The Maintenance of an Equatorially Asymmetric State in a Hybrid Coupled GCM

Abstract

An ocean general circulation model is coupled with a simple atmosphere model to investigate the formation mechanism of the intertropical convergence zone in the eastern Pacific, which is observed in the Northern Hemisphere. The coupled model develops an asymmetric state under conditions symmetric about the equator. The zonal variation in equatorial upwelling leads to pronounced differences between the western and other parts of the ocean. In the western warm water pool region, where the cooling effect of the equatorial upwelling is suppressed, both atmospheric and oceanic surface conditions are symmetric about the equator. On the other hand. in the central region where the upwelling cools the equatorial ocean, a single ITCZ forms off the equator in the Northern or Southern Hemisphere, depending on the initial condition. A strong contrast exists in the sea surface temperature SST between the hemispheres; SST is much higher at the latitude of the ITCZ than that on the other side of the equator. This high SST is crucial for the development of deep convection in the ITCZ. An air-sea interaction mechanism. where the wind speed-dependent surface evaporation plays a crucial role, maintains the asymmetric state. confirming the results from a previous two-dimensional model study.