Quasi-Biennial Oscillation

The Quasi-Biennial Oscillation (abbreviated QBO) is a quasi-periodic, downward-propagating oscillation of the equatorial zonal wind in the tropical stratosphere between roughly 16 and 50 kilometers altitude (100 to 1 hPa pressure levels), in which belts of prevailing easterly (westward) and westerly (eastward) winds alternate with an average period of ~28 months (ranging from 22 to 34 months). Discovered independently in 1960–1961 by British meteorologist Richard J. Reed and German meteorologist Richard A. Ebdon from equatorial radiosonde balloon soundings (such as those launched at Canton Island and Singapore), the QBO is the most regular, predictable internal oscillation of Earth's atmosphere not directly locked to an astronomical solar or lunar cycle. Its physical explanation by Richard S. Lindzen and James R. Holton (1968, 1972) established the modern paradigm of wave–mean flow interaction in geophysical fluid dynamics.

Continuous daily rawinsonde observations from Singapore (1.3°N, 103.9°E) and Canton/Gan Island since 1953 reveal five defining kinematic properties of the QBO:

Why do zonal winds on the equator reverse every 14 months and descend downward against the upward Brewer–Dobson circulation? Because the angular momentum per unit mass of a ring of air at the equator (M = a[Ωa + ū]) exceeds the angular momentum of the planetary surface (Ωa²) whenever the wind is westerly (ū > 0), Hide's theorem proves that no axisymmetric meridional overturning circulation or diffusive mixing can produce equatorial superrotation (ū > 0); only non-axisymmetric eddies (waves) propagating upward from below can transport eastward momentum into the equatorial stratosphere.

In 1968 and 1972, Richard S. Lindzen and James R. Holton formulated the wave-driven QBO theory (demonstrated in a laboratory Rotating-Annulus water-tank experiment by Alan Plumb and A. D. McEwan in 1978):

1. Upward-Propagating Tropical Waves: Deep cumulonimbus convection in the Intertropical Convergence Zone (ITCZ) and the Madden–Julian Oscillation excites a broad spectrum of vertically propagating atmospheric waves that enter the lower stratosphere:
   • Eastward-Propagating Waves (c > 0, carrying positive/westerly Eliassen–Palm momentum flux u'w' > 0): Equatorial Kelvin waves (period 10–20 days, zonal wavenumber k = 1–3, phase speed c ≈ +20 to +30 m s⁻¹) and eastward inertia–gravity and small-scale gravity waves.
   • Westward-Propagating Waves (c < 0, carrying negative/easterly momentum flux u'w' < 0): Mixed Rossby–Gravity (Yanai) waves (period 4–5 days, k = 4, c ≈ −20 to −25 m s⁻¹), equatorial Rossby waves, and westward gravity waves.
2. Selective Wave Absorption and Radiative Damping: As an eastward wave with phase speed c > 0 propagates upward into a descending zone of westerly vertical wind shear (∂ū/∂z > 0) where the background mean flow ū(z) approaches c, its intrinsic Doppler-shifted frequency ω̂ = k(c − ū) and vertical group velocity c_gz ∝ (c − ū)² drop toward zero. Approaching this critical level (ū → c), the wave's vertical wavelength compresses, enabling Newtonian infrared radiative cooling (radiative damping by CO₂ and O₃) and mechanical wave breaking to dissipate the wave.
3. Downward Descent of the Shear Zone: By the Eliassen–Palm non-acceleration theorem, wave dissipation deposits the wave's horizontal momentum directly into the mean flow (∂ū/∂t ≈ −(1/ρ₀) ∂(ρ₀ u'w')/∂z) just below the altitude of maximum westerly wind. This accelerates the air directly below the westerly jet core into a westerly flow, causing the westerly regime to descend downward with time.
4. Shadowing and Phase Reversal: While the low-altitude westerly belt is present, it absorbs and filters out upward-propagating eastward waves, leaving westward waves (c < 0) free to propagate unimpeded into the upper stratosphere (~30–40 km), where they dissipate and build a new easterly wind belt aloft. Once the lower westerly belt reaches the tropopause (~17 km) and dissipates by turbulent friction, the upper easterly belt descends in turn, blocking westward waves and allowing eastward Kelvin and gravity waves to penetrate aloft and initiate the next westerly cycle.

