CLIMATIC IMPLICATIONS OF THE SEASONAL-VARIATION OF UPPER TROPOSPHERE WATER-VAPOR

Satellite observations indicate that the humidity of the upper troposp here is higher in summer than in winter. We use general circulation mo del (GCM) simulations to explore the processes that maintain upper tro posphere water vapor and determine its seasonal cycle. In the subtropi cs, drying by Hadley cell subsidence and stratiform condensation is of fset primarily by moistening by eddies, with moist convection playing a minor role. Elsewhere, both mean meridional circulation and eddies m oisten the upper troposphere and are balanced primarily by stratiform condensation drying. The effect of the seasonal shift of the Hadley ce ll is limited to latitudes equatorward of 30 degrees. At higher latitu des where the largest observed summer moistening occurs, eddy moisture fluxes are primarily responsible despite the eddies being weaker in s ummer than winter. The same mechanism causes upper level humidity to i ncrease in GCM climate warming simulations. The observed seasonal vari ation may thus be a good proxy for decadal climate change. This sugges ts that upper troposphere water vapor feedback is positive at all lati tudes, consistent with GCM predictions.