A northern hemispheric climatology of cross‐tropopause exchange for the ERA15 time period (1979–1993)

Abstract

A 15‐year climatology of cross‐tropopause exchange in the extratropical Northern Hemisphere is presented based upon a previously developed Lagrangian methodology and reanalysis data from the European Centre for Medium‐Range Weather Forecasts (ECMWF) for the years 1979–1993. For this time period, no apparent trends exist for upward (troposphere‐to‐stratosphere, TST) and downward (stratosphere‐to‐troposphere, STT) cross‐tropopause transport. The seasonal cycles of hemispherically integrated TST and STT and the zonally integrated meridional exchange distributions show relatively little interannual variability. The 15‐year averaged geographical distribution of STT reveals significant deviations from zonal symmetry and pronounced seasonal variability. STT occurs predominantly over the Atlantic and Pacific storm track regions during winter, spring, and autumn and also over the Mediterranean in winter and spring. In contrast, during summer, STT maxima are located over the continents, particularly over southeastern Europe and central Asia. TST is less variable and is strongest near the southern tip of Greenland and the Aleutian Islands during all seasons. Aspects of the interannual variability of the geographical exchange distributions are investigated. Over the Atlantic, STT varies significantly in line with the changing storm tracks during different phases of the North Atlantic Oscillation (NAO), whereas TST does less so. Special consideration is given to vertically deep exchange, where the exchange air parcels transfer rapidly from the lowermost stratosphere to the boundary layer (or vice versa). These processes occur mainly during winter and exhibit pronounced maxima over the North American west (deep STT) and east (deep TST) coasts.