Although the QBO's primary wind oscillation is confined to 15°S–15°N, it exerts three global teleconnections across the stratosphere and troposphere:

1. The Holton–Tan Effect on the Winter Polar Vortex

Discovered by James R. Holton and Hsiu-Chi Tan in 1980, the Holton–Tan effect links the equatorial QBO phase at 40–50 hPa to the wintertime strength of the extratropical Northern Hemisphere stratospheric polar vortex:

• During the Easterly QBO (QBO-E) phase, the zero-wind boundary line (ū = 0) for stationary planetary Rossby waves is shifted into the winter subtropics (~15°N–20°N). This narrowing of the extratropical waveguide refracts upward-propagating planetary Rossby waves poleward into the high-latitude stratosphere, increasing wave breaking and triggering more frequent Sudden Stratospheric Warming (SSW) events, a weaker polar vortex, and a negative North Atlantic Oscillation (NAO−) winter regime over Europe and North America.
• During the Westerly QBO (QBO-W) phase, the zero-wind line lies in the summer hemisphere, allowing planetary Rossby waves to propagate equatorward away from the pole, maintaining a cold, undisturbed, and strong polar vortex and favoring a positive NAO+ winter regime.

2. Modulation of the Madden–Julian Oscillation (QBO–MJO Link)

In 2016, Seok-Woo Son, Chidong Zhang, and colleagues discovered that the QBO modulates the wintertime amplitude and predictability of the tropospheric Madden–Julian Oscillation (explaining up to 40%–50% of interannual MJO variance during boreal winter): during the Easterly QBO (QBO-E) at 50 hPa, the cold temperature anomaly (−2 to −3 K) induced at the tropical tropopause layer (TTL, ~100–70 hPa) destabilizes the upper-tropospheric static stability and promotes high-altitude cirrus cloud radiative feedbacks, making winter MJO events stronger, slower, and predictable 1 to 2 weeks longer than during QBO-W.

3. Stratospheric Ozone and the "Tape Recorder" Effect

Because easterly and westerly QBO shear zones drive a secondary meridional circulation (the QBO-induced meridional circulation) that modulates the speed of tropical upwelling across the tropopause, the QBO imprints a ~28-month oscillation onto total column ozone (±8 to ±12 Dobson Units in the tropics and subtropics) and modulates the upward ascent speed of the stratospheric water-vapor "tape recorder" discovered by Philip W. Mote and colleagues (1996).

From the start of continuous radiosonde measurements in 1953 until 2015—spanning more than 27 consecutive cycles over 62 years—the QBO never failed to propagate steadily downward from 10 hPa to 100 hPa. However, in January–February 2016 (during the strong 2015–2016 El Niño) an unexpected QBO disruption occurred (Osprey et al., 2016): an anomalous belt of easterlies developed inside the descending westerly jet near 40 hPa (~22 km), splitting the westerly phase in two and forcing the upper westerly branch upward. Momentum-budget diagnostics demonstrated that such disruptions are triggered by lateral intrusion of extratropical planetary Rossby waves propagating from the winter hemisphere into the equatorial stratosphere and depositing westward momentum.

In January 2020, equatorial radiosonde soundings at Singapore and satellite limb profiles documented the second QBO disruption in the 70-year observational record, in which equatorward-propagating Southern Hemisphere planetary Rossby waves deposited westward momentum near 40 hPa and severed the descending westerly jet into an anomalous easterly band (Anstey et al., 2021).

In August 2021, multi-model projections from the Coupled Model Intercomparison Project (CMIP6) and the Quasi-Biennial Oscillation initiative (QBOi) ensemble demonstrated that greenhouse-gas-driven acceleration of tropical Brewer–Dobson upwelling is projected to weaken QBO wind amplitudes in the lower stratosphere (~50–70 hPa) and increase the risk of Rossby-wave-induced QBO disruptions over the twenty-first century (Anstey et al., 2021).

In September 2022, a synthesis in Nature Reviews Earth & Environment quantified the tropospheric teleconnections of the QBO across subseasonal-to-seasonal numerical weather prediction systems, confirming that the 50 hPa Easterly QBO phase amplifies boreal-winter Madden–Julian Oscillation convection and extends MJO skill horizons by 7 to 14 days via upper-tropospheric static stability reductions (Anstey et al., 2022).

In December 2022, NASA's Aura Microwave Limb Sounder (MLS) and MERRA-2 reanalysis diagnostics showed that the ~150 Tg stratospheric water-vapor plume injected by the January 2022 Hunga Tonga–Hunga Haʻapai eruption induced a −3 to −4 K radiative cooling anomaly in the tropical middle stratosphere, perturbing equatorial thermal-wind shear balance and altering the secondary meridional circulation of the QBO (Coy et al., 2022).

Why do QBO wind belts propagate downward even though tropical stratospheric air rises upward?

In the tropical stratosphere, the mean Brewer–Dobson circulation transports air parcels upward at ~0.2 to 0.3 mm s⁻¹, yet QBO easterly and westerly wind belts descend downward at ~1 km per month (~0.4 mm s⁻¹). This occurs because the descending wind belts are wave-driven phase patterns, not descending air parcels. Upward-propagating tropical Kelvin and gravity waves are absorbed at critical levels just below the core of the existing wind jet, depositing their momentum at the base of the shear zone and continuously shifting the level of maximum wind downward over time.

What is the Holton–Tan effect and how does the equatorial QBO affect winter weather in Europe and North America?

Formulated by James Holton and Hsiu-Chi Tan in 1980, the Holton–Tan effect describes how the equatorial QBO phase at 40–50 hPa modulates the wintertime Northern Hemisphere stratospheric polar vortex. Because stationary planetary Rossby waves cannot propagate into easterly winds, an Easterly QBO (QBO-E) shifts the zero-wind boundary into the winter subtropics and focuses Rossby wave breaking onto the polar vortex, increasing the frequency of Sudden Stratospheric Warming events and cold negative-NAO winter outbreaks across northern Eurasia and eastern North America.

What caused the 2016 and 2020 QBO disruptions?

During both 2015–2016 and 2019–2020, anomalous quasi-stationary Rossby wave trains propagated horizontally from the mid-latitude winter hemisphere across the subtropical waveguide into the equatorial lower stratosphere (~40 hPa). When these extratropical Rossby waves broke near the equator, they deposited westward (easterly) momentum right inside the core of the descending westerly QBO jet, splitting the westerly phase and interrupting the normal 28-month downward cycle.

1. Lindzen, R. S., & Holton, J. R. (1968). A theory of the quasi-biennial oscillation025<1095:ATOTQB>2.0.CO;2). Journal of the Atmospheric Sciences, 25(6), 1095–1107.
2. Holton, J. R., & Tan, H.-C. (1980). The influence of the equatorial quasi-biennial oscillation on the global circulation at 50 mb037<2200:TIOTEQ>2.0.CO;2). Journal of the Atmospheric Sciences, 37(10), 2200–2208.
3. Baldwin, M. P., Gray, L. J., Dunkerton, T. J., Hamilton, K., Haynes, P. H., Randel, W. J., Holton, J. R., Alexander, M. J., Hirota, I., Horinouchi, T., Jones, D. B. A., Kinnersley, J. S., Marquardt, C., Sato, K., & Takahashi, M. (2001). The Quasi-Biennial Oscillation. Reviews of Geophysics, 39(2), 179–229.
4. Osprey, S. M., Butchart, N., Knight, J. R., Scaife, A. A., Hamilton, K., Anstey, J. A., Schenzinger, V., & Zhang, C. (2016). An unexpected disruption of the atmospheric quasi-biennial oscillation. Science, 353(6306), 1424–1427.
5. Anstey, J. A., Banyard, T. P., Butchart, N., Coy, L., Newman, P. A., Osprey, S., & Wright, C. J. (2021). Prospects for future disruptions of the stratospheric Quasi-Biennial Oscillation. Geophysical Research Letters, 48(15), e2021GL093058.
6. Anstey, J. A., Osprey, S. M., Alexander, J., Baldwin, M. P., Butchart, N., Gray, L., Kawatani, Y., Newman, P. A., & Richter, J. H. (2022). Impacts, processes and projections of the quasi-biennial oscillation. Nature Reviews Earth & Environment, 3(9), 588–603.
7. Coy, L., Newman, P. A., Wargan, K., Partyka, G., Strahan, S. E., & Pawson, S. (2022). Stratospheric circulation changes associated with the Hunga Tonga–Hunga Ha'apai eruption. Geophysical Research Letters, 49(22), e2022GL100982.