ENSO phase space dynamics in CMIP models

This study analyses the El Niño-Southern Oscillation (ENSO) phase space as simulated by the Coupled Model Intercomparison Projects 5 and 6 (CMIP5 and CMIP6) models. The ENSO phase space describes the ENSO cycle between the sea surface temperature (SST) anomaly in the eastern equatorial Pacific ( T ) and the equatorial mean thermocline depth anomaly ( h ). We find that the characteristics out-of-phase cross-correlation between T and h is shifted to negative values in CMIP models, suggesting that the coupling between T and h is regionally sifted to the east compared to the observed central Pacific. If we consider the CMIP models with an eastward shifted h then the models have better agreements with the observed characteristics. While the models can capture some of the non-linear aspects with high correlations, they do largely underestimate the strength of non-linear ENSO aspects. They underestimate the likelihood of extreme El Niño and discharge states, they cannot capture the enhanced growth rates during the recharge state, the enhanced decay after the discharge state nor the reduced phase transitions after the La Niña phases. Weaker than observed wind-SST feedback and weaker h variability are likely some of the reasons why models cannot fully capture the non-linear ENSO phase space dynamics. Further, we found no indication of significant improvements from the CMIP 5 to 6 ensemble, suggesting that the two ensembles are essentially the same in terms of their ENSO dynamics. There is, however, a large spread within the model ensembles, leading to models with quite different ENSO dynamics.