Stratospheric ozone depletion and tropospheric ozone increases drive Southern Ocean interior warming

Atmospheric ozone has undergone distinct changes in the stratosphere and troposphere during the second half of the twentieth century, with depletion in the stratosphere and an increase in the troposphere. Until now, the effect of these changes on ocean heat uptake has been unclear. Here we show that both stratospheric and tropospheric ozone changes have contributed to Southern Ocean interior warming with the latter being more important. The ozone changes between 1955 and 2000 induced about 30% of the net simulated ocean heat content increase in the upper 2,000 m of the Southern Ocean, with around 60% attributed to tropospheric increases and 40% to stratospheric depletion. Moreover, these two warming contributions show distinct physical mechanisms: tropospheric ozone increases cause a subsurface warming in the Southern Ocean primarily via the deepening of isopycnals, while stratospheric ozone causes depletion via spiciness changes along isopycnals. Our results highlight that tropospheric ozone is more than an air pollutant and, as a greenhouse gas, has been pivotal to the Southern Ocean warming.