A study of the leakage of the Antarctic polar vortex in late austral winter and spring using isentropic and 3‐D trajectories

Abstract

The permeability of the Antarctic polar vortex is investigated in late austral winter and spring by comparing isentropic and three‐dimensional (3‐D) trajectories. Trajectory computations were performed with the help of the Royal Dutch Meteorological Institute (KNMI) trajectory model, using data from the European Centre for Medium‐Range Weather Forecasts (ECMWF) from August to November 1998. Large numbers of air parcels were initially released inside and outside the polar vortex on the 350, 450, and 550 K isentropic surfaces. They were integrated 4 months forward in time in an isentropic mode, as well as in a 3‐D mode that uses all three wind components from the ECMWF and takes into account diabatic heating and cooling effects. For the isentropic trajectory calculations, very little transport (0.37%/week) was found for August and September, while October and November gave somewhat higher transport rates (1.95%/week). The 3‐D trajectory calculations for October gave much more exchange between the vortex and midlatitudes than the isentropic ones owing to a significant number of parcels that descended inside the vortex. Descent rates were calculated for 350 K (October), 450 K (August–October) and 550 K (October). Overall, the results show that 3‐D trajectories will provide more accurate leakage rates than the isentropic ones. Also, despite the large‐scale mixing in the polar vortex or in midlatitudes, little ozone‐depleted air leaks from the ozone hole into the midlatitude stratosphere.