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

Climate Dynamics, Aug 2016

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.

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

The South Atlantic Anticyclone as a key player for the representation of the tropical Atlantic climate in coupled climate models William Cabos 0 1 2 3 4 5 6 7 8 9 Dmitry V. Sein 0 1 2 3 4 5 6 7 8 9 Joaquim G. Pinto 0 1 2 3 4 5 6 7 8 9 Andreas H. Fink 0 1 2 3 4 5 6 7 8 9 Nikolay V. Koldunov 0 1 2 3 4 5 6 7 8 9 Francisco Alvarez 0 1 2 3 4 5 6 7 8 9 Alfredo Izquierdo 0 1 2 3 4 5 6 7 8 9 Noel Keenlyside 0 1 2 3 4 5 6 7 8 9 Daniela Jacob 0 1 2 3 4 5 6 7 8 9 0 Alfred Wegener Institute for Polar and Marine Research , Bremerhaven , Germany 1 Department of Physics, University of Alcalá, Alcalá de Henares , Madrid , Spain 2 William Cabos 3 Geophysical Institute and Bjerknes Centre for Climate Research, University of Bergen , Bergen , Norway 4 Marine Science and Technological Center of Andalusia, University of Cadiz , Cádiz , Spain 5 Climate Service Center, Helmholtz Zentrum Geesthacht , Hamburg , Germany 6 Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology , Karlsruhe , Germany 7 Institute for Geophysics and Meteorology, University of Cologne , Cologne , Germany 8 Department of Meteorology, University of Reading , Reading , UK 9 P. P. Shirshov Institute of Oceanology RAS , St.Petersburg , Russia 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. South Atlantic Anticyclone; Southeastern tropical Atlantic; Seasonal cycle; SST biases 1 Introduction The tropical oceans are one of the key areas of weather and climate variability. Coupled atmosphere–ocean general circulation models (AOGCMs) typically show warm biases in sea surface temperature (SST) over both the tropical Pacific and Atlantic oceans, particularly in their eastern parts (Mechoso et al. 1995) . Richter et al. (2012b) reported that the tropical Atlantic biases persist in most AOGCMs participating in the Coupled Model Intercomparison Project (CMIP) Phase 3 and 5 (Meehl et al. 2007; Taylor et al. 2012) . Several studies have explored the causes and structure of these biases: for example, Li and Xie (2012) suggested that the biases could be both of oceanic or atmospheric origin. They proposed the existence of two predominant types of tropical-wide errors in CMIP models. The first can be traced back to biases in atmospheric simulation of cloud cover and shows a broad meridional structure of the same sign across all basins, with models with enhanced cloud cover featuring lower biases in tropical-wide SST. The second originates from differences in the simulation of the thermocline depth, with models with a deeper thermocline typically showing a warmer equatorial cold tongue. Over the tropical Atlantic, the oceanic and atmospheric processes are strongly coupled and this coupling is a major determinant of climate variability over the adjacent tropical continents. For example, the year-toyear variability of the seasonal development of Atlantic cold tongue has strong impacts on rainfall variability over West Africa in general, particularly on the onset of the West African monsoon (Kushnir et al. 2006; Brandt et al. 2011) . Thus, the warm SST biases in coupled models severely inhibits the ability of these models to correctly reproduce both the observed SST variability in the equatorial Atlantic and the dynamics governing that variability (Ding et al. 2015). The present paper focuses on the southeastern tropical Atlantic (SETA; Fig. 1a). Various mechanisms have been proposed for the development of warm bia (...truncated)


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William Cabos, Dmitry V. Sein, Joaquim G. Pinto, Andreas H. Fink, Nikolay V. Koldunov, Francisco Alvarez, Alfredo Izquierdo, Noel Keenlyside, Daniela Jacob. The South Atlantic Anticyclone as a key player for the representation of the tropical Atlantic climate in coupled climate models, Climate Dynamics, 2017, pp. 4051-4069, Volume 48, Issue 11-12, DOI: 10.1007/s00382-016-3319-9