The Role of Ozone in the Secondary Circulation of the QBO: Linear Theory

Abstract The secondary circulation is central to understanding how changes in ozone (or other radiatively active species) might affect the dynamics of the quasibiennial oscillation (QBO). We develop here a linearized scaling theory to quantify the potential role of ozone in determining the secondary circulation, and validate it against a radiative/photochemical/convective equilibrium column model that reproduces the observed temperature and ozone anomalies of the QBO. Although the potential for radiative heating due to ozone to affect the secondary circulation is generally appreciated, theoretical discussions of the secondary circulation have not included an explicit representation of the ozone budget. For realistic parameter values, ozone heating can play a dominant role in determining the strength and structure of the secondary circulation. In particular, the photochemical sink in the lower stratosphere where ozone is conventionally thought to be determined by transport is key to obtaining a realistic circulation: in its absence the theory predicts a much weaker circulation in which vertical motion is aligned with the core of the jets rather than the shear zones. The photochemical sink (and thus the secondary circulation) depends nonlocally on the ozone column above. We further apply this theory to estimate the component of the secondary circulation that is due to ozone heating, confirming a previously inferred over‐estimate of the secondary circulation in ERA 5. These results have implications for understanding the role of interactive ozone in model‐generated QBOs. Plain Language Summary The quasibiennial oscillation is a mode of climate variability in the tropical lower stratosphere characterized by descending eastward and westward jets that alternate with a period of about 28 months. It features a secondary circulation in which air ascends over the equator where the jets are transitioning from eastward to westward, then moves poleward before descending in the subtropics. This secondary circulation is key to understanding how long term changes in ozone might affect the dynamics of the QBO. Previous theories have found that radiative heating is the major driver of this secondary circulation. We have extended these theories to include the radiative effects of ozone, and show that it can play a major role in modulating the secondary circulation. The theory implies that subtle, nonlocal photochemical effects are essential; these are often neglected by theories because they are weak and introduce additional complexity. Our results show that ozone variability is essential to understand the secondary circulation. They will help to interpret the representation of the quasibiennial oscillation in global climate models, and are a step toward predicting how this mode of climate variability might respond to a changing climate. Key Points Novel scaling arguments are derived for the quasibiennial oscillation (QBO) secondary circulation which indicate the ozone budget can play a major role Linearized coefficients for the ozone and thermodynamic budgets appropriate for the QBO are estimated The ozone QBO and thus the secondary circulation are highly sensitive to nonlocal processes in the lower stratosphere