A dynamics-based separation of deep and shallow stratospheric circulation branches

Abstract. The wave-driven Brewer–Dobson circulation plays a crucial role in determining the transport of trace gases and aerosols in the stratosphere. We examine the structure of the circulation based on reanalysis data (ERA5, ERA-Interim, MERRA2, and JRA55) and 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 planetary waves with wavenumbers 1–3, and a shallow circulation branch mainly driven by smaller-scale waves with wavenumbers greater than 3. We propose a definition of the separation level between shallow and deep branches as the lowest level where outflow from planetary waves is larger than outflow from smaller-scale waves. We show that this level occurs at approximately 22 km (43 hPa) with 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 planetary waves, while the variability in the shallow branch is related to both smaller-scale and planetary waves. Trends in the circulation over the period 1980–2017 show an upward shift of the deep branch related to planetary waves and a downward shift of the shallow branch related to both planetary 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 explain the spread in model inter-comparisons.