Seasonal transitions of zonally symmetric circulations

A series of zonally symmetric (two-dimensional) models of the atmosphere is used to examine possible mechanisms involved in the seasonal evolution of Hadley-type circulations. When a model with dry dynamics is forced by relaxation of atmospheric temperatures to prescribed distributions which vary with a smooth seasonal cycle, the resulting circulations vary on the same seasonal timescales without sudden transitions between summer and winter states. Transitions between solsticial circulations do occur on timescales considerably shorter than the seasonal in a zonally symmetric model configured with representations of moist convection, long- and shortwave radiation, and a land surface poleward of the equator in one hemisphere. In this model, such comparatively rapid transitions occur primarily because of a wind-evaporation feedback, in which the poleward gradient of ocean surface enthalpy fluxes is enhanced as surface zonal winds increase accompanying the onset of a thermally direct circulation in the coastal region. The effect of this wind-evaporation feedback is found to dominate those of SST gradients for SST gradients in the range of those found in observations of the South Asian region. Moisture-radiation feedbacks are found to delay the onset of the summer circulation and enhance its peak intensity. The possible relevance of these solutions to the onset and withdrawal of monsoon circulations is discussed.