Group seminar on 28. May, 14:15
Barotropic instability in the QBO region and its effect on mixed Rossby-gravity wave growth
Sandor Maho
According to observational as well as modelling studies, barotropic instability can occur in the QBO region (at 30-50 hPa), which can influence the growth of mixed Rossby-gravity (MRG) waves. These studies also suggest that instead of contributing to the zonal mean acceleration of the flow in the stratosphere, MRG waves are generated locally by the barotropic instability development of the QBO jet. Here I will present a climatological survey for the barotropic instability of the QBO region by applying different stability conditions and the linearized shallow water equations. The shallow water system allows us to give prediction for the MRG wave growth by barotropic instability, which we compare with kinetic energy derived from ERA5. We find that synoptic and sub-synoptic MRG scales (zonal wavenumber 6-12) are most affected by the barotropic instability of the QBO jet, especially in the westerly QBO phase. Unstable background eigenmodes also indicate larger MRG wave growth in the westerly phase. Nevertheless, the overall effect of barotropic instability on the MRG growth in the QBO region is estimated to be only about 10-20%.