QBO

The QBO, Quasi-Biennial Oscillation, is the alternation of easterly and westerly zonal winds in the equatorial stratosphere, descending from about 10 hPa to 70 hPa with a mean period near 28 months. It is driven by upward-propagating equatorial waves (Kelvin and mixed Rossby-gravity waves plus gravity waves) that deposit momentum and force the wind reversal. The QBO modulates the strength of the Arctic polar vortex through the Holton-Tan effect, influences MJO activity, and affects tracer transport. Its regular descent was disrupted in 2015 to 2016 and again in 2019 to 2020.