Dynamical tropopause based on isentropic potential vorticity gradients

Since its inception, the dynamical tropopause based on potential vorticity (PV) is identified by the PV gradient on isentropes. Conceptually, significant isentropic gradients shown on the middle world PV maps reflect the underlying transport barrier associated with the tropopause, formed by jet streams that separate tropospheric air masses at low latitudes and stratospheric air masses at high latitudes. Largely owing to the lack of a general method, the dynamical tropopause has often been represented by a PV value chosen ad hoc without any temporal or spatial differentiation. In this work, we present a method for determining the PV isoline of the dynamical tropopause based on the isentropic PV gradients. Using 1 year of data from the European Centre for Medium–Range Weather Forecasts, the spatial and temporal variability of this PV gradient-based dynamical tropopause is examined. The results show that in general there is a broad distribution of PV values at the dynamical tropopause, ranging from 1.5 to 5 potential vorticity units. Therefore, a fixed PV surface for all isentropes and seasons does not accurately represent the location of the tropopause barrier. The PV at the dynamical tropopause increases with increasing potential temperature. This increase is more pronounced in the Southern Hemisphere than in the Northern Hemisphere. The seasonal cycle shows higher PV values at the dynamical tropopause during summer than during winter. This seasonal cycle is larger on higher isentropes. The dispersion of the PV at the dynamical tropopause about its mean is twofold larger during summer and autumn than during winter and spring in both hemispheres.