Wind–Evaporation Feedback and Abrupt Seasonal Transitions of Weak, Axisymmetric Hadley Circulations

For an imposed thermal forcing localized off the equator, it is known that conservation of absolute angular momentum in axisymmetric flow produces a nonlinear response once the forcing exceeds a critical amplitude. It is shown here that, for a moist atmosphere in convective quasi-equilibrium, the combination of wind-dependent ocean surface enthalpy fluxes and zonal momentum advection can provide a separate feedback that causes the meridional flow to evolve nonlinearly as a function of a sea surface temperature (SST) forcing, even if an angular momentum–conserving response is not achieved. This wind–evaporation feedback is examined in both an axisymmetric primitive equation model and a simple model that retains only a barotropic and single baroclinic mode. Only SST forcings that do not produce an angular momentum–conserving response are examined here. The wind–evaporation feedback is found to be inhibited in models with linear dynamics because the barotropic component of the Hadley circulation, which is coupled to the baroclinic circulation via surface drag, keeps surface winds small compared to upper-level winds. In models with nonlinear dynamics, the convergence of zonal momentum into the ascending branch of the cross-equatorial Hadley cell can create barotropic westerlies that constructively add to the baroclinic wind at the surface, thereby eliminating the inhibition of the wind–evaporation feedback. The possible relevance of these results to the onset of monsoons is discussed.