Towards understanding ENSO's non-monotonic projections in the CESM2 climate model

There is an emerging consensus that ENSO amplitude may change non-monotonically in response to external forcing, increasing over the course of the 20th century and decreasing in the 22nd and 23rd centuries. While the physical explanations for these asymptotic short-term and long-term cases are well-supported by climate model simulations, projections of ENSO’s 21st century changes vary widely between models. In this work, we investigate why ENSO amplitude begins to decline in the mid-21st century in the CESM2 model. We find that almost all of the amplitude decrease results from a weakening of the strongest El Niño events. While strong El Niños' intensity begins decreasing in ~2010, La Niñas’ intensity continues increasing for several decades afterwards. The net result of these opposing changes is a ~2030 maximum in overall ENSO amplitude and a reverse in ENSO asymmetry by the 21st century (La Niñas become stronger than El Niños, opposite to the 20th century). We show that this asymmetric change in ENSO intensity is consistent with a weakening of the zonal temperature gradient, which increases the zonal variability of the Walker circulation and limits the potential intensity of El Niños but not La Niñas. Overall, our analysis suggests that changes in asymmetry may have a leading-order effect on overall ENSO amplitude in the 21st century, and that El Niño intensity may not always be a useful upper bound on La Niña intensity.