Balloon‐Borne Observations of Short Vertical Wavelength Gravity Waves and Interaction With QBO Winds

Abstract

The quasi‐biennial oscillation (QBO), a ubiquitous feature of the zonal mean zonal winds in the equatorial lower stratosphere, is forced by selective dissipation of atmospheric waves that range in periods from days to hours. However, QBO circulations in numerical models tend to be weak compared with observations, probably because of limited vertical resolution that cannot adequately resolve gravity waves and the height range over which they dissipate. Observations are required to help quantify wave effects. The passage of a superpressure balloon (SPB) near a radiosonde launch site in the equatorial Western Pacific during the transition from the eastward to westward phase of the QBO at 20 km permits a coordinated study of the intrinsic frequencies and vertical structures of two inertia‐gravity wave packets with periods near 1 day and 3 days, respectively. Both waves have large horizontal wavelengths of about 970 and 5,500 km. The complementary nature of the observations provided information on their momentum fluxes and the evolution of the waves in the vertical. The near 1 day westward propagating wave has a critical level near 20 km, while the eastward propagating 3‐day wave is able to propagate through to heights near 30 km before dissipation. Estimates of the forcing provided by the momentum flux convergence, taking into account the duration and scale of the forcing, suggests zonal force of about 0.3–0.4 m s−1 day−1 for the 1‐day wave and about 0.4–0.6 m s−1 day−1 for the 3‐day wave, which acts for several days.

How Can We Improve the Driving of the Quasi‐Biennial Oscillation in Climate Models?

“…Vincent and Alexander (2020) neatly illustrates how observations covering virtually the whole vertical domain between wave sources and dissipation heights can be used to advance our understanding of wave forcing in the tropical stratosphere. The forthcoming ∼50 SPB flights that will be performed in 2019–2025 within Strateole‐2 (Haase et al, 2018) should provide many new opportunities to extend this work and explore the role of all equatorial waves in driving the QBO.…”

“…The study by Vincent and Alexander (2020) most importantly highlights that efforts to improve our confidence in future climate projections in part rely on a better understanding of fundamental processes in the atmosphere. It is thus critical that funding agencies worldwide continue to support the networks and campaigns that provide the detailed observations required to address these processes.…”

“…The wave packets studied in Vincent and Alexander (2020) correspond to inertia-gravity with relatively large horizontal scales that would likely favor their explicit representation in modern climate models. Yet their method can certainly extend to pure gravity waves with shorter wavelengths (with perhaps special care at the very high frequency end Vincent & Hertzog, 2014), which are essentially handled by parameterizations in those models.…”

“…Vincent and Alexander (2020) have focused on two gravity‐wave packets with periods near 1 and 3 days and horizontal wavelengths near 970 and 5,500 km, which were observed while an SPB flight was flying close to Manus Island in the western Pacific. SPB observations are particularly well suited to described wave‐packet characteristics (Boccara et al, 2008; Geller et al, 2013; Vincent & Hertzog, 2014), and Vincent and Alexander (2020) used SPB observations to derive the wave momentum fluxes and phase speeds. They were then able to identify the same wave packets in the radiosoundings performed on Manus Island and to provide an estimate of their height of dissipation, which occurs when the wave phase speed matches the background wind (the so‐called critical level).…”

“…They are advected by the wind and are thus able to circumnavigate the Earth while collecting observations with a temporal resolution sufficient to resolve the whole atmospheric wave spectrum (Podglajen et al, 2016). Vincent and Alexander (2020) have focused on two gravity-wave packets with periods near 1 and 3 days and horizontal wavelengths near 970 and 5,500 km, which were observed while an SPB flight was flying close to Manus Island in the western Pacific. SPB observations are particularly well suited to described wave-packet characteristics (Boccara et al, 2008;Geller et al, 2013;Vincent & Hertzog, 2014), and Vincent and Alexander (2020) used SPB observations to derive the wave momentum fluxes and phase speeds.…”