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, with the surface layer attached to a slab ocean. The model is forced by solar heating, and the ITCZ is located over the warmest sea surface temperature (SST). Atmospheric dynamics allows 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 mixed-layer depth and ITCZ width, the model exhibits an abrupt seasonal transition of the ITCZ. 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 temperature at the equator. This inhibits convection, while favoring a rapid seasonal transition of the ITCZ between the warmer surface water on either side of this latitude.