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In the tropical stratosphere, deep layers of eastward and westward winds encircle the globe and descend regularly from the upper stratosphere to the tropical tropopause. With a complete cycle typically lasting almost 2.5 years, this quasi-biennial oscillation (QBO) is arguably the most predictable mode of atmospheric variability that is not linked to the changing seasons. The QBO affects climate phenomena outside the tropical stratosphere, including ozone transport, the North Atlantic Oscillation and the Madden–Julian Oscillation, and its high predictability could enable better forecasts of these phenomena if models can accurately represent the coupling processes. Climate and forecasting models are increasingly able to simulate stratospheric oscillations resembling the QBO, but exhibit common systematic errors such as weak amplitude in the lowermost tropical stratosphere. Uncertainties about the waves that force the oscillation, particularly the momentum fluxes from small-scale gravity waves excited by deep convection, make its simulation challenging. Improved representation of the processes governing the QBO is expected to lead to better forecasts of the oscillation and its impacts, increased understanding of unusual events such as the two QBO disruptions observed since 2016, and more reliable future projections of QBO behaviour under climate change.

The quasi-biennial oscillation (QBO) was unexpectedly disrupted for only the second time in the historical record during the 2019/20 boreal winter. As the dominant mode of atmospheric variability in the tropical stratosphere, and a significant source of seasonal predictability globally, understanding the drivers behind this unusual behaviour is very important. Here, novel data from Aeolus, the first Doppler wind lidar in space, is used to observe the 2019/20 QBO disruption. Aeolus is the first satellite able to observe winds at high resolution on a global scale, and is therefore a uniquely capable platform for studying the evolution of the disruption and the broader circulation changes triggered by it. This study therefore contains the first direct wind observations of the QBO from space, and exploits measurements from a special Aeolus scanning mode, implemented to observe this disruption as it happened. Aeolus observes easterly winds of up to 20 ms−1 in the core of the disruption jet during July 2020. By co-locating with radiosonde measurements from Singapore and ERA5 reanalysis, like-for-like comparisons of the observed wind structures in the tropical stratosphere are produced, showing equatorial Kelvin wave activity and key parts of the Walker Circulation during the disruption period. The onset of the disruption easterly jet occurs 5 days earlier in Aeolus observations compared with the reanalysis. This analysis highlights how Aeolus and future Doppler wind lidar satellites can deepen our understanding of the QBO, its disruptions, and the tropical upper-troposphere lower-stratosphere region more generally.

The January 2022 Hunga Tonga-Hunga Ha'apai eruption was one of the most explosive volcanic events of the modern era(1,2), producing a vertical plume that peaked more than 50 km above the Earth(3). The initial explosion and subsequent plume triggered atmospheric waves that propagated around the world multiple times(4). A global-scale wave response of this magnitude from a single source has not previously been observed. Here we show the details of this response, using a comprehensive set of satellite and ground-based observations to quantify it from surface to ionosphere. A broad spectrum of waves was triggered by the initial explosion, including Lamb waves(5,6) propagating at phase speeds of 318.2 +/- 6 m s(-1) at surface level and between 308 +/- 5 to 319 +/- 4 m s(-1) in the stratosphere, and gravity waves(7) propagating at 238 +/- 3 to 269 +/- 3 m s(-1) in the stratosphere. Gravity waves at sub-ionospheric heights have not previously been observed propagating at this speed or over the whole Earth from a single source(8,9). Latent heat release from the plume remained the most significant individual gravity wave source worldwide for more than 12 h, producing circular wavefronts visible across the Pacific basin in satellite observations. A single source dominating such a large region is also unique in the observational record. The Hunga Tonga eruption represents a key natural experiment in how the atmosphere responds to a sudden point-source-driven state change, which will be of use for improving weather and climate models. Royal Society University Research Fellowship [UF160545]; NERC [NE/S00985X/1]; NASA Heliophysics DRIVE Science Center [80NSSC20K0628]; NERC Fellowship [NE/P006450/1]; European Research Council (ERC) under the European Union [742909]; NASA AIM Small Explorer Program [NAS5-03132] Published version C.J.W. is supported by a Royal Society University Research Fellowship, reference no. UF160545. C.J.W. and N.P.H. are supported by NERC grant no. NE/S00985X/1. M.J.A. and C.E.R. were supported by a NASA Heliophysics DRIVE Science Center (grant no. 80NSSC20K0628). C.N.M. was supported by NERC Fellowship NE/P006450/1 for work underpinning this research. C.C. and M.B. received funding from the European Research Council (ERC) under the European Union's Horizon 2020 and innovation programme (grant agreement no. 742909, IASI-FT advanced ERC grant). J.C. was supported by the NASA AIM Small Explorer Program, contract no. NAS5-03132. The Australian Institute of Marine Sciences, the Australian Bureau of Meteorology and the Tongan Met Office are thanked for provision of surface station pressure data. We thank I. Krisch, N. Kaifler and B. Kaifler (all at the DLR, Oberpfaffenhofen, Germany) for assistance with preliminary data analysis, A. Boynard (LATMOS, Paris, France) for providing the H2O IASI data, S. Proud (RAL) for correcting some details of the geostationary imager measurements and E. Gryspeerdt (Imperial College, London, UK) for independent confirmation of the Lamb wave trigger time.

Stratospheric variability is examined in a vertically extended version of the Met Office global climate model. Equatorial variability includes the simulation of an internally generated quasi-biennial oscillation (QBO) and semiannual oscillation (SAO). Polar variability includes an examination of the frequency of sudden stratospheric warmings (SSW) and annular mode variability. Results from two different horizontal resolutions are also compared. Changes in gravity wave filtering at the higher resolution result in a slightly longer QBO that extends deeper into the lower stratosphere. At the higher resolution there is also a reduction in the occurrence rate of sudden stratospheric warmings, in better agreement with observations. This is linked with reduced levels of resolved waves entering the high-latitude stratosphere. Covariability of the tropical and extratropical stratosphere is seen, linking the phase of the QBO with disturbed NH winters, although this linkage is sporadic, in agreement with observations. Finally, tropospheric persistence time scales and seasonal variability for the northern and southern annular modes are significantly improved at the higher resolution, consistent with findings from other studies.