Governing Processes of Extreme El Niño and Implications for Future Projections

The El Niño Southern Oscillation is the most dominant year-to-year climate phenomenon on earth. Sourced in the equatorial Pacific Ocean, its impacts reverberate around the globe, affecting occurrences of weather and marine extremes with serious consequences on society. El Niño, the warm phase of ENSO, can attain much stronger magnitude than the cold phase, La Niña. From time to time, it can reach an extraordinarily large amplitude, leading to dramatic alterations of ocean and atmospheric circulations. This poses a significant risk to the environment and economy, especially in a climate that is changing as greenhouse gas emission continues unabated. A better understanding of extreme El Niño processes is thus critical, and we emphasize that this will help us anticipate how ENSO behaviour would change in the future.

The processes behind extreme El Niño are not yet completely understood. But thanks to sustained observations and modeling efforts we are able to gradually put some of the puzzle pieces together. Here we present the salient processes which are inherently nonlinear, involving changes in equatorial Pacific rainfall, winds, ocean currents, and thermal stratification. We find that not all climate models are able to simulate these processes. Using the capable models however, we find that greenhouse forcing tends to reinforce these processes, leading to elevated occurrences of extreme El Niño, as well as extreme La Niña, in a warming climate under business-as-usual emission scenarios.