Compensation between Resolved and Unresolved Wave Driving in the Stratosphere: Implications for Downward Control

Perturbations to the orographic gravity wave parameterization scheme in an idealized general circulation model reveal a remarkable degree of compensation between the parameterized and the resolved wave driving: when the orographic gravity wave driving is changed, the resolved wave driving tends to change in the opposite direction, so there is little impact on the Brewer–Dobson circulation. Building upon earlier observations of such compensation, an analysis based on quasigeostrophic theory suggests that the compensation between the resolved and parameterized waves is inevitable when the stratosphere is driven toward instability by the parameterized gravity wave driving. This instability, however, is quite likely for perturbations of small meridional length scale in comparison with the Rossby radius of deformation. The insight from quasigeostrophic theory is confirmed in a systematic study with an idealized general circulation model and supported by analyses of comprehensivemodels. The compensation between resolved and unresolvedwaves suggests that the commonly used linear separation of the Brewer–Dobson circulation into components (i.e., resolved versus parameterized wave driving) may provide a potentially misleading interpretation of the role of different waves. It may also, in part, explain why comprehensive models tend to agree more on the total strength of the Brewer–Dobson circulation than on the flow associated with individual components. This is of particular relevance to diagnosed changes in the Brewer–Dobson circulation in climate scenario integrations as well.