in coupled runs. The strong SST austral summer biases are associated with a weaker SAA, which weakens the winds over the southeastern tropical Atlantic, deepens the thermocline and prevents the local coastal upwelling of colder water. The biases in the basins interior in this season could be related to the advection and eddy transport of the coastal warm anomalies. In winter, the deeper thermocline and atmospheric fluxes are probably the main biases sources. Biases in incoming solar radiation and thus cloudiness seem to be a secondary effect only observed in austral winter. We conclude that the external prescription of the SAA south of 20°S improves the simulation of the seasonal cycle over the tropical Atlantic, revealing the fundamental role of this anticyclone in shaping the climate over this region

Recent progress in understanding and predicting Atlantic decadal climate variability

Recent Atlantic climate prediction studies are an exciting new contribution to an extensive body of research on Atlantic decadal variability and predictability that has long emphasized the unique role of the Atlantic Ocean in modulating the surface climate. We present a survey of the foundations and frontiers in our understanding of Atlantic variability mechanisms, the role of the Atlantic Meridional Overturning Circulation (AMOC), and our present capacity for putting that understanding into practice in actual climate prediction systems

Sahel rainfall strength and onset improvements due to more realistic Atlantic cold tongue development in a climate model

The simulation of Sahel rainfall and its onset during the West African Monsoon (WAM) remains a challenge for current state-of-the-art climate models due to their persistent biases, especially in the tropical Atlantic region. Here we show that improved representation of Atlantic Cold Tongue (ACT) development is essential for a more realistic seasonal evolution of the WAM, which is due to a further inland migration of the precipitation maximum. The observed marked relationship between ACT development and Sahel rainfall onset only can be reproduced by a climate model, the Kiel Climate Model (KCM), when sufficiently high resolution in its atmospheric component is employed, enabling enhanced equatorial Atlantic interannual sea surface temperature variability in the ACT region relative to versions with coarser atmospheric resolution. The ACT/Sahel rainfall relationship in the model critically depends on the correct seasonal phase-locking of the interannual variability rather than on its magnitude. We compare the KCM results with those obtained from climate models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5)