A wind-evaporation mechanism for the abrupt seasonal transition of Hadley Circulations

The seasonal evolution of Hadley circulations is examined in a series of zonally symmetric (non-eddy permitting) models. When subject to a seasonally varying thermal forcing, nonlinearities are found to be weak in classical dry zonally symmetric models because the combination of a high static stability and low diabatic heating used in these models produces advection of absolute angular momentum on a timescale much longer than that of the forcing. In a moist model with a stratification closer to that of the Earth's tropics, momentum advection occurs more quickly and the circulation does exhibit a nonlinear response to a seasonally varying SST forcing. However, meridional gradients of free-tropospheric absolute angular momentum in this model are considerably weaker than observations unless surface enthalpy fluxes are allowed to depend on wind speed. We find that a wind-evaporation feedback provides a nonlinearity separate from that of angular momentum conservation, and that purely baroclinic Hadley circulations are linearly unstable to this feedback. Although the barotropic component of the Hadley circulation is found to strongly damp this instability in a linear model by reducing surface winds, meridional homogenization of free-tropospheric absolute angular momentum produces a barotropic mode that amplifies, rather than damps, the surface wind near the SST maximum. The positive correlation between the wind-driven surface enthalpy fluxes and the temperature field results in a circulation that intensifies nonlinearly with the SST forcing while only mildly deforming the angular momentum field. The relevance of these results to both monsoons and the zonal mean circulation is discussed.