The wave driven Brewer-Dobson circulation plays a crucial role in determining the transport of trace gases andaerosols in stratosphere. We examine the structure of the circulation based on reanalyses data (ERA5, ERA-Interim, MERRA2,JRA55), using the Transformed Eulerian Mean and downward control framework, aiming for a dynamical separation ofdifferent circulation branches in terms of outflow generated by wave driving. The results show the existence of differentcirculation regimes, with a deep circulation branch mainly driven by large-scale waves with wavenumbers 1-3, and a shallow5circulation branch mainly driven by smaller-scale waves with wavenumbers 4-180. We propose a definition of the separationlevel between a shallow and deep branch as the lowest level where outflow from waves 1-3 is larger than from waves 4-180. Weshow that this level occurs at approximately 22km (43hPa) and exhibits a weak annual cycle. This climatological structure isrobust in various reanalyses. The variability of the circulation in the deep branch above the separation level is mainly related tolarge-scale waves 1-3, while the variability in the shallow branch is related to both smaller-scale and large-scale waves. Trends10in the circulation over the period 1980-2017 show an upward shift of the deep branch related to waves 1-3 and a downwardshift of the shallow branch related to both large and smaller scale waves. The height of the separation level shows no significanttrend. Taking into account differences in wave driving between the branches of the circulation could reduce the spread in modelinter-comparisons.
Baikhadzhaev et al. (Wed,) studied this question.