Thermally-forced mean mass circulations in the Southern Hemisphere

Abstract

The three-dimensional distribution of the thermally-forced time-averaged horizontal mass transport in isentropic coordinates is determined for the Southern Hemisphere from modeled diabatic heating fields. The model, based on the time-averaged isentropic continuity equation, specifies the horizontal mass transport in terms of the gradient of a mass transport potential on a vertical array of isentropic surfaces. The mass transport potential is derived by numerical solution of a two-dimensional Poisson equation on a hemispheric grid. The Southern Hemisphere heating fields are based on estimates of the normal summer and winter columnar radiative, latent and sensible heating and simplified models of their vertical apportionment.

The results highlight the role of radiative cooling over the cold elevated Antarctic ice cap, sensible heat flux from the warm circumpolar ocean and, in particular, the zonal and meridional contrasts of latent heat release in forcing large-scale mean mass circulations in the Southern Hemisphere. The inferred mass circulations include strong meridional (Hadley) and zonal (Walker) components in the tropical belt extending into the middle and high latitudes of the Southern Hemisphere. Of particular interest is a large toroidal mass circulation cell transporting heat away from the heavy rainfall belt lying southeast from Papua-New Guinea to regions of net radiative cooling to the east, west, north and south.