An isentropic analysis of the wintertime vorticity exchange in relation to extratropical heat sources and sinks

Abstract

The influence of heat sources and sinks on the time-averaged atmospheric circulation is studied through an isentropic analysis of the balance of dynamic vorticity. The property of isentropic dynamic vorticity, the vertical component of the curl of the momentum (k▽₀× pJ₀U), is based on Bjerknes' definition of circulation, his theorems and their application to problems of physical hydrodynamics including the general circulation of the earth's atmosphere. The diagnostics emphasize the three-dimensional exchange of dynamic vorticity occurring within migratory cyclone-scale waves of the Northern Hemisphere extratropics. Analysis reveals that the maintenance of the dynamic vorticity balance in the wintertime Northern Hemisphere is directly coupled to the three-dimensional distribution of heating and cooling. An isentropic mass circulation previously identified transports energy from extratropical oceanic heat sources to polar continental heat sinks. A corresponding systematic vorticity exchange occurs through air-mass exchange within mid-latitudes between continents and oceans. In the lowest isentropic layers, the balance of dynamic vorticity is maintained by absolute vorticity transport from anticyclonic circulations over Asia, North America and Greenland to cyclonic circulations over the North Pacific and Atlantic Oceans in concert with solenoidal sources and sinks within these regions. The scale of dynamic vorticity transport and solenoidal processes that occur within the isentropic mass circulation correspond with the scale of extratropical heat sources and sinks. As such, the commonality among the scales of the isentropic mass circulation, energy exchange and vorticity exchange reveals a direct response to differential heating in the maintenance of the time-averaged extratropical circulation of the Northern Hemisphere.