The role of Eastern Pacific Coastal Warm Bias on the ENSO Nonlinearity Bias in CMIP6 Models

Most state-of-the-art climate models (CMIP6) struggle to simulate the non-linear asymmetry of the El Niño-Southern Oscillation (ENSO), specifically the positive skewness where El Niño events are stronger than La Niña. While previous studies have attributed this deficiency to atmospheric parameterizations or equatorial cold tongue biases, we identify a distinct oceanic driver originating in the Eastern Pacific. We show that a systematic coastal warm bias in the eastern tropical Pacific is actively transported westward by the South Equatorial Current, creating an "advective bridge" that warms the central Pacific cold tongue. This advected heat elevates the background state, saturating the deep convection threshold and effectively removing the thermodynamic "floor" required for non-linear atmospheric feedbacks. Critically, we demonstrate a physical dissociation: while central Pacific trade wind biases primarily control ENSO amplitude, it is the upstream advective warming that determines ENSO skewness. These results suggest that improving the representation of eastern boundary current dynamics is a prerequisite for capturing the non-linear character of future ENSO variability.