Mechanisms for interactions between resolved and parameterized waves in the stratosphere

The Brewer-Dobson circulation describes a slow overturning circulation of the stratosphere, which, in concert with chemical processes, sets the distribution of stratospheric ozone and water vapor. This circulation can be understood as a response to mechanical wave driving through the “downward control” principal. Planetary-scale Rossby waves and small-scale gravity waves are the primary sources of wave driving, but as the latter cannot be properly resolved in most models, their influence must be parameterized. Comprehensive climate models almost uniformly project an increase in the Brewer-Dobson circulation in response to anthropogenic forcing, but differ significantly in explaining how this change is effected. Some models suggest that resolved waves are primarily responsible for the increase, while others suggest parameterized waves play an important role. Given this uncertainty, there has been justifiable concern about the model projections.

An idealized atmospheric model allows us to explore the interaction between resolved Rossby waves and parameterized gravity waves. We find that these interactions are significant, to the degree that the use of downward control to linearly partition the influence of different waves is not well posed. We identify three mechanisms for these interactions and explore them both analytically and numerically. 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 torques' location and the interaction time scale that determines the dominant mechanism.

In light of these interactions we suggest a new framework for interpreting the Brewer-Dobson circulation dynamics, one that explicitly considers the impact of the wave driving on the potential vorticity of the stratosphere. While this approach blurs the relative roles of Rossby and gravity waves, it provides more intuition into how perturbations to each component impact the circulation as a whole.