Future Projections of the El Ni?o-Southern Oscillation and Tropical Pacific Mean State in CMIP6

The El Nino-Southern Oscillation (ENSO) is an important mode of tropical Pacific atmosphere-ocean variability that drives teleconnections with weather and climate globally. However, prior studies using state-of-the-art climate models lack consensus regarding future ENSO projections and are often impacted by tropical Pacific sea-surface temperature (SST) biases. We used 173 simulations from 29 climate models participating in the Coupled Model Intercomparison Project, version 6 (CMIP6) to analyze model biases and future ENSO projections. We analyzed two ENSO indices, namely the ENSO Longitude Index (ELI), which measures zonal shifts in tropical Pacific deep convection and accounts for changes in background SST, and the Nino 3.4 index, which measures SST anomalies in the central-eastern equatorial Pacific. We found that the warm eastern tropical-subtropical Pacific SST bias typical of previous generations of climate models persists into many of the CMIP6 models. Future projections of ENSO shift toward more El Nino-like conditions based on ELI in 48% of simulations and 55% of models, in association with a future weakening of the zonal equatorial Pacific SST gradient. On the other hand, none of the models project a significant shift toward La Nina-like conditions. The standard deviation of the Nino 3.4 index indicates a lack of consensus on whether an increase or decrease in ENSO variability is expected in the future. Finally, we found a possible relationship between historical SST and low-level cloud cover biases in the ENSO region and future changes in ELI; however, this result may be impacted by limitations in data availability.