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

Abstract. The wave driven Brewer-Dobson circulation plays a crucial role in determining the transport of trace gases and aerosols 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 of different circulation branches in terms of outflow generated by wave driving. The results show the existence of different circulation regimes, with a deep circulation branch mainly driven by large-scale waves with wavenumbers 1–3, and a shallow circulation branch mainly driven by smaller-scale waves with wavenumbers 4–180. We propose a definition of the separation level between a shallow and deep branch as the lowest level where outflow from waves 1–3 is larger than from waves 4–180. We show that this level occurs at approximately 22 km (43 hPa) and exhibits a weak annual cycle. This climatological structure is robust in various reanalyses. The variability of the circulation in the deep branch above the separation level is mainly related to large-scale waves 1–3, while the variability in the shallow branch is related to both smaller-scale and large-scale waves. Trends in the circulation over the period 1980–2017 show an upward shift of the deep branch related to waves 1–3 and a downward shift of the shallow branch related to both large and smaller scale waves. The height of the separation level shows no significant trend. Taking into account differences in wave driving between the branches of the circulation could reduce the spread in model inter-comparisons.