ISENTROPIC CROSS-TROPOPAUSE MASS-EXCHANGE IN THE EXTRATROPICS

The extratropical stratosphere-troposphere SIT exchange is investigated on isentropic surfaces that intersect the tropopause, using a semi-Lagrangian transport model and analyzed winds from the European Center for Medium-Range Weather Forecasts (ECMWF). It is found that the characteristics of extratropical SIT exchange exhibit two distinctive regimes. On the 330 K isentropic surface and below, S/T exchange occurs vigorously in all seasons, mainly caused by the irreversible mixing and transport by breaking synoptic-scale baroclinic disturbances. However, on and above the 340 K isentropic surface, S/T exchange exhibits a strong annual cycle where very little S/T exchange takes place in the winter hemisphere, but significant S/T exchange occurs in the summer hemisphere, particularly in the northern summer. It is argued that the weak SIT exchange in the winter hemisphere is mainly due to the barrier effect of the strong potential vorticity (PV) gradient at the tropopause and the active S/T exchange in the summer hemisphere is mainly associated with the summer Asian and Mexican monsoons in the northern summer and the Australia and South American monsoons in the southern summer.