Wind-evaporation feedback, angular momentum conservation, and the abrupt onset of monsoons

Abstract

This thesis examines the mechanisms responsible for the abrupt onset of monsoon circulations, focusing on the role played by wind-evaporation feedback and its interaction with angular momentum conserving flow. The first half of the thesis examines the effect of wind induced surface heat exchange (WISHE) on nonlinear, axisymmetric Hadley circulations in a convective quasi-equilibrium framework. For thermal forcings localized off the equator, WISHE is found to reduce the critical forcing amplitude needed to produce angular momentum conserving flow. For forcings that are subcritical even with the effects of WISHE, the combination of WISHE and momentum advection is shown to nonlinearly enhance the circulation strength for all but the weakest forcings. These results hold for the time-dependent response to seasonally varying forcings: for forcings of intermediate strength, WISHE produces an abrupt onset of solsticial flow when only a linear response would otherwise occur, while for strong forcings WISHE shifts the abrupt onset to an earlier time in the seasonal cycle. The second half of the thesis examines the consistency of these idealized results with the onset of the South Asian monsoon in both observations and a detailed three-dimensional model. Observational composites of monsoon onset are consistent with a wind-evaporation feedback in that the increase in baroclinic flow during onset is accompanied by a large increase in surface enthalpy flux over the off-equatorial ocean. This increase in surface enthalpy flux is collocated with the peak increase in deep tropospheric ascent. Results from the three-dimensional numerical model were less conclusive in that this model did not successfully simulate an abrupt monsoon onset even with WISHE, although WISHE did strongly control the intensity and spatial structure of the model's mean summer circulation.

(cont) In particular, a version of the model integrated without WISHE failed to produce a strong, angular momentum conserving monsoon circulation. Combined with the axisymmetric model results from the first half of the thesis, this suggests that the thermal forcing of the South Asian land mass may not be sufficiently strong in the absence of WISHE to produce angular momentum conserving monsoon flow.