Upper-Tropospheric Potential Vorticity Fluctuations and the Dynamical Relevance of the Time Mean

Analysis of frequency distribution functions of potential vorticity (PV) on middleworld isentropic surfaces that intersect the tropopause reveals that dynamics on these surfaces are dominated by the tropopause separating subtropical and polar PV pools with definitive characteristic values. The dominance of the tropopause on these surfaces suggests that the subseasonal transient variance should be a function solely of the time mean PV. Exploiting this relation, it is shown that knowledge of the annual mean PV distribution allows quite skillful deduction of interannual anomalies in transient variance. This analysis is extended to the dynamic tropopause, where it is shown that polar and subtropical jets clearly emerge in fields of the most frequent tropopause potential temperature. Implications of these results on the current understanding of how patterns of interannual variance arise are discussed.