Large uncertainty exists in hydrologic sensitivity (HS), the global-mean precipitation increase per degree warming, across global climate model (GCM) ensembles. Meanwhile, the global circulation and hence global precipitation are sensitive to variations of surface temperature under internal variability. The El Niño Southern Oscillation (ENSO) is the most dominant mode of global temperature variability and hence of precipitation variability. Here we show in the Coupled Model Intercomparison Project 6 that the strength of HS under ENSO is predictive of HS in the climate-change context (r=0.56). This correlation increases to 0.62 when only Central-Pacific ENSO events are considered, suggesting that they are a better proxy for HS under future warming than East-Pacific ENSO events. GCMs with greater HS are associated with greater weakening of the Walker Circulation and expansion of the Hadley Circulation under ENSO. Observations of HS under ENSO suggest that it is significantly underestimated by the GCMs, with the lower bound of observational uncertainty almost double even the highest-HS GCMs. The ENSO-related transformation of the tropical circulation holds clues into how the GCMs may be improved in order to more reliably simulate future hydrological-cycle intensification.