Increasing ENSO–rainfall variability due to changes in future tropical temperature–rainfall relationship

Abstract

Intensification of El Niño-Southern Oscillation (ENSO)-rainfall variability in response to global warming is a robust feature across Coupled Model Intercomparison Project (CMIP) iterations, regardless of a lack of robust projected changes in ENSO-sea-surface temperature (SST) variability. Previous studies attributed this intensification to an increase in mean SST and moisture convergence over the central-to-eastern Pacific, without explicitly considering underlying nonlinear SST–rainfall relationship changes. Here, by analyzing changes of the tropical SST–rainfall relationship of CMIP6 models, we present a mechanism linking the mean SST rise to amplifying ENSO–rainfall variability. We show that the slope of the SST–rainfall function over Niño3 region becomes steeper in a warmer climate, ~42.1% increase in 2050–2099 relative to 1950–1999, due to the increase in Clausius–Clapeyron-driven moisture sensitivity, ~16.2%, and dynamic contributions, ~25.9%. A theoretical reconstruction of ENSO–rainfall variability further supports this mechanism. Our results imply ENSO’s hydrological impacts increase nonlinearly in response to global warming.