제목:  Toward better predictability of the quasi-biennial oscillation: Understanding period fluctuations​​

​​​​​​​​​​​​​​​​​​연사: 김영하 박사 (서울대학교)​​​​​​​​​​​​​​​​

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일시: 2026년 4월 14일 화요일 16:00

장소: 과학관 B102호​​

Abstract:

​The tropical stratospheric flow alternates between westerly and easterly regimes with periods varying between 20 and 35 months, and these wind regimes descend toward the tropopause over time. This so-called quasi-biennial oscillation (QBO) exerts global impacts on large-scale circulations across the stratosphere, troposphere, and surface climate, providing an important source of predictability on seasonal to interannual timescales. However, prediction models have difficulty in reproducing the observed variations in the QBO period, and the underlying mechanisms of these variations remain poorly understood. 

A notable feature of the QBO is the seasonality of its phase-descent speed, which in turn contributes to the period variability. Previous studies have attributed this to the annual cycle of the Brewer–Dobson circulation in the stratosphere: i.e., stronger tropical upwelling during boreal winter should slow the phase descent. Yet, models fail to reproduce the observed seasonality in the QBO descent speed, even though the annual cycle of the circulation can be reasonably simulated. Variability in tropical wave activity is another possibility, given that the QBO is driven by wave-induced momentum forcing. However, direct evidence linking the waves to the observed QBO period and descent variations has been lacking. 

This study shows that the seasonally varying activity of mesoscale waves (gravity waves) in the tropical tropopause layer primarily explains the observed variability in the QBO descent speed, using ERA5 reanalysis. In particular, gravity waves that carries westward momentum modulate the descent of the easterly QBO phase with a semiannual cycle, thereby controlling the period variations. Compared to this gravity-wave variability, stratospheric upwelling variability plays a relatively minor role above ~40 hPa, although it becomes comparable below that level. The semiannual variation in gravity wave activity arises from the seasonality of tropical convection and tropopause-layer winds. The implications of these findings are discussed.