How well do momentum diagnostics explain extratropical stratospheric variability in reanalysis datasets?
The residue is large in early-generation reanalyses (~1m/s/day) but is substantially reduced in modern reanalysis dataset, especially in the lower stratosphere where gravity wave drag is smaller than the drag exerted by resolved waves. In that region, zonal wind and zonal wind acceleration are well correlated among reanalyses.
The residue and forcing terms of the momentum equation are evaluated during the vacillation cycle of the stratospheric polar vortex: quasi-periodic intensification and weakening of the stratospheric polar winds. The magnitude of residue varies in function of the vacillation cycle, especially in older-generation reanalyses, indicating that some dynamical regimes are harder to capture than others.
Throughout the vacillation cycle, the largest discrepancies in momentum forcing come from the Coriolis torque acting on the Eulerian-mean circulation. The latter is strongly correlated to the residue. TEM diagnostics of wave-mean flow interaction, such as the EP-flux divergence, are generally consistent across reanalyses. Those results indicate that a good representation of the large-scale meridional circulation is crucial for the accuracy of momentum diagnostics.