Impacts, processes and projections of the quasi-biennial oscillation

Abstract

In the tropical stratosphere, deep layers of eastward and westward winds encircle the globe and descend regularly from the upper stratosphere to the tropical tropopause. With a complete cycle typically lasting almost 2.5 years, this quasi-biennial oscillation (QBO) is arguably the most predictable mode of atmospheric variability not linked to the changing seasons. The QBO affects climate phenomena outside the tropical stratosphere, including ozone transport, the North Atlantic Oscillation and Madden-Julian Oscillation, and its high predictability could enable better forecasts of these phenomena if models can accurately represent the coupling processes. Climate and forecasting models are increasingly able to simulate stratospheric oscillations resembling the QBO, but exhibit common systematic errors such as weak amplitude in the lowermost tropical stratosphere. Uncertainties about the waves that force the oscillation, particularly the momentum fluxes from small-scale gravity waves excited by deep convection, make its simulation challenging. Improved representation of processes governing the QBO is expected to lead to better forecasts of the oscillation and its impacts, increased understanding of unusual events such as the two QBO disruptions observed since 2016, and more reliable future projections of QBO behaviour under climate change.

The stratosphere: a review of the dynamics and variability

“…In addition, the relative importance of the different wave dissipation mechanisms (e.g. damping or critical layer wave breaking that occurs when the phase speed is close to the wind speed) is poorly understood (Anstey et al, 2022b).…”

“…These impacts also extend down into the troposphere (Sect. 6.3) and upward into the mesosphere and are commonly referred to as "QBO teleconnections" (Anstey et al, 2022b).…”

“…As the extratropical planetary wave activity is further modulated by the strength of the vortex (Sect. 2.4), this has, so far, hampered attempts to isolate which of these two proposed mechanisms for the HT effect is more relevant (Anstey et al, 2022b). More recently, Yamazaki et al (2020) have suggested an alternative tropospheric pathway for the boreal winter in which a convection anomaly resulting from the QBO effect on the tropical troposphere (see Sect.…”

“…Interactions found between the QBO stratospheric teleconnections and the solar cycle (Naito and Hirota, 1997) and El Niño-Southern Oscillation (ENSO; Garfinkel and Hartmann, 2007) make the causes of these differences uncertain as both the QBO and polar vortex variability are, separately, impacted by these longer timescale modes of variability. The QBO altitudes which exert the strongest influence on the extratropics is another uncertain aspect (Anstey et al, 2022b).…”

“…Despite waves from the troposphere being instrumental in most of the dynamical variability seen throughout the stratosphere, fluctuations in the wave flux from the troposphere are not essential to obtain the variability. Indeed, the QBO arises mostly from the feedback between the waves and the mean flow rather than an oscillation in the wave sources and filtering within the troposphere (Anstey et al, 2022b). Likewise, Hardiman et al (2020) argue that, for monthly means, vacillations between the waves and mean flow can explain much of the sub-seasonal and interannual variability in the strength of the NH polar vortex.…”