Stability of equatorially symmetric and asymmetric climates under annual solar forcing

A simple coupled ocean‐atmosphere model is developed to investigate the maintenance of the latitudinal asymmetries of the sea surface temperature (SST) field and the intertropical convergence zone (ITCZ). the model is of the minimum degree of freedom necessary to describe latitudinal asymmetries, computing only the difference between the hemispherical SST maxima. A wind‐evaporation‐SST feedback is found to destabilize the equatorial symmetry established by the annual‐mean solar radiation. By considering the nonlinear SST threshold for deep convection, equatorially asymmetric steady states are reached, with a single ITCZ off the equator. The annual component of solar radiation is anti‐symmetric about the equator, acting to force convection in the summer hemisphere independent of symmetry‐breaking ocean‐atmosphere interactions. A multiple time‐scales analysis is applied to separate the annual cycle and the slowly evolving annual‐mean climatology. It is found that the stability of an annual‐mean state varies with annual forcing amplitude. As forcing amplitude varies, the model displays three flow regimes: (1) an asymmetric regime for small seasonal forcing, featuring a permanent ITCZ in one hemisphere and a seasonal one in the other; (2) a mixed regime for intermediate forcing, with both symmetric and asymmetric solutions being stable; and (3) a symmetric regime for large annual forcing where the ITCZ migrates between the hemispheres following the seasonal march of the sun.