Control of the South Atlantic Convergence Zone by extratropical thermal forcing

Abstract

The response of the South Atlantic Conver- gence Zone (SACZ) to an extratropical thermal forcing is investigated in a series of simulations performed with an atmospheric general circulation model coupled to a slab ocean model. Three sets of experiments are performed, varying the extratropical forcing. In the first the forcing consists of warming of the Northern Hemisphere (NH) and cooling of the Southern Hemisphere, with zero global aver- age. In the second and third experiments, the former forc- ing is divided into its northern and southern components to asses their relative roles in affecting the SACZ. In all the cases realistic surface boundary conditions are imple- mented. We found that during its peak in austral summer the SACZ weakens in response to the extratropical forcing and that such weakening is mostly due to the NH compo- nent of the forcing. We found that 75% of the SACZ signal in response to the forcing is linked to the generation of a secondary tropical convergence zone in the Atlantic Ocean around 20°N–30°N, which generates an anomalous Hadley circulation with subsidence over the SACZ. This mechanism appears to be dependent on the upper level changes and tropical ocean response, as it weakens significantly when the simulation is repeated not allowing the tropical sea surface temperatures to change in response to the forcing. The remaining 25% of the signal can be explained through the development of a Walker-type of circulation between western tropical Africa and the SACZ, being this mechanism dependent on the African land surface temperature reaction to the remote forcing.