Towards understanding the robust strengthening of ENSO and more frequent extreme El Niño events in CMIP6 global warming simulations
The El Niño Southern Oscillation (ENSO) has profound impacts on weather patterns across the globe, yet there is no consensus on its response to global warming. State-of-the-art climate models have been used to make projections of ENSO changes for several decades now, but vigorous debates on the future of ENSO still continue.
This study shows that the most recent climate model intercomparison (CMIP6) indicates a robust strengthening of ENSO with global warming, accompanied by a pronounced increase in the frequency of extreme El Niño events. Potential mechanisms of these changes are analyzed.
Using a broad range of models from the Coupled Model Intercomparison Project phase 6 (CMIP6) and four types of warming experiments, here we show that the majority of the models predict a stronger ENSO by century-end in Shared Social Pathway (SSP) experiments, and in idealized 1pctCO2 and abrupt 4xCO2 experiments. The frequency of extreme El Niño events and precipitation variability increase as well. Some of these changes in ENSO sea surface are correlated with the projected future reduction in the mean zonal SST gradient. A stronger atmospheric noise, including westerly wind bursts, is another factor. Nevertheless, we argue that a robust inter-model mechanism that could consistently explain a stronger ENSO simulated with global warming is still lacking. Therefore, caution should be exercised when considering ENSO changes based on a single model or warming scenario.