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Nearly all chemistry‐climate models (CCMs) have a systematic bias of a delayed springtime breakdown of the Southern Hemisphere (SH) stratospheric polar vortex, implying insufficient stratospheric wave drag. In this study the Canadian Middle Atmosphere Model (CMAM) and the CMAM Data Assimilation System (CMAM-DAS) are used to investigate the cause of this bias. Zonal wind analysis increments from CMAMDAS reveal systematic negative values in the stratosphere near 608S in winter and early spring. These are interpretedas indicatinga bias in the model physics, namely, missing gravity wave drag (GWD). The negative analysisincrementsremainatanearlyconstantheightduringwinteranddescendasthevortexweakens,much like orographic GWD. This region is also where current orographic GWD parameterizations have a gap in wave drag, which is suggested to be unrealistic because of missing effects in those parameterizations. These findings motivate a pair of free-running CMAM simulations to assess the impact of extra orographic GWD at 608S. The control simulation exhibits the cold-pole bias and delayed vortex breakdown seen in the CCMs. In thesimulationwithextraGWD,the cold-polebiasis significantly reducedandthe vortexbreaks downearlier. Changes in resolved wave drag in the stratosphere also occur in response to the extra GWD, which reduce stratospheric SH polar-cap temperature biases in late spring and early summer. Reducing the dynamical biases, however, results in degraded Antarctic column ozone. This suggests that CCMs that obtain realistic columnozoneinthepresenceofanoverlystrongandpersistentvortexmaybedoingsothroughcompensating errors.

Is Missing Orographic Gravity Wave Drag near 60°S the Cause of the Stratospheric Zonal Wind Biases in Chemistry–Climate Models?