Zonally symmetric monsoon dynamics in a general circulation model

Abstract

The MIT general circulation model is used with simplified setup to study steady zonally averaged monsoon circulations. Two dimensional model runs are made with a zonally symmetric continent north of 15N and a slab ocean of uniform sea surface temperature to study the applicability of axisymmetric theory. Forcing to drive the monsoon is applied by heating the subtropical land surface. The dynamical constraints of axisymmetry prevent low-level cross-equatorial flow and inhibit the northward transport of moisture onto the continent when there is no temperature gradient across the equator. The ocean cannot supply adequate moisture to feed the monsoon, and the ground hydrology strictly controls the behavior of the monsoon. A second set of two dimensional runs with similar continent, but with an SST gradient across the equator, result in a viable steady monsoon with low-level cross-equatorial flow providing moisture to the monsoon. The surface forcing required to induce a monsoon is reasonable given the constraints of the axisymmetric model setup. A series of three dimensional model runs with a zonally symmetric continent are made to study the role of zonally asymmetric flow on the zonal mean monsoon. It is found that greater land surface forcing is required to induce a zonally averaged monsoon circulation in the three dimensional runs than in similar axisymmetric runs. The behavior of the monsoon disturbances in the three dimensional runs is similar to the observed Asian monsoon in that there is low-level cross-equatorial flow which is southwesterly along the coastline, and in that a large-scale angular momentum conserving meridional circulation develops with ascent over the continent and subsidence in the opposite hemisphere. Moisture transport is found to play a very strong role in the monsoon dynamics in all of the model runs.