ENSO Response to Greenhouse Forcing

Abstract:

The response of El Niño Southern Oscillation (ENSO) to increasing concentration of atmospheric greenhouse gases has remained an elusive issue for decades. Here we provide a review of progress on this topic and an update in light of recent advances. Climate models have produced widely diverging results based on the traditional sea surface temperature (SST) metrics of ENSO. 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 owing to the relatively strong intermodel agreement in projected changes of the Pacific mean climate, marked by equatorially enhanced warming and weakened Walker Circulation. The mean state changes facilitate increased frequency of extreme El Niño events in models that better simulate nonlinear ENSO properties. In this ensemble of models, the frequency of extreme La Niña events is also projected to increase. A projected increase in upper‐ocean stratification further favors enhanced variability and occurrences of Eastern Pacific El Niño. While ENSO representation in climate models still needs further improvements, these projections nonetheless underscore a possible heightened impact of ENSO‐driven changes in a warming climate.