A dynamical separation of deep and shallow branches in the stratospheric circulation

The Brewer-Dobson circulation is the wave driven meridional circulation of the stratosphere it plays an important role in determining the transport of trace gases and aerosols within the stratosphere, affecting the lifetimes of ozone depleting substances, and the global radiation budget. The overturning part of the circulation is usually split into a deep and a shallow branch. Here, we investigate the dynamical driving of these circulation branches, aiming for a dynamical separation of different circulation regimes.

For that purpose, we use data from fifth generation atmospheric reanalysis ERA5 by the European Centre for Medium-range Weather Forecasts (ECMWF) and apply the Transformed Eulerian Mean approach to estimate the overturning mass flux and the related wave forcing in the stratosphere. We find that reducing the horizontal resolution of the data from 0.3 to 1 degree does not affect results significantly. However, reducing temporal resolution from 1 to 6 hours has a significant effect on the structure of daily mean upwelling, but this effect is much less pronounced for monthly means. Eliassen-Palm flux divergence is used as a diagnostic to estimate the wave propagation in the stratosphere. Using Fourier transformation for spectral decomposition we estimate the contribution from different waves to the driving of the deep and shallow branches. In particular, it is found that the deep branch has a strong seasonality with maximum in winter, while the shallow branch is less affected by the change of seasons. On the one hand, we find a strong correlation between variability in residual circulation velocity along the deep branch and large scale waves with wavenumber 3 or smaller. On the other hand, variability of medium and small scale waves with wavenumber 4 or greater correlates strongly with variability in the shallow branch circulation velocities. The change between these two dynamical regimes happens at a level close to 37 hPa. These results were further tested by applying a downward control calculation, showing that indeed the large-scale, planetary waves (wavenumber less than 4) account for the largest part of deep branch variability, while smaller waves (wavenumber larger than 3) account for the largest part of shallow branch variability. Based on these results, we propose a physical definition of the different Brewer-Dobson circulation branches, with the deep branch defined as driven by planetary waves (wave numbers 1-3) and located above 35 hPa, whereas the shallow branch being located below that level and driven by smaller-scale waves (wave numbers 4 and greater).