Formation and variability of a northerly ITCZ in a hybrid coupled AGCM: Continental forcing and oceanic-atmospheric feedback

Abstract

Despite the equatorial symmetry of the annual-mean insolation, the intertropical convergence zone (ITCZ) and the collocated band of high sea surface temperature (SST) assume perennial northern latitudes over the eastern Pacific and Atlantic. An atmospheric general circulation model is coupled with an intermediate ocean model to study continental forcing and oceanic-atmospheric interaction that act to break the equatorial symmetry. The model reaches a statistically symmetric mean state under perfectly symmetric conditions with the continental coasts running along meridians. When a bulge of landmass is added to the eastern continent north of the equator, it initiates a coupled ocean-atmosphere wave front that propagates westward across the ocean basin, cooling the ocean surface and suppressing deep convection on and south of the equator. As a result, the ITCZ shifts into the Northern Hemisphere. In contrast to this basinwide response, little latitudinal asymmetry develops in the coupled model when the same land bulge is moved to the western continent, with the ITCZ staying on the equator. These model experiments demonstrate that the meridional structure of the oceanic ITCZ is largely determined by the continental geometry in the east, lending support to a westward control hypothesis based on simple linear wave dynamics. Under steady solar forcing, the model ITCZ displays substantial interannual variability in both intensity and latitude. A coherent interhemispheric SST pattern with opposing polarities is found to be associated with this ITCZ variability. Various oceanic-atmospheric feedback mechanisms involved in the formation and variability of the ITCZ are examined.