Diabatic Sources of Potential Vorticity in the General Circulation

Abstract

A form of the potential vorticity (PV) budget is proposed that facilitates analysis on the role of global heat sources and sinks in the general circulation. A local diabatic source of PV occurs due to vertical variations of heating. Additionally, since the irrotational mass circulation in isentropic coordinates is uniquely linked to diabatic heating, the associated horizontal advection of PV may be viewed as a diabatic source. The sum of these processes constitutes an effective ?baroclinic wave source? due to diabatic processes, and is analogous to the effective barotropic Rossby wave source due to divergence as discussed by Sardeshmukh and Hoskins. Diagnostic results are presented for the upper-tropospheric PV balance at 350 K during northern winter. When the PV budget is diagnosed in its conventional form, the midlatitude flow appears insulated from the influence of tropical heating in the sense that diabatic sources and sinks are mainly due to vertical variations of extratropical heat sources and sinks. In the NH, these sources/sinks are balanced by the mean horizontal advection of PV by the total flow, which acts to transport PV from reservoirs of large values over eastern Asia and Canada to small values over the central North Pacific and western North Atlantic oceans. Analysis of the effective baroclinic wave source reveals that the midlatitude PV balance depends strongly on the distribution of tropical heating, a result that agrees more favorably with empirical and numerical studies on tropical?extratropical interactions. Sinks due to horizontal PV advection by the irrotational flow occur throughout the eastern hemisphere along 30° latitude, and exceed the local sources associated with in situ diabatic cooling. The implied poleward transport of low PV air from the tropics occurs in the outflow branch of the regional Hadley circulation, revealing the large influence of the Australasian monsoon.