Sensitivity of the thermocline feedback to global warming in coupled GCMs: Implications for changes in ENSO amplitude

The sensitivity of the thermocline feedback to a warmer climate is investigated using a coordinated set of climate model experiments. The coupled general circulation models (GCMs) simulate robust changes in the time-mean climate of the tropical Pacific to when GHG forcing is increased. Notwithstanding, their simulated ENSO variability shows diverging responses, which have been widely reported in the literature. The answer for this lack of agreement has remains elusive. We investigate the origin of this discrepancy through a budget analysis of the upper tropical Pacific on ENSO time scales in each model, and compare the changes in ENSO feedbacks between the preindustrial control climate and a quasi-equilibrated climate with doubled atmospheric CO2 concentrations. The changes in the thermocline feedback are traced back to the changes in the time-mean thermocline. The time-mean equatorial thermocline shoals in response to the weakening of the Walker circulation and sharpens in response to surface warming. Evidence is provided showing that this two processes compete to strengthen and weaken the thermocline feedback respectively providing an explanation for the divergent ENSO changes in response to global warming.