Anthropogenic Weakening of the Atmospheric Circulation During the Satellite Era

Abstract

Climate models predict a slowing of the atmospheric overturning circulation with warming. In models, this slowing manifests primarily as a weakening of the Walker Circulation (WC). However, observational studies indicate a strengthened Pacific WC over the past several decades, raising questions about the models' ability to represent critical energetic and hydrologic constraints responsible for the predicted weakening. This discrepancy is closely tied to differences in the warming pattern over the Pacific during this period. We show that model simulations with either observed or model-projected warming patterns predict a robust weakening of atmospheric overturning circulation, despite having opposing changes in the Pacific WC strength. This weakening occurs in the zonally asymmetric circulation, rather than in the zonal-mean Hadley cell. Weakening inferred from satellite observations is reproduced in coupled models only when anthropogenic forcing is included, suggesting that a human-induced weakening of the global atmospheric circulation is already detectable in observations.