How Will El Nino Change Under Global Warming?

How El Niño-Southern Oscillation (ENSO) will change under global warming affects extreme events around the world. The change of ENSO amplitude is investigated based on the historical simulations and Representative Concentration Pathway (RCP) 8.5 experiments in the Coupled Model Intercomparison Project (CMIP5) phase 5. The projected change in ENSO amplitude is highly uncertain with large inter-model uncertainty. By using the relative sea surface temperature (SST) as a measure of convective instability, this study finds that the spatial pattern of tropical Pacific surface warming is the major source of inter-model uncertainty in ENSO amplitude change. In models with an enhanced mean warming in the eastern equatorial Pacific, the barrier to deep convection is reduced, and the intensified rainfall anomalies of ENSO amplify the wind response and hence SST variability. In models with a reduced eastern Pacific warming, conversely, ENSO amplitude decreases. Corroborating the mean SST pattern effect, inter-model uncertainty in changes of ENSO-induced rainfall variability decreases substantially in atmospheric simulations forced by a common ocean warming pattern. Thus, reducing the uncertainty in the Pacific surface warming pattern helps improve the reliability of ENSO projections. To the extent that correcting model biases favors an El Niño-like mean warming pattern, this study suggests an increase in ENSO-related SST variance likely under global warming.

The talk will also discuss the relative importance of internal variability and radiative forcing based on the NCAR CESM large ensemble simulations.