Estimation of gravity-wave parameters to alleviate the delay in the Antarctic vortex breakup in general circulation models

The impact of optimal parameters in a non-orographic gravity-wave drag parametrization on the middle atmosphere circulation of the Southern Hemisphere is examined. Optimal parameters are estimated using a data assimilation technique. The proposed technique aims to reduce the delay in the winter vortex breakdown of the Southern Hemisphere found in general circulation models, which may be associated with a poor representation of gravity-wave activity. We introduce two different implementations of the parameter estimation method: an offline estimation method and a sequential estimation method. The delay in the zonal-mean zonal-wind transition is largely alleviated by the optimal gravity-wave parameters. The sequential method diminishes the model biases during winter vortex evolution, through gravity-wave drag alone. On the other hand, the offline method accounts better for unresolved-resolved wave interactions and the zonal-wind transition. We show that the final warmings in the lower mesosphere are driven mainly by planetary-wave breaking. These are affected by changes in the gravity-wave drag that are responsible for stratospheric preconditioning. Parameter estimation during the vortex breakdown is a challenging task that requires the use of sophisticated estimation techniques, because there are strong interactions between unresolved gravity-wave drag and planetary waves.