Formation of a Northward Displaced ITCZ in a Hybrid Coupled AGCM

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 (GCM) is coupled with an intermediate ocean model to study continental forcing and ocean-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 land mass 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 basin-wide 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. This contrast between the response to an eastern and western continental forcing is attributed to the westward-only propagation of wind-evaporation-SST (WES) wave as a linear theory suggests. Possible application of this westward control mechanism to the real Pacific and Atlantic climates will be discussed.