Ch. 13 ENSO Response to Greenhouse Forcing

Abstract

How ENSO responds to an increasing concentration of greenhouse gases in the atmosphere has remained an elusive issue for decades. Climate models produce widely diverging results based on the traditional sea surface temperature (SST) metrics of ENSO. Some models project stronger ENSO SST variability, some weaker, some show no clear change. Steering away from these static measures, but more carefully examining the underlying processes and the associated key physical characteristics of ENSO, a clearer picture begins to emerge. Due to the nonlinear response of the atmosphere to SSTs, climate models project an increase in ENSO‐driven precipitation. Such a response tends to be robust across models linked to the relatively strong intermodel agreement in projected changes of the Pacific mean climate, marked by equatorially enhanced warming and weakened Walker Circulation. These mean‐state changes facilitate increased frequency of extreme El Niño events in models that are able to simulate nonlinear properties of ENSO closer to observations. In this ensemble of selected models, the frequency of extreme La Niña events is also projected to increase, as facilitated by faster warming of the Maritime Continent than the surrounding ocean waters. A projected increase in upper‐ocean stratification further favors increased variability and occurrences of Eastern Pacific El Niño. Uncertainties, however, remain due to persistent model biases, highlighting the need to further improve climate models, as well as sustain reliable observations to constrain model projections. Nonetheless, these projections underscore a possible heightened impact of ENSO‐driven changes in a warming climate.