Abstract
AbstractThe quasi‐biennial oscillation (QBO) of tropical stratospheric winds was disrupted during the 2019/20 Northern Hemisphere winter. We show that this latest disruption to the regular QBO cycling was similar in many respects to that seen in 2016, but initiated by horizontal momentum transport from the Southern Hemisphere. The predictable signal associated with the QBO's quasi‐regular phase progression is lost during disruptions and the oscillation reemerges after a few months significantly shifted in phase from what would be expected if it had progressed uninterrupted. We infer from an increased wave‐momentum flux into equatorial latitudes seen in climate model projections that disruptions to the QBO are likely to become more common in future. Consequently, it is possible that in the future, the QBO could be a less reliable source of predictability on lead times extending out to several years than it currently is.
Highlights
The quasi-biennial oscillation (QBO) consists of alternating layers of eastward and westward wind that gradually descend through the tropical stratosphere before dissipating near the tropopause (Baldwin et al, 2001; Tegtmeier et al, 2020)
It has been argued that horizontally propagating waves from the midlatitudes into the tropics play a minor part in the QBO's evolution (O’Sullivan, 1997), which is consistent with the QBO's remarkable cycle-to-cycle consistency and predictability extending out to a few years (Scaife, Athanassiadou, et al, 2014)
The QBO has been disrupted again for only the second time since its discovery. Both disruptions occurred near 40 hPa and were initiated by historically large forcing from extratropical waves
Summary
The quasi-biennial oscillation (QBO) consists of alternating layers of eastward and westward wind that gradually descend through the tropical stratosphere before dissipating near the tropopause (Baldwin et al, 2001; Tegtmeier et al, 2020). It has been argued that horizontally propagating waves from the midlatitudes into the tropics play a minor part in the QBO's evolution (O’Sullivan, 1997), which is consistent with the QBO's remarkable cycle-to-cycle consistency and predictability extending out to a few years (Scaife, Athanassiadou, et al, 2014). This highly predictable QBO signal is encapsulated by the time evolution of the amplitudes of the leading two empirical orthogonal functions (EOFs) of zonal wind vertical structure, which capture ∼90% of the month-to-month variability (Wallace et al, 1993). We use future projections from climate models to assess whether QBO disruptions could potentially be an emerging signal of climate change
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