Multiple equilibria in an axisymmetric model of the tropical circulation with prognostic BL moisture

An axisymmetric tropical circulation model with prognostic boundary layer (BL) moisture is used to study the annual variation of the ITCZ. Multiple equilibria, whereby the ITCZ can have more than one location with the same solar heating depending upon the initial condition, exist for certain versions of the model, depending upon between-cloud subsidence in the convecting region and ocean mixed layer depth, for example. The off-equatorial equilibrium is found to be associated with a large BL moisture gradient between the convecting and subsiding regions, which is maintained by the large-scale circulation, which converges moisture over the ITCZ and dries other areas via large-scale subsidence. Processes which maintain a large moisture gradient favor the existence of multiple equilibria. The model identifies processes that might contribute to the observed persistence of the eastern Pacific ITCZ within the Northern Hemisphere.