Towards a PV-θ view of the general circulation

In recent years, there has been a resurgence of interest in using isentropic coordinates and Rossby-Ertel potential vorticity (PV) for diagnosing the behaviour of middle latitude synoptic systems. Such a PV-θ analysis may also prove important in providing insight into the global circulation of the atmosphere. Apart from the isentropic diagnostic of D. Johnson and collaborators, some quasi-geostrophic studies and recent studies of stratospheric behaviour, there has been little work in this area and our present understanding is very limited. The object of the present paper is to stimulate such studies by presenting some initial results from continuing research. A three-fold division of the atmosphere is discussed. The “Overworld” is the region encompassed by isentropic surfaces that are everywhere above the tropopause. In the “Middleworld”, the region with isentropes crossing the tropopause but not striking the Earth's surface, the isentropic zonal and time mean of PV exhibits interesting regions of enhanced and diminished gradients. The isentropic transient eddy advection of PV exhibits a dipolar distribution about the tropopause, suggestive of PV mixing. The marked PV signature of the Asian summer monsoon on one particular Middleworld isentrope is shown and the mean isentropic advection of PV shows interesting features. For the “Underworld”, in which isentropic surfaces intercept the surface of the Earth, a PV-θ analysis yields a novel constraint linking lowlevel drag and diabatic heating. This constraint links “westerlies” and “cooling”, and “easterlies” and “heating” in some average sense. The result is discussed in terms of the Southern Hemisphere strong surface westerlies and the circulation associated with the Asian summer and winter monsoons.