Stratospheric dynamics modulates ozone layer response to molecular oxygen variations

Photolysis of molecular oxygen (O-2) sustains the stratospheric ozone layer and is thereby protecting living organisms on Earth by absorbing harmful ultraviolet radiation. In the past, atmospheric O-2 levels were not constant, and their variations are thought to be responsible for the extinction of some species due to the thinning of the ozone layer. Over the Phanerozoic Eon (last similar to 500 Mio years), the O-2 volume mixing ratio ranged between 10% and 35% depending on the level of photosynthetic activity of plants and oceans. Previous estimates, mostly performed by simplified 1-D models, showed different ozone (O-3) responses to atmospheric O-2 changes within this range, such as monotonically positive or negative correlations, or displaying a maximum in the O-3 column around a certain O-2 level. Here, we assess the ozone layer sensitivity to atmospheric O-2 varying between 5% and 40% with a state-of-the-art 3-D chemistry-climate model (CCM). Our findings show that the O-3 layer thickness maximizes around the current mixing ratio of O-2, 21% +/- 5%, while lower or higher levels of O-2 result globally in a reduction of total column O-3. At low latitudes, the total column O-3 is less sensitive to O-2 variations, because of the "self-healing" effect, namely, a vertical dipole in the tropical ozone response. Mid- and high-latitude O-3 columns that are largely affected by transport of O-3 from the tropics, however, are much more sensitive to O-2 with changes up to 20 DU even for small (+/- 5%) O-2 perturbations. We show that these variations are largely driven by the radiative impact of O-3 on stratospheric temperatures and on the strength of the Brewer-Dobson circulation (BDC), indicating chemistry-radiation-transport feedback. High O-2 cases result in an acceleration of the BDC and vice versa, which always works in favor of the negative part of the O-3 anomaly dipole in the tropics being more effectively transported to the mid- and high-latitudes than the positive one. Although there are other factors strongly influencing O-3/O-2 relationship on the Phanerozoic Eon timescales that have not been considered here, our results and the presented mechanism bring useful insights for other studies focusing on the long-term O-3/O-2 relationship.