Local Processes

Abstract

Stratosphere-troposphere exchange (STE) refers to the processes whereby mass and chemical species are transported across the tropopause and exchanged between the troposphere and the stratosphere. This exchange is important to the chemistry of both regions as it regulates the transport of species with tropospheric sources (e.g., pollutants) into the stratosphere and of species with stratospheric sources into the troposphere. In this article we identify and describe the local processes occurring in the vicinity of the tropopause that govern this exchange. We distinguish between these processes and the large-scale regulation of the exchange by the global-scale circulation. The local processes influence precisely where and when STE of mass and chemical species occurs. As the tropopause can be represented by a surface of constant potential temperature in the tropics and by a surface of constant potential vorticity in the extratropics, STE requires a change in these variables and this, in turn, requires the occurrence of diabatic or mixing processes. Thus, STE in the tropics is associated with diabatic heating, while STE in the extratropics can occur diabatically and along isentropic surfaces crossing the extratropical tropopause. Particular local processes that are known to be related to significant diabatic heating and mixing and to cause significant STE include convection in the tropics and to a lesser degree in the extratropics and especially monsoon circulations in the tropics. In the extratropics mixing processes can be related to large-scale Rossby waves and associated flow features such as PV streamers, cut-off lows and tropopause folds as well as to small-scale turbulent dynamics. Because of the different processes involved, the description of STE by local processes is split between the tropics and extratropics, and finally it is discussed what role the different processes play for dehydration and moistening of the stratosphere.