Robust increase of the equatorial Pacific rainfall and its variability in a warmed climate

Regional pattern of the mean precipitation changes in the latter half of the 21st century (Delta(P) over bar) has been interpreted in terms of mean precipitation in current climate and a magnitude of increase in mean sea surface temperature (SST). Here we use state-of-the-art climate model ensembles from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and present that the amplitude of the precipitation variability, relative to slowly varying mean precipitation, coherently increases with Delta(P) over bar, anchored over the central equatorial Pacific where a large SST swing occurs during El Nino-Southern Oscillation (ENSO) cycles. This increase owes to skewed probability distribution of precipitation as well as an asymmetrical precipitation response to positive and negative in situ SST anomalies and is robust despite uncertainty in the future ENSO amplitude change. The CMIP5 model ensembles also give a robust estimate of the projected Delta(P) over bar over the central equatorial Pacific, showing a 7% increase per unit increase of global-mean surface temperature. Observational constraints applied to the above relationship suggest that the amounts of increases in both mean precipitation and variability should be even larger than the model averages.