2025 Articles

CMIP6 models cannot capture long-term forced changes in the tropical Paci c sea surface temperature gradient

Byrne, Hannah; Seager, Richard; Smerdon, Jason

The observed zonal sea surface temperature gradient in the tropical Pacific has strengthened over the last 150 years, but many CMIP6 models simulate a forced weakening of this gradient over the same period. This has spurred a multi-decade debate over whether models are correctly representing dynamics in the tropical Pacific and whether the observed strengthening is a forced response. We comprehensively assess all observed and modeled gradient trends of 20 years or longer from 1870-2024 across five observational datasets and 14 CMIP6 large ensembles and find that models are not able to match many long-term trends in the observed gradient, especially those that end more recently. Models that are able to match these trends do so through excessive internal variability that compensates for their gradient-weakening forced responses. We additionally find that trends in the observed gradient strengthen at an increasing rate with time, a forced response that is in contrast to the behavior of most models.