A Stronger Versus Weaker Walker: Understanding Model Differences in Fast and Slow Tropical Pacific Responses to Global Warming

Abstract The tropical Pacific response to radiative forcing remains uncertain as projected future changes to the Walker circulation and SST patterns vary substantially among climate models. Here, we study what sets the magnitude and timescales of the response and why they differ across models. Specifically, we compare the fast and slow responses of the tropical Pacific to abrupt CO2 increases (2,4,8,16xCO2) in two configurations of the same model family (CESM) that differ in horizontal resolution and mean biases. We find that the model with a higher resolution shows a transient ocean thermostat-like response to CO2-forcing, with a stronger Walker cell and lack of warming in the eastern Pacific trade wind belts. This fast response lasts for about 50 years and is followed by a slight Walker cell weakening and equatorial warming. The second model, with a coarser resolution, shows a weak and short-lasting ocean thermostat response, followed by pronounced Walker cell weakening and eastern equatorial Pacific warming, similar to the long-term pattern noted in previous studies. These fast and slow responses also manifest in gradual CO2 increase experiments. We relate the magnitude of the fast ocean-thermostat response to the structure of the equatorial thermocline, setting the strength of the Bjerknes feedback. The magnitude and timing of the eastern equatorial Pacific warming are, is turn, related to the competition of the wind-evaporation-SST feedback amplifying the ocean-thermostat against the slowdown of oceanic subtropical cells and extra-tropical warming eroding the thermostat. Different balances between these effects could explain the large spread in the future projections for the tropical Pacific.