Mean climate controls on the simulated response of ENSO to increasing greenhouse gases

Climate model experiments are analyzed to elucidate if and how the changes in mean climate in response to doubling of atmospheric CO₂ (2xCO₂) influence ENSO. The processes involved the development, transition, and decay of simulated ENSO events are quantified through a multimodel heat budget analysis. The simulated changes in ENSO amplitude in response to 2xCO2 are directly related to changes in the anomalous ocean heat flux convergence during the development, transition, and decay of ENSOevents. The weakening of the Walker circulation and the increased thermal stratification, both robust features of the mean climate response to 2xCO₂,play opposing roles in ENSO-mean climate interactions. Weaker upwelling inresponse to a weaker Walker circulation drives a reduction in thermocline-driven ocean heat flux convergence (i.e., thermocline feedback) and, thus, reduces the ENSO amplitude. Conversely, a stronger zonal subsurface temperature gradient, associated with the increased thermal stratification, drives an increase in zonal-current-induced ocean heat flux convergence (i.e., zonal advection feedback) and, thus, increases the ENSO amplitude. These opposing processes explain the lackof model agreement in whether ENSO is going to weaken or strengthen in response to increasing greenhouse gases, but alsowhy ENSO appears to be relatively insensitive to 2xCO₂ in most models.