Wind-evaporation feedback and the transition to axisymmetric, angular momentum conserving Hadley flow

The effect of wind-induced surface heat exchange (WISHE) on axisymmetric Hadley circulations is examined for off-equatorial forcings strong enough to produce angular momentum conserving meridional flow. Such forcings are known to produce a narrow, off-equatorial ascent zone in the summer hemisphere where the convergence of zonal momentum is balanced by drag on surface westerlies. A quasi-equilibrium model with two vertical modes is used here to show that surface entropy fluxes induced by these westerlies can narrow and intensify the ascent zone, and shift both it and the peak subcloud layer entropy toward the equator. This equatorward shift of the entropy peak is associated with a reduction in the forcing amplitude needed to produce angular momentum conserving meridional flow. A previous theory of frontogenesis in tropical cyclones is adapted to axisymmetric Hadley circulations to show that WISHE shifts the peak subcloud layer entropy toward the equator by exerting a frontogenetic tendency on the zonal wind field.

These effects are seen for forcings that vary in a seasonal cycle, with the precise effect of WISHE depending on the amplitude of the forcing. For weak seasonally-varying forcings, WISHE can increase the intensity of a local, viscous circulation, while for moderate forcings WISHE produces a transition to angular momentum conserving flow when such a transition would not otherwise occur. For the strongest forcings, WISHE shifts the transition to angular momentum conserving flow to a time earlier in the seasonal cycle. The possible relevance of these results to the seasonal cycle of monsoons is discussed.