Eastern Pacific coastal warm bias as a key control on ENSO nonlinearity in climate models

The El Niño–Southern Oscillation (ENSO) exhibits a pronounced positive skewness, with El Niño events typically stronger than La Niña events, reflecting nonlinear air–sea coupling in the tropical Pacific. However, most coupled climate models fail to reproduce this asymmetry. Here we show that ENSO skewness errors in CMIP6 models are primarily linked to mean-state biases along the eastern Pacific coast rather than to local equatorial SST biases. Inter-model diagnostics reveal that warm SST biases in the eastern Pacific coastal region are strongly associated with reduced ENSO skewness, coincident with a flattened zonal SST gradient and weakened Walker circulation. These mean-state distortions linearize the atmospheric component of the Bjerknes feedback, yielding a symmetric wind–SST response. Targeted flux-correction experiments with the ICON XPP coupled model provide causal support: reducing the eastern coastal bias restores asymmetric wind–SST coupling and recovers approximately half of the observed ENSO skewness, despite persistent equatorial SST biases.