The Role of Gravity Wave Drag Optimization in the Splitting of the Antarctic Vortex in the 2002 Sudden Stratospheric Warming

The impact of gravity wave drag on the Antarctic sudden stratospheric warming (SSW) in 2002 is examined through a mechanistic middle atmosphere model combined with a variational data assimilation system. Significant differences in the SSW representation are found between a model integration that uses reference gravity wave parameters and one that uses parameters estimated using data assimilation. Upon identical wave forcings at 100 hPa, the vortex breakdown may arise as either a vortex splitting event or a displacement vortex event depending on gravity wave parameters. A local enhancement of Rossby waves is found in the integration with estimated parameters, leading to a split SSW. The changes in the vortex breakdown are associated with changes in the vortex geometry caused entirely by modifying the gravity wave parameters. Gravity wave drag proved to play an instrumental role in preconditioning the stratosphere near a resonant excitation point that triggers the split SSW. Plain Language Summary Sudden stratospheric warming (SSW) events are manifestations of an abrupt change of the wintertime polar jet stream circulation, with consequences on tropospheric weather. The associated vortex breakdown is characterized by either a displacement or split of the vortex. This work focuses on the unprecedented 2002 Antarctic SSW and compares a model integration using standard model parameters, with an integration using parameters optimized using data assimilation methods. The optimized parameters are the ones in the gravity wave drag parameterization. Only the integration with optimal parameters is able to accurately reproduce the vortex splitting. Our results show that gravity wave drag parameterizations play an essential role when modeling SSWs. Data assimilation methods may be a useful tool to properly adjust these types of parameterizations.