Stratospheric Subtropical Transport Barriers in CESM1‐WACCM and Observations: Climatology, Variability, and Trends

Abstract Accurate representation of large‐scale stratospheric transport in chemistry–climate models is essential for understanding and interpreting observed atmospheric variability and for making reliable projections. The subtropical transport barriers (STBs) separate the tropical stratosphere, influenced by slow upwelling, from the midlatitudes, where rapid mixing occurs. Long‐lived tracer contours reflect the combined effects of advection and mixing, making them valuable for identifying these barriers. This study compares a set of tracer‐based and dynamical diagnostics of the STBs in the CESM1‐WACCM chemistry‐climate model with various observational data sets. Tracer‐based barriers in the model show excellent agreement with observations between approximately 50‐5 hPa in seasonal climatology, variability associated with the Quasi‐Biennial Oscillation (QBO), and trends. The STBs exhibit a QBO‐driven displacement due to secondary meridional circulation in the winter hemisphere and due to the enhanced isentropic mixing associated with waves crossing the equator in the summer hemisphere. Tracer‐based barriers reveal a statistically significant southward shift of the tropical pipe from 2005 to 2012 consistent with prior findings. We show that this southward shift is a non‐linear effect of the QBO and disappears over longer periods. Dynamical estimates of the STBs show overall good agreement between model and reanalysis in terms of the climatology but are unreliable in the summer hemisphere. Moreover, they fail to capture the QBO‐related variability and trends observed in the tracer‐based boundaries. This discrepancy highlights that dynamical metrics isolate specific instantaneous transport processes, whereas tracer‐based metrics capture their cumulative impact, displaying a lagged and damped seasonality compared to the dynamical estimates.