Hysteresis in global atmospheric river activity under carbon dioxide removal

Atmospheric rivers (ARs) are key agents regulating global hydroclimate and extreme precipitation. Climate models project the increase and intensification of ARs in a warming climate, but their responses to CO 2 mitigation remain unclear. Based on large-ensemble climate model experiments in which CO 2 concentrations are systematically increased and then decreased, we show that AR frequency and intensity do not fully return to their present-day states when CO 2 concentrations are reduced. Instead ARs are projected to remain more frequent and intense, particularly along the western coasts of North America, Europe and South America, in East Asia, and along the Antarctic coast, leading to increased extreme precipitation in the midlatitudes and potential impacts on Antarctic ice mass balance. The hysteresis pattern of AR frequency results from the competition between effects of thermodynamic and dynamic processes, both of which are closely related to the delayed recovery of the Atlantic meridional overturning circulation and the Southern Ocean temperature.