A revisit and comparison of the quasi-biennial oscillation (QBO) disruption events in 2015/16 and 2019/20

As the most dominant variability of the tropical stratospheric zonal winds on the interannual timescale, the quasi-biennial oscillation (QBO) has a significant impact on weather and climate on the hemispheric and even global scales. In contrast to the process of gradual downward propagation of the alternated easterly and westerly winds from the upper to the lower stratosphere, the downward propagation of westerly winds can be abruptly cut off by a rapid development of equatorial easterly winds in the lower stratosphere, which is known as the westerly QBO disruption event. Such an event has been observed twice since the discovery of the QBO in 1953. The QBO disruption has been widely studied in past years, which is hypothesized to be driven by strong extratropical Rossby waves and abrupt strengthening of equatorial waves under a strong westerly background in the tropical lower stratosphere. Various types of waves were active during the two events (2015/16 and 2019/20). During the 2015/16 QBO disruption, the sudden reversal of the QBO winds was attributed to the combination of the westerly QBO phase and a strong Brewer-Dobson circulation. During the 2020 QBO disruption, extratropical waves were enhanced especially in the Southern Hemisphere and propagated equatorward. In addition to the wind disruption, the usual evolution of the stratospheric temperature and trace gases was also disrupted and exhibited anomalously irregular behaviors. Climate models suggest that QBO disruption events might increase with global warming, which pose unprecedented challenges for the QBO prediction and understanding.