Unstable transition of the tropical climate to an equatorially asymmetric state in a coupled ocean-atmosphere model

Over a large zonal extent of the central and eastern Pacific, the intertropical convergence zone (ITCZ) is located to the north of the equator. Collocated with this ITCZ is a zonal band of warm sea surface, where the highest sea surface temperatures (SST) along a meridian are found. A one-dimensional coupled ocean-atmosphere model that neglects zonal variations is used to investigate this problem of latitudinal asymmetry in the tropical climate. The equatorially symmetric model solution is found to be unstable to infinitesimal disturbances and equatorial asymmetries develop spontaneously. A linear instability that is stationary in space and antisymmetric about the equator is responsible for the unstable transition of the model from the symmetric state. The destabilizing mechanism involves a positive feedback between the scalar wind speed and SST through surface evaporation, which is illustrated with a simple low-order model that contains only two SST grid points, one in each hemisphere. The existence of the equatorially antisymmetric instability indicates that in a zonally uniform setting, a latitudinally asymmetric climate with a single ITCZ off the equator could emerge on a symmetric planet.