Stratospheric subtropical transport barriers in CESM-WACCM and observations: climatology, variability, and trends

Accurate representation of the large-scale stratospheric transport in chemistry-climate models is crucial for interpreting observed variability in chemical species, such as ozone, and making reliable projections regarding future changes. Subtropical transport barriers separate the tropical stratosphere, influenced by slow upwelling, from the surf zones, where rapid mixing occurs due to wave breaking. Long-lived tracer contours reflect the combined effects of advection and mixing and can be used to identify the location of the subtropical transport barriers. This study comprehensively compares tracer- and dynamics-based diagnostics of the subtropical transport barriers in the CESM-WACCM4 chemistry-climate model and various observational datasets. The model tracer-based estimates show excellent agreement with observations regarding seasonal climatology and variability linked to the Quasi-Biennial Oscillation (QBO). The chemical boundaries shift due to the secondary meridional circulation induced by the QBO in the winter hemisphere and due to the enhanced isentropic mixing associated with waves crossing the equator in the summer hemisphere. Consistent with previous studies, the observational tracer metrics feature a southward shift of the tropical pipe over 2005–2012. The model, which nudges the QBO to observations, captures qualitatively the shift over this period. Dynamical metrics represent individual transport processes and thus fail to capture the variability and trends in the tracer-based boundaries.