The sensitivity of the tropical hydrological cycle to ENSO

Satellite observations of temperature. water vapor, precipitation and longw ave radiation are used to characterize the variation of the tropical hydrol ogic and energy budgets associated with the El Nino-Southern Oscillation (E NSO). As the tropical oceans warm during an El Nino event, the precipitatio n intensity, water vapor mass. and temperature of the tropical atmosphere a re observed to increase, reflecting a more vigorous hydrologic cycle. The e nhanced latent heat release and resultant atmospheric warming lead to an in crease in the emission of longwave radiation. Atmospheric global climate mo dels, forced with observed sea surface temperatures (SSTs), accurately repr oduce the observed tropospheric temperature, water vapor, and outgoing long wave radiation changes. However. the predicted variations in tropical-mean precipitation rate and surface longwave radiation are substantially smaller than observed. The comparison suggests that either (i) the sensitivity of the tropical hydrological cycle to ENSO-driven changes in SST is substantia lly underpredicted in existing climate models or (ii) that current satellit e observations are inadequate to accurately monitor ENSO-related changes in the tropical-mean precipitation. Either conclusion has important implicati ons for current efforts to monitor and predict changes in the intensity of the hydrological cycle.