Laboratory Analogue of the Quasi-Biennial Oscillation

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In the equatorial stratosphere. between 5-100 hPa and 15 N-15 S, the dominant source of variability is an alternating pattern of descending eastward and westward wind regimes known as the Quasi-Biennial Oscillation (QBO). These zonal winds alternate with a period that varies between 22 and 34 months, averaging 28 months. The QBO is of significant interest to long term forecasting due to its direct and indirect influence on a wide range of phenomena including the stratospheric polar vortex (Baldwin et al., 2021), subtropical and midlatitude jet (Kidston et al., 2015), and tropical convection including the MJO (Son et al., 2017). Generally, an easterly QBO increases the chances of a weak midlatitude jet, sudden stratospheric warmings, and cold winters in the UK, while a westerly QBO increases the chances of a stronger jet and milder but stormier UK winters. Kelvin, Rossby-gravity, and shorter-length interval gravity waves produced at the equatorial tropopause propagate upwards through the stratosphere, and the associated wave-mean flow interaction generates the oscillating pattern. There is much existing theoretical framework for understanding these fundamental mechanisms, but there are many outstanding problems in current outstanding. The behaviour of the QBO and its underlying mechanisms is also relevant to the atmosphere of other planets. Similar oscillations (which pose similar questions) are observed on Jupiter (Li and Read, 2000) and Saturn (Fouchet et al., 2008).