Mechanisms Controlling the Annual Cycle of Precipitation in the Tropical Atlantic Sector in an Atmospheric GCM*

Abstract

Abstract A set of AGCM experiments is used to study the annual cycle of precipitation in the region surrounding the tropical Atlantic Ocean. The experiments are designed to reveal the relative importance of insolation over land and the (uncoupled) SST on the annual cycle of precipitation over the tropical Atlantic Ocean, Africa, and the tropical Americas. SST variations impact the position of the maritime ITCZ by forcing the hydrostatic adjustment of the atmospheric boundary layer and changes in surface pressure and low-level convergence. The condensation heating released in the ITCZ contributes substantially to the surface circulation and the maintenance of the SST-induced ITCZ anomalies. The remote influence of SST is felt in equatorial coastal areas and the Sahel. The circulation driven by condensation heating in the maritime ITCZ extends to the coastal regions, thus communicating the SST signal onshore. Conversely, the Sahel responds to variations in SST through boundary layer processes that do not involve the maritime ITCZ. The atmospheric response to changes in subtropical SST is advected inland and forces changes in sea level pressure and low-level convergence across a large part of tropical Africa. The impact of local insolation on continental precipitation can be explained by balancing net energy input at the top of the atmospheric column with the export of energy by the divergent circulation that accompanies convection. Increased insolation reduces the stability of the atmosphere in the main continental convection centers, but not in monsoon regions. Insolation over land impacts the intensity of the maritime ITCZ via its influence on precipitation in Africa and South America. Reduced land precipitation induces the cooling of the Atlantic upper troposphere and the enhancement of convective available potential energy in the maritime ITCZ.