Influences from outside of the tropics explain recent Walker circulation strengthening

Abstract The Pacific Walker circulation and associated sea surface temperature (SST) distribution in the tropical Pacific significantly impact global climate. However, climate model historical simulations cannot capture the observed Walker circulation enhancement since around 1980. Although a number of mechanisms have been proposed to explain the observed change, quantitative discussion and clues for reconciling the model-observation discrepancy have not yet been obtained. Here we show that the Walker circulation strengthening for 1980–2020 can be quantitatively explained by remote influence from subtropical and extratropical SST changes. This conclusion is obtained from climate model pacemaker experiments in which SST anomalies outside the tropics are restored toward observations. Influence from the Southeastern Pacific, which cools the eastern tropical Pacific, is especially crucial for the Walker circulation strengthening. This equatorward influence occurs mostly through the atmosphere and its thermal coupling with the ocean. We further show that current generation climate models have biases in the Southeastern Pacific SST changes, which can cause the failure in reproducing the Walker circulation trend. Our results suggest that improved representation of air-sea coupling in this region can be key to better projections of future climate.