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| Home > Publications database > Dynamical drivers of deep and shallow branches in the stratospheric circulation and transport |
| Book/Dissertation / PhD Thesis | FZJ-2026-04285 |
2026
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-997-8
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Please use a persistent id in citations: doi:10.34734/FZJ-2026-04285
Abstract: The wave-driven Brewer-Dobson circulation plays a crucial role in transporting radiatively active trace gases and aerosols through the stratosphere, which, in turn, impact the Earth’s radiation budget. A detailed understanding of this transport, particularly within a changing climate, is therefore critical. This thesis aims to achieve a dynamical separation of the BDC into distinct branches based on their wave driving, examining the circulation’s structure in terms of residual circulation and isentropic mixing. The analysis in this work is based on multiple meteorological reanalysis datasets (ERA5, ERA-Interim, MERRA2, JRA55) and data from the Lagrangian chemistry transport model (CLaMS), utilizing the Transformed Eulerian Mean (TEM) framework, the Downward Control Principle, and tracer continuity equation-based transport diagnostics. The analysis relies on two metrics generated by wave drag: residual circulation is quantified by the outflow (the total mass flow across turn-around latitudes), while isentropic mixing is assessed via the horizontal mixing tendency in the tracer continuity equation (the horizontal component of tracer transport by eddy mixing). The results reveal the existence of distinct circulation regimes: a deep branch mainly driven by planetary waves (wavenumbers 1–3) and a shallow branch primarily driven by smaller-scale waves(wavenumbers > 3). It is shown that the separation level between the shallow and deep branches can be robustly defined as the lowest altitude where outflow or isentropic mixing from planetary waves exceeds that from smaller-scale waves. Based on analysis of both residual circulation and eddy mixing, this separation level is found at approximately 22 km (about 45 hPa pressure, or about 500 K potential temperature), and shows a weak annual cycle with the highest levels during boreal winter. This climatological structure is robust across the various reanalyses. Above this separation level, the variability of residual circulation and isentropic mixing in the deep branch is mainly related to planetary waves. In the shallow branch, smaller-scale waves play a more important role in the variability of the residual circulation and mixing compared to the deep branch. Trends in the isentropic mixing and outflow over the period 1980–2017 indicate a weakening of the Brewer-Dobson circulation below the separation level and a strengthening at altitudes slightly above it. In climate model inter-comparisons, the strength and wave driving of the BDC are usually compared at fixed pressure levels, where different circulation branches likely dominate stratospheric transport in different models. Past and present model intercomparisons generally show a large spread regarding the strength of the simulated BDC. Taking into account differences in wave driving between the circulation branches and in the separation level will likely reduce the spread in model inter-comparisons.
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