Mechanisms of Fast Walker Circulation Responses to CO2 Forcing.

The Walker circulation (WC) responds to CO2 forcing at both short and long timescales. In climate models, the fast response accounts for a substantial portion of the total responses, but its mechanisms, particularly those pertaining to air‐sea interactions, remain unclear. We find contrasting fast WC responses in the first 2 years of abrupt CO2 forcing, determined by the models' air‐sea coupling strength in the equatorial Pacific. In models with a strong coupling, wind anomalies induced by the instantaneous land‐sea thermal contrast trigger a Bjerknes feedback, leading to cooling in the equatorial Pacific and WC strengthening. The WC weakens gradually as the Bjerknes feedback wanes and the subsurface warm pool water migrates eastward as Kelvin waves. In models with a weaker coupling, the WC weakens monotonically. Our results suggest that the inter‐model discrepancy in WC changes is associated with the uncertainty in the fast component. Plain Language Summary: The Walker circulation (WC), a large‐scale tropical air flow in zonal and vertical directions, is a product of the atmosphere‐ocean interaction. A strong WC is associated with a more positive air‐sea coupling: as the westward trade winds accumulate warm water in the western Pacific and draw up cold water in the eastern Pacific, the zonal sea surface temperature (SST) gradient and atmospheric convection above warm regions are enhanced. The zonal SST and sea level pressure gradient reinforce the trade winds. We use multiple climate models to study the response of the WC to CO2 at different time scales. Two different WC responses are found and the difference is determined by simulated air‐sea coupling strength among models. In the first class of models, a strong air‐sea coupling causes a strengthening of WC right after imposed forcing—opposite to the longterm response. In the second class of models, the WC weakens from the beginning of the simulation. Key Points: Climate models disagree on the fast response of the Pacific Walker circulation to CO2The disagreement is determined by models' air‐sea coupling strengthModels simulating stronger air‐sea coupling establish a strengthening of the Walker circulation during fast response [ABSTRACT FROM AUTHOR]