Asymmetric hysteresis response of mid-latitude storm tracks to CO2 removal

In a warming climate, storm tracks are projected to intensify on their poleward side. Here we use large-ensemble CO2 ramp-up and ramp-down simulations to show that these changes are not reversed when CO2 concentrations are reduced. If CO2 is removed from the atmosphere following CO2 increase, the North Atlantic storm track keeps strengthening until the middle of the CO2 removal, while the recovery of the North Pacific storm track during ramp-down is stronger than its shift during ramp-up. By contrast, the Southern Hemisphere storm track weakens during ramp-down at a rate much faster than its strengthening in the warming period. Compared with the present climate, the Northern Hemisphere storm track becomes stronger and the Southern Hemisphere storm track becomes weaker at the end of CO2 removal. These hemispherically asymmetric storm-track responses are attributable to the weakened Atlantic meridional overturning circulation and the delayed cooling of the Southern Ocean.