The influence of convective outflow on water vapor mixing ratios in the tropical upper troposphere: An analysis based on UARS MLS measurements

The source of increased water vapor mixing ratios over the central and east ern tropical Pacific region during the 1992 El Nino event is examined using measurements of upper tropospheric water vapor provided by the Microwave L imb Sounder (MLS) on board the Upper Atmosphere Research Satellite. Horizon tal winds on isentropic surfaces are combined with ISCCP cloud information to provide back-trajectory calculations free of high clouds. These calculat ions show that the water vapor mixing ratio of an air parcel in the cloud-f ree regions of the eastern Pacific decreases to approximately one-half of i ts original value within the first 30 hours after encountering deep convect ion. This analysis also finds a larger number of air parcels encountering d eep convection within 30 hours of observation, and therefore having higher mixing ratios, in March-April 1992 compared to March-April 1994. Hence, inc reased deep convection over the equatorial central and eastern Pacific in 1 992 contributed to the moistening of the downstream tropical upper troposph ere.