Understanding CMIP6 Multi-Model Ensemble Projected Pacific Warming Pattern Variability

Abstract

Most CMIP6 climate models simulate a Pacific SST warming pattern through the 20th and 21st centuries that is “El Niño-like,” with a weakened zonal equatorial gradient. However, observed trends are “La Niña-like,” displaying a strengthened zonal equatorial gradient, raising concerns about the accuracy of these projections. Here we explore multi-model ensemble variability in the projected Pacific warming pattern. This pattern variability is largely explained by two dominant patterns: one linked to hemispheric warming asymmetries, and the other to zonal equatorial gradients. Crucially, model differences in projected zonal equatorial gradients are strongly tied to the model representation of the mean state in the off-equatorial east and west Pacific. This implies that mean state biases partly control the projected warming pattern. Moreover, models with more realistic mean states tend to produce: (a) historical trends with stronger gradients, aligning better with observations, and (b) more “El Niño-like” future projections.