ENSO phase space dynamics in CMIP models

Abstract This study analyses the El Niño-Southern Oscillation (ENSO) phase space as simulated by the Coupled Model Intercomparison Projects (CMIP) 5 and 6 models. In the framework of the ENSO phase space, the ENSO cycle is described as a two-dimensional representation of the sea surface temperature 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 shifted. If we consider the CMIP models with h estimated shifted further to the eastern Pacific, then the models have better agreements with the observed characteristics. While the models can capture some of the observed asymmetries 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 slower phase transitions after the La Niña phase. 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, with only a few CMIP models accurately simulating the observed asymmetries of the ENSO phase space, leading to models with quite different ENSO dynamics.