Wind–Evaporation Feedback and the Axisymmetric Transition to Angular Momentum–Conserving Hadley Flow

The effect of wind-induced surface heat exchange (WISHE) on axisymmetric, solstitial Hadley circulations is examined for forcings strong enough to produce meridional flow that nearly conserves absolute angular momentum in the free troposphere. Such forcings are known to produce an off-equatorial ascent zone in the summer hemisphere where the convergence of zonal momentum is balanced by drag on surface westerlies. Here, a convective quasi-equilibrium model with two vertical modes is used to show that enhanced surface entropy fluxes induced by these westerlies can intensify and shift both this ascent zone and the subcloud-layer entropy peak toward the equator. The equatorward shift of the subcloud entropy peak is associated with a reduction in the forcing amplitude needed to produce angular momentum–conserving (AMC) meridional flow. A previous theory of frontogenesis in tropical cyclones is adapted to axisymmetric Hadley circulations to show how WISHE shifts the peak subcloud entropy toward the equator. These effects also occur for forcings that vary in a seasonal cycle, with the precise effect of WISHE depending on the peak amplitude of the forcing. For weak seasonally varying forcings, WISHE can abruptly increase the intensity of a local, viscous circulation, whereas for forcings of intermediate strength WISHE produces a transition to AMC flow when such a transition would not otherwise occur. For the strongest forcings, WISHE shifts the transition to AMC flow to a time earlier in the seasonal cycle. The possible relevance of these results to monsoon dynamics is discussed, as are possible effects of processes not represented in these axisymmetric models.