The Annual Cycle over the Tropical Atlantic, South America, and Africa

Abstract

The annual cycle over land can be thought of as being forced locally by the direct action of the sun and remotely by circulations forced by regions of persistent precipitation organized primarily by SST and, secondarily, by land. This study separates these two sources of annual variability in order to indicate where and when the remote effects are important. Two main sets of AGCM experiments were performed: one with fixed SST boundary conditions and seasonally varying insolation, another with fixed insolation and seasonally varying SST. For each experiment, the evolution of the annual cycle is presented as the differences from the reference month of March. The comparison of other months to March in the fixed-SST runs separates out the direct response of the land?atmosphere system to the annual insolation changes overhead. Similarly, the same comparison in the annual cycle of the fixed-insolation runs reveals the response of the land?atmosphere system to changes in SST. Over most of the domain, insolation is the dominant forcing on land temperature during June and December, but SST dominates during September. Insolation determines the north?south displacement of continental convection at the solstices and greatly modulates the intensity of precipitation over the tropical Atlantic Ocean. The SST determines the location of the ITCZ over the oceans and influences continental precipitation in coastal regions and in the Sahel/Sudan region. In September, when SST deviations from the March reference values are largest, the SST influence on both precipitation and surface air temperature extends to most of the tropical land. SST is an important forcing for the surface air temperature in the Guinea highlands and northeast Brazil throughout the year.