The source of increased water vapor mixing ratios over the central and eastern tropical Pacific region during the 1992 El Ni¿o event is examined using measurements of upper tropospheric water vapor provided by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite. Horizontal winds on isentropic surfaces are combined with ISCCP cloud information to provide back-trajectory calculations free of high clouds. These calculations show that the water vapor mixing ratio of an air parcel in the cloud-free regions of the eastern Pacific decreases to approximately one-half of its original value within the first 30 hours after encountering deep convection. This analysis also finds a larger number of air parcels encountering deep convection within 30 hours of observation, and therefore having higher mixing ratios, in March-April 1992 compared to March-April 1994. Hence, increased deep convection over the equatorial central and eastern Pacific in 1992 contributed to the moistening of the downstream tropical upper troposphere. ¿ 2000 American Geophysical Union