An atmospheric mechanism for ENSO amplitude changes under an abrupt quadrupling of CO2 concentration in CMIP5 models

We investigate the impact of a quadrupled CO2 concentration on the simulated El Nino-Southern Oscillation (ENSO) amplitudes in 19 Coupled Model Intercomparison Project phase 5 (CMIP5) climate models. The amplitude of ENSO-related sea surface temperature (SST) variability decreases in 11 of these models, and increases in the rest, in response to the enhanced radiative forcing. These opposing amplitude changes are predominantly explained by opposite changes in the time-lagged SST response to a given central Pacific zonal wind stress (ZWS) forcing, with the net heat flux forcing and the SST-ZWS feedback playing smaller roles. We find a robust relationship between the changes in the ZWS forcing efficiency and those in the ZWS-deep convection coupling in the central-western Pacific, indicating an important role for this coupling in ENSO amplitude changes. Indeed, the projected change in this coupling is indicative of the projected change in ENSO-related SST variability.