Strong Eastern Pacific El Niño events largely determine the spread in ENSO Amplitude and Asymmetry in Climate Models

Abstract The El Niño/Southern Oscillation (ENSO) exhibits a considerable asymmetry in Sea Surface Temperature anomalies (SSTa), as El Niño events tend to be stronger and centered further east than La Niña events. Here we show by observations and climate models that the southward shift of the Intertropical Convergence Zone (ITCZ) onto the equator during strong Eastern Pacific (EP) El Niño events is essential for ENSO asymmetry, as this shifts the wind-SST feedback eastward by up to 15° compared to Central Pacific (CP) El Niño events, explaining the more eastern center in SSTa. Further, the thermocline feedback is enhanced in the east and in turn the Bjerknes feedback, which explains the large amplitude of the EP El Niño events. These results are confirmed by the CESM1 Large Ensemble. Further, it shows a statistically significant and strong (r ~ 0.8) link between internally generated differences in the fraction of strong EP events in 50-years long realizations and their overall ENSO amplitude and asymmetry. Finally, we analyze ENSO amplitude and asymmetry in preindustrial control simulations of 32 models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6). The spread in ENSO amplitude across the model ensemble can be largely explained by the fraction of the strong EP El Niños. Half of the models strongly underestimate the ENSO asymmetry, which can be partly explained by the equatorial Pacific cold SST bias that hampers the southward migration of the ITCZ onto the equator during El Niño events, leading to fewer strong EP El Niño events.