Quasi-biennial oscillation steers Ural blocking and Eurasian winter weather

One of the most consequential atmospheric phenomena of Eurasian winter—Ural blocking—does not act alone. