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

Area of Science:

• Climate Science
• Oceanography
• Atmospheric Science

Background:

• The tropical Pacific sea surface temperature (SST) gradient has strengthened over the past 150 years.
• Coupled Model Intercomparison Project Phase 6 (CMIP6) models often simulate a weakening of this gradient.
• This discrepancy fuels debate on model accuracy and the nature of observed SST trends.

Purpose of the Study:

• To comprehensively assess long-term trends in the observed and modeled tropical Pacific SST gradient.
• To evaluate the ability of CMIP6 models to capture observed gradient dynamics.
• To investigate whether observed strengthening is a forced response and how models represent it.

Main Methods:

• Analysis of gradient trends (≥20 years) from 1870-2024.
• Utilized five observational datasets and 14 CMIP6 large ensembles.
• Compared model-simulated trends against observational data.

Main Results:

• CMIP6 models largely fail to match observed long-term tropical Pacific SST gradient trends, particularly recent ones.
• Models matching observed trends often rely on excessive internal variability to mask a weakening forced response.
• Observed gradient trends exhibit an accelerating strengthening rate over time, a forced response absent in most models.

Conclusions:

• Significant discrepancies exist between observed and CMIP6 modeled tropical Pacific SST gradient trends.
• Current climate models may not accurately represent the dynamics driving the observed strengthening gradient.
• The accelerating strengthening of the observed gradient suggests a robust forced response not captured by most simulations.