A pause in Southern Hemisphere circulation trends due to the Montreal Protocol

Observations and reanalyses have shown trends in Southern Hemisphere tropospheric circulation at the end of the last century, with a poleward shift in the midlatitude jet, a positive trend in the Southern Annular Mode, and an expansion of the Hadley Cell. These trends have impacted weather systems, ocean circulation and carbon uptake, and Antarctic sea ice. Here we show the emergence of a pause, or slight reversal, in these circulation trends around the turn of the century. A large ensemble of simulations from one climate model and multiple chemistry-climate model simulations demonstrate the essential role of the Montreal Protocol-controlled ozone-depleting substances in driving observed trend changes in the position of the midlatitude jet in austral summer, acting in concert with increasing greenhouse gases to shift the jet poleward before 2000, but canceling their effects after 2000.