Anthropogenic Weakening of the Atmospheric Circulation During the Satellite Era

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. The atmospheric circulation transfers moisture and energy from the tropics to the polar regions and regulates the distribution of rainfall in the tropics which is home to around 40% of the world's population. In recent years, observations show a strengthening of the Pacific Walker Circulation (WC), a regional-level circulation, which contrasts the weakening of the circulation predicted by climate models. This discrepancy questions the climate models' ability to predict future changes in overturning circulations. Using various strength indices, we find a consistent "weakening with warming" of the global overturning circulation in both observations and climate models for the common period, regardless of the behavior of regional-level circulations. Moreover, we find that it is highly unlikely that this observed weakening is due to natural variations in the climate. The global atmospheric overturning circulation has weakened in recent decades despite a strengthening of the Walker CirculationClimate models predict a circulation weakening consistent with observations, manifested primarily as a weakening of the zonally asymmetric overturning circulationThe observed weakening is reproduced in coupled climate models only when anthropogenic forcing is included