Correcting Underestimated Internal Variability Fails to Reconcile Models With Observed Pacific SST Gradient Strengthening

Abstract The equatorial Pacific zonal sea surface temperature (SST) gradient has strengthened since 1980, yet fewer than 1% of CMIP6 simulations reproduce this trend. We test whether underestimated internal variability explains this mismatch. Extreme El Niño events enhance interdecadal variability of the gradient, but CMIP6 models simulate them too infrequently, producing low Niño3 SST skewness that correlates (∼0.65) with interdecadal gradient variability. Applying this observational constraint to statistically correct modeled interdecadal gradient variability increases the fraction of simulations exceeding the observed trend to only ∼6%, indicating that enhancing variability alone cannot resolve the discrepancy. One‐third of models with large ensembles display a weak but forced recent strengthening that later reverses. Beyond +1.7°C of global warming, over 95% of simulations project a ∼15% weakening. Together, these results suggest that the recent strengthening includes a substantial externally forced transient component poorly simulated by models, while the projected long‐term weakening is robust.