Eastern equatorial Pacific warming delayed by aerosols and thermostat response to CO2 increase

The tropical Pacific east-west temperature gradient intensified recently, but climate models do not reproduce this, and they also predict future weakening. This discrepancy is attributed to a competition between long-term weakening and transient strengthening from aerosols and ocean equatorial upwelling. Understanding the tropical Pacific response to global warming remains challenging. Here we use a range of Coupled Model Intercomparison Project Phase 6 greenhouse warming experiments to assess the recent and future evolution of the equatorial Pacific east-west temperature gradient and corresponding Walker circulation. In abrupt CO2-increase scenarios, many models generate an initial strengthening of this gradient resembling an ocean thermostat, followed by a small weakening; other models generate an immediate weakening that becomes progressively stronger, establishing a pronounced eastern equatorial Pacific warming pattern. The initial response in these experiments is a strong predictor for the intensity of this pattern simulated in both abrupt and realistic warming scenarios, but not in historical simulations showing no multi-model-mean warming trend in this region. The likely explanation is that the recent CO2-driven changes in the tropical Pacific are masked by aerosol effects and a potential ocean-thermostat-related delay, while the eastern equatorial Pacific warming pattern will emerge as greenhouse gases overcome aerosol forcing.

Automated summary

This study examines the discrepancy between recent intensification of the tropical Pacific east-west temperature gradient and climate model predictions of future weakening. Results suggest that increased CO2 may lead to an eastern equatorial Pacific warming pattern as greenhouse gases overcome aerosol forcing.