Climate change simulations of the 21^?? century have been converging towards enhanced ENSO SST variability and more frequent extreme El Niño events. In fact, a broad range of models from CMIP6 predict a stronger ENSO by century-end in the Shared Social Pathway (SSP) experiments, and in idealized 1pctCO₂ and abrupt 4xCO₂ experiments (e.g., Heede and Fedorov 2023). Yet, the projections for the future ENSO amplitude exhibit a large spread, and a robust physical mechanism explaining the future change across different models is still lacking. Most commonly, to explain ENSO variations, changes in the background state of the tropical Pacific are invoked. Indeed, we find correlations of about 0.6 between increases in ENSO amplitude and the weakening of the mean zonal SST gradient in SSP5-8.5 experiment across the models. However, this linkage between ENSO variability and the tropical mean state change is relatively modest. Going beyond the effect of background state, we find that changes in the structure of wind-stress anomalies associated with ENSO are potentially as important. Specifically, changes in the magnitude, meridional width, and zonal structure of these anomalies can explain approximately 57% of the inter-model variance in the projected change of ENSO magnitude through the 21st century, providing a robust constraint of the CMIP6 representation of ENSO. Among the changes, the meridional widening of wind-stress anomalies plays the most important role. To demonstrate that these changes are indeed critical, we use a hybrid model of ENSO (a dynamical ocean coupled to a simplified statistical atmosphere), which allows us to implement and examine the effect of the CMIP6 projected change in tropical Pacific wind-stress anomalies on ENSO. In the absence of external forcing and corresponding mean state changes, the imposed change in the structure of wind-stress anomalies in this model results in an increase in ENSO amplitude of about 10%. Thus, future changes in ENSO wind-stress anomalies, together with changes in the mean state, will play a critical role in the projected strengthening of ENSO (Stuivenvolt-Allen et al. 2024).

Heede, U.K. and Fedorov, A.V., 2023: Towards understanding the robust strengthening of ENSO and more frequent extreme El Niño events in CMIP6 global warming simulations. Climate Dyn.,61, 3047–3060.

Stuivenvolt-Allen, J., Fedorov, A.V., Heede, U.K. and Minmin Fu, 2024: Widening of wind stress anomalies drives ENSO strengthening in a warming climate. In revision J. of Climate