What drives the Brewer-Dobson circulation?

Recent studies have revealed strong interactions between resolved Rossby-wave and parameterized gravity-wave driving in stratosphere-resolving atmospheric models. Perturbations to the parameterized wave driving are often compensated by opposite changes in the resolved wave driving, leading to ambiguity in the relative roles of these waves in driving the Brewer-Dobson circulation. Building on previous work, we identify three mechanisms for these interactions and explore them in multiple atmospheric models (idealized and comprehensive) as well as using reanalysis data. The three mechanisms are associated with a stability constraint, a potential vorticity mixing constraint, and a nonlocal interaction driven by modifications to the refractive index of planetary-wave propagation. While the first mechanism is likely for strong-amplitude and meridionally-narrow parameterized torques, the second is most likely for parameterized torques applied inside the winter-hemisphere surf-zone region, a key breaking region for planetary waves. The third mechanism, on the other hand, is most relevant for parameterized torques just outside the surf zone. It is likely for multiple mechanisms to act in concert, and it is largely a matter of the torque's location and the interaction time scale that determines the dominant mechanism.

In light of these interactions, the conventional paradigm for separating the relative roles of Rossby- and gravity-wave driving by downward control is critiqued. A modified approach is suggested, which explicitly considers the impact of wave driving on the potential vorticity of the stratosphere. While this approach blurs the roles of Rossby and gravity waves, it provides more intuition into how perturbations to each component impact the circulation as a whole.