제목: Role of tropical lower stratosphere winds in quasi-biennial oscillation disruptions

The quasi-biennial oscillation (QBO) in the tropical stratosphere is the dominant form of interannual variability in the zonal-mean zonal wind from 5 to 100 hPa, which consists of alternating, downward-propagating easterly and westerly winds with an irregular period of 20 to 35 months. It has been shown that the QBO is generated by wave-mean flow interaction, where the waves include equatorial Kelvin, Rossby, mixed Rossby-gravity (MRG), inertia gravity (IG), and small-scale gravity waves (GWs). Since radiosonde observations in the equatorial stratosphere began to be recorded, the QBO has shown downward propagation with time. In 2016, however, the westerly quasi-biennial oscillation (WQBO) in the equatorial stratosphere was unprecedentedly disrupted by westward forcing near 40 hPa; this was followed by another disruption in 2020. Strong extratropical Rossby waves propagating toward the tropics were considered the main cause of the disruptions, but why the zonal wind is reversed only in the middle of the WQBO remains unclear. Here, we show that strong westerly winds in the equatorial lower stratosphere (70-100 hPa) help to disrupt the WQBO by hindering the wind reversal at its base. They also help equatorial westward waves propagate further upward, increasing the negative forcing at around 40 hPa that drives the QBO disruptions. Westerly winds in the equatorial lower stratosphere have been increasing in the past, and are projected to increase in a warmer climate. Such background wind changes may allow more frequent QBO disruptions in the future, leading to less predictability in atmospheric weather and climate systems.