Seasonal behavior of tropical to midlatitude upper tropospheric water vapor from UARS MLS

Measurements of upper tropospheric water vapor made during 1991–1997 with the Microwave Limb Sounder instrument on the Upper Atmospheric Research Satellite are described. Zonal mean results versus day of year are presented for tropical to midlatitudes on pressure surfaces 316, 215, 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 latitudes, coincident with the Indian monsoon and with a June‐August maximum of water vapor transport to the lower stratosphere Rosenlof et al , 1997. Comparison with lower stratospheric studies supports the Rosenlof et al. 1997 conclusion that water vapor transport is a maximum from the summer northern hemisphere tropical troposphere. Seasonally adjusted UTH is higher in the northern hemisphere than at equivalent southern hemisphere latitudes. The midlatitude secondary maximum in relative humidity seen in other (lower altitude) data sets is seen on the 316 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 distribution analysis of tropical measurements shows the peak (mode) of the frequency distribution to be much drier than mean and median values at 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 wetter in the tropical wet than in the dry season at 215 and 147 hPa. The wettest values of the frequency distribution mode occur in April–May, corresponding to neither the tropical wet nor the dry season.