SEASONAL BEHAVIOR OF TROPICAL TO MIDLATITUDE UPPER-TROPOSPHERIC WATER-VAPOR FROM UARS MLS

Measurements of upper tropospheric water vapor made during 1991-1997 w ith the Microwave Limb Sounder instrument on the Upper Atmospheric Res earch Satellite are described. Zonal mean results versus day of year a re presented for tropical to midlatitudes on pressure surfaces 316, 21 5, and 147 hPa. The latitude of greatest upper tropospheric humidity ( UTH) varies with season, following the Intertropical Convergence Zone. Annual maximum UTH occurs in northern summer at north tropical latitu des, coincident with the Indian monsoon ana with a June-August maximum of water vapor transport to the lower stratosphere [Rosenlof et al., 1997]. Comparison with lower stratospheric studies supports the Rosenl of et al. [1997] conclusion that water vapor transport is a maximum fr om the summer northern hemisphere tropical troposphere. Seasonally adj usted UTH is higher in the northern hemisphere than at equivalent sout hern hemisphere latitudes. The midlatitude secondary maximum in relati ve humidity seen in other (lower altitude) data sets is' seen on the 3 16 hPa surface throughout the year, only in northern hemisphere spring -summer at 215 hPa, and does not occur at 147 hPa. These observations characterize seasonal and interhemispheric differences in strengths of midlatitude convection and of subtropical subsidence. Frequency distr ibution analysis of tropical measurements shows the peak (mode) of the frequency distribution to be much drier than mean and median values a t 316 and 215 hPa and marginally drier than mean and median values at 147 hPa. The frequency distribution mode is drier in the tropical wet than in the dry season at 316 hPa, consistent with other data sets at 300-500 hPa [Spencer and Braswell, 1997; Chiou et al., 1997] but is we tter in the tropical wet than in the dry season at 215 and 147 hPa. Th e wettest values of the frequency distribution mode occur in April-May , corresponding to neither the tropical wet nor the dry season.