Abrupt Seasonal Migration of the ITCZ into the Summer Hemisphere

Abstract

Although the maximum of solar radiation at the top of the atmosphere moves gradually from one hemisphere to the other as part of the seasonal cycle, the intertropical convergence zone (ITCZ) moves abruptly into the summer hemisphere. An axisymmetric circulation model is developed to study this rapid transition. The model consists of an upper and lower layer of the Hadley circulation (HC), with the surface layer attached to a slab ocean and the lower layer connected to the upper layer by a constant lapse rate. The model is forced by solar heating, and the ITCZ is prescribed to coincide with the warmest sea surface temperature (SST). The collocation of tropical rainfall with warm SST allows the model ITCZ migration to be understood in terms of the relative influence of solar heating and atmospheric dynamics upon ocean temperature. Atmospheric dynamics allow the ITCZ to move off the equator by flattening the meridional temperature gradient that would exist in radiative?convective equilibrium. For the present-day tropical oceanic mixed layer depth and ITCZ width, the model exhibits an abrupt seasonal transition of the ITCZ across the equator. It is found that there are two determinative factors on the abrupt transition of the ITCZ: the nonlinear meridional advection of angular momentum by the circulation and ocean thermal inertia. As a result of nonlinear dynamics, angular momentum is well mixed, resulting in minimum atmospheric temperature at the equator and a similar equatorial minimum in SST. This inhibits convection over the equator while favoring a rapid seasonal transition of the ITCZ between the warmer surface water on either side of this latitude.