ISENTROPIC MASS-EXCHANGE BETWEEN THE TROPICS AND EXTRATROPICS IN THE STRATOSPHERE

The isentropic mass exchange between the Tropics and extratropics in t he stratosphere is investigated with a semi-Lagrangian transport model for the periods from 1 June to 31 October 1992 and from 1 December 19 92 to 30 April 1993 using winds from the U.K. Meteorological Office da ta assimilation system. Calculations with an idealized, initially zona lly symmetric tracer show that in the middle and upper stratosphere th e bulk of tropical air is transported into the midlatitudes of the win ter hemisphere although there exist quasi-permeable barriers in the su btropics. The transport takes place in the form of planetary-scale ''t ongues'' of material that are drawn poleward in association with the e pisodic amplification of planetary-scale waves in high latitudes of th e winter hemisphere. Once air of tropical origin is transported to the midlatitudes it is irreversibly mixed with the midlatitude air in the ''surf zone.'' Air of tropical origin can, however, hardly penetrate into the interior of the winter polar vortex until the breakdown of th e vortex. Transport of tropical air into the midlatitudes of the summe r hemisphere is strongly inhibited. In the lower stratosphere, tropica l air is transported into the northern and southern midlatitudes. Duri ng the period from 1 June to 31 October 1992, the amount of tropical a ir transported into the Northern Hemisphere is, however, much smaller than that transported into the Southern Hemisphere, and there exist st rong gradients in the tracer field in the equatorial region, indicatin g that there is a quasi-permeable barrier to cross-equator mass exchan ge. During the period from 1 December 1999 to 30 April 1993, on the ot her hand, roughly the same amounts of tropical air are transported int o the northern midlatitudes and into the southern midlatitudes, and th ere exist no significant transport barriers in the equatorial area.