STRATOSPHERE-TROPOSPHERE EXCHANGE

In the past, studies of stratosphere-troposphere exchange of mass and chemical species have mainly emphasized the synoptic- and small-scale mechanisms of exchange. This review, however, includes also the global -scale aspects of exchange, such as the transport across an isentropic surface (potential temperature about 380 K) that in the tropics lies just above the tropopause, near the 100-hPa pressure level. Such a sur face divides the stratosphere into an ''overworld'' and an extratropic al ''lowermost stratosphere'' that for transport purposes need to be s harply distinguished. This approach places stratosphere-troposphere ex change in the framework of the general circulation and helps to clarif y the roles of the different mechanisms involved and the interplay bet ween large and small scales. The role of waves and eddies in the extra tropical overworld is emphasized. There, wave-induced forces drive a k ind of global-scale extratropical ''fluid-dynamical suction pump,'' wh ich withdraws air upward and poleward from the tropical lower stratosp here and pushes it poleward and downward into the extratropical tropos phere. The resulting global-scale circulation drives the stratosphere away from radiative equilibrium conditions. Wave-induced forces may be considered to exert a nonlocal control, mainly downward in the extrat ropics but reaching laterally into the tropics, over the transport of mass across lower stratospheric isentropic surfaces. This mass transpo rt is for many purposes a useful measure of global-scale stratosphere- troposphere exchange, especially on seasonal or longer timescales. Bec ause the strongest wave-induced forces occur in the northern hemispher e winter season, the exchange rate is also a maximum at that season. T he global exchange rate is not determined by details of near-tropopaus e phenomena such as penetrative cumulus convection or small-scale mixi ng associated with upper level fronts and cyclones. These smaller-scal e processes must be considered, however, in order to understand the fi ner details of exchange. Moist convection appears to play an important role in the tropics in accounting for the extreme dehydration of air entering the stratosphere. Stratospheric air finds its way back into t he troposphere through a vast variety of irreversible eddy exchange ph enomena, including tropopause folding and the formation of so-called t ropical upper tropospheric troughs and consequent irreversible exchang e. General circulation models are able to simulate the mean global-sca le mass exchange and its seasonal cycle but are not able to properly r esolve the tropical dehydration process. Two-dimensional (height-latit ude) models commonly used for assessment of human impact on the ozone layer include representation of stratosphere-troposphere exchange that is adequate to allow reasonable simulation of photochemical processes occurring in the overworld. However, for assessing changes in the low ermost stratosphere, the strong longitudinal asymmetries in stratosphe re-troposphere exchange render current two-dimensional models inadequa te. Either current transport parameterizations must be improved, or el se, more likely, such changes can be adequately assessed only by three -dimensional models.