On the potential vorticity balance on an isentropic surface during normal and anomalous winters

Data for 39 winters are used to compute the potential vorticity (PV) budget on the theta = 315 K isentropic surface over the Northern Hemisphere. The object is to compare the mechanisms that maintain the PV balance during nor mal winters with those that maintain the balance during winters with anomal ies of the North Atlantic Oscillation (NAO) and Pacific-North American (PNA ) types. On an isentropic surface that does not intersect the ground, which is usually the case for the 315 K surface, the mean seasonal flow must be such as to maintain a simple local balance between the diabatic and frictio nal sources/sinks of PV, the isentropic advection of PV by the mean seasona l flow, and the mean seasonal PV advection by the subseasonal transients. The climatology over the 39 winters shows that the main positive PV centers over the east coasts of Asia and Canada are maintained through a three-way balance among the upstream diabatic/frictional sources of PV, the PV advec tion by the mean seasonal flow, and that by the subseasonal transients. The transients with periods between 2 and 10 days and those with periods betwe en 10 and 90 days are found to contribute about equally to the PV balance. The PV balance of NAO and PNA winter anomalies reveals that the PV advectio n by the subseasonal transients more systematically opposes the advection b y the seasonal mean flow, so that the local PV source term is proportionate ly much less important than it is in the maintenance of climatological PV c enters. The calculations were also made on the theta = 350 K and 450 K isentropes. The results are presented only briefly to highlight the main similarities a nd differences with those obtained at 315 K.