ENSO diversity in CMIP models within the recharge oscillator framework

This study is focused on analysing the phase space dynamics of the El Niño-Southern Oscillation (ENSO) for two different types of ENSO, Eastern Pacific (EP) and Central Pacific (CP) in the Coupled Model Intercomparison Project (CMIP) 5 and 6 model simulations. The phase space is defined by a 2-dimensional representation of the eastern equatorial sea surface temperature anomaly (T) and the thermocline depth anomaly (h). We find that the dynamics of h in CMIP models appear to be regionally shifted to the east (h_shift). The results, when considering h_shift, suggest that CMIP models successfully capture the key aspects of observed ENSO diversity, including the asymmetries in the mean phase space, the extremes, and the phase speed, with distinct differences between EP and CP. We find significantly weaker interaction between CP and h as compared to EP and h, suggesting that the EP mode is more dynamically coupled to h than the CP mode, as it is observed. CMIP models reproduce the faster phase transition speed for EP, whereas, for CP, they replicate the weaker and slower observed phase transitions. However, CMIP model ensembles have substantial limitations and reproduce most observed characteristics with much weaker intensities. There is a large spread within the model ensemble, with only a few CMIP models accurately simulating the observed asymmetry of the ENSO phase space for CP and EP. Further, we found no significant improvement from CMIP 5 to CMIP 6 models in simulating the observed phase space dynamics of ENSO diversity.