The Vorticity Budget of the Hemispheres

Abstract

The cyclonic vorticity budget of the hemispheres is investigated using the output of the CCC GCM and objective analyses for the FGGE period. Analysis of GCM output gives direct estimates of the “topographic” and “frictional” vorticity sources and sinks, which are generally otherwise unavailable. In a broad sense, vorticity flows from source regions associated with high mean sea level pressure to sink regions associated with low mean sea level pressure. Direct estimates of the topographic source/sink term show a strong vorticity source associated with drainage flow from the elevated cold Antarctic plateau in Southern Hemisphere winter and, the reverse situation, a strong vorticity source associated with upward motion over the elevated warm Tibetan plateau in Northern Hemisphere summer. Direct estimates of the frictional source/sink term show strongest values over oceans in winter and over portions of the hot continental land masses of the Northern Hemisphere in summer. In both cases the instability of the lower layers of the atmosphere means that there is a good frictional connection between the surface and the free atmosphere. The seasonality of this behavior is especially notable over the southern oceans. From the zonally averaged point-of-view there is an interesting duality in the vorticity budget of the Southern Hemisphere. An almost complete cancellation occurs between the vorticity flux associated with the upper branches of the meridional circulation and that associated with the transient eddies. The vorticity transport is thus accomplished by the lower branches of the meridional circulation. When the budget terms are vertically integrated, however, the vorticity flux associated with the upper and lower branches of the meridional circulation tends to cancel so that the result is completely dominated by the transient eddy term.