Contour lengthening rates near the tropopause

We present the results of contour advection calculations using winds derived from the UK Universities Global Atmospheric Modelling Programme (UGAMP) model. Contours are initialised using PV values on a set of potential temperature surfaces which span the stratosphere and troposphere in order to examine the mixing properties of stratospheric and tropospheric air on a time-scale of a few days. We show that the contour stretching rates do not convey any information about the location of the tropopause, although the lower layers of the stratosphere are vigorously mixed with the troposphere over this time. The results suggest that the stratosphere becomes more isolated from the troposphere with increasing potential temperature, with mixing confined to a narrow region in the subtropics at the higher levels (greater than 330K).