Group seminar on 17. January, 14:15 CET

Barotropic instability generated MRG waves and their potential relation to QBO

Sandor Maho

In this talk I will discuss the relationship between barotropic instability and the equatorial mixed Rossby-gravity (MRG) waves, which were studied in QBO-like environments. There is evidence that instabilities in the QBO jet give rise to the local generation of MRG waves. This was demonstrated by Garcia and Richter (2019) who studied the momentum budget of the QBO in numerical simulations with the WACCM-110L model. They diagnosed MRG waves via the combination of wavenumber-frequency decomposition and coherence analysis of the 3D velocity and temperature field. They showed that MRG waves generated locally by the instability of the jet contribute to the easterly acceleration of the QBO and these waves are likely to redistribute zonal-mean vorticity. Kang and Chun (2021) also found evidence for MRG waves generated locally by the instability of the QBO jet during the 2019/2020 QBO disruption period, which was characterized by unexpected easterly wind development during the descending westerly phase of the QBO. Their study relied on MERRA-2 re-analysis data from which they calculated spectrally decomposed Eliassen-Palm flux divergence and found anomalously energetic MRG waves during the disruption. This MRG wave activity shown to be stronger than climatological mean was caused partly by anomalously strong instabilities in the QBO region and enhanced convective activity in the tropical troposphere.

After discussing these two studies, I will compare them to the findings of idealized TIGAR experiments that were performed to study the energetic response of MRG waves to barotropic instability, which demonstrate that the MRG mode gain substantial amount of energy and becomes the dominant non-IG mode during the instability development of a QBO-like equatorial jet.