This paper focuses on the longitudinal variations in tropopause properties evident in the historical record of tropical Pacific radiosonde data. Over 30 years of radiosonde temperature soundings from tropical Pacific stations (Ponape, Yap, Guam, Majuro, Koror, and Eniwetok) have been analyzed to yield tropopause height and tropopause potential temperature. Both tropopause height and tropopause potential temperature show a clear year-to-year variation in response to changes in tropical convection associated with El Ni¿o--Southern Oscillation (ENSO) events. In particular, the tropopause potential temperature appears to be very sensitive to the longitude of major convective activity. As the locus of convective activity moves eastward during an El Ni¿o, there is a concurrent change in the longitudinal gradient of tropopause potential temperature. During an El Ni¿o year the tropopause potential temperature is systematically warmer at Koror as compared to Majuro. This is the reverse of the normal tropopause potential temperature difference between these two stations. The difference in the tropopause potential temperature between these two stations forms an ENSO index with interannual variations well correlated with the conventional ENSO index formed by the Tahiti-Darwin surface pressure.