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 subt ropical and polar PV pools with definitive characteristic values. The domin ance of the tropopause on these surfaces suggests that the subseasonal tran sient variance should be a function solely of the time mean PV. Exploiting this relation, it is shown that knowledge of the annual mean PV distributio n allows quite skillful deduction of interannual anomalies in transient var iance. This analysis is extended to the dynamic tropopause, where it is sho wn that polar and subtropical jets clearly emerge in fields of the most fre quent tropopause potential temperature. Implications of these results on th e current understanding of how patterns of interannual variance arise are d iscussed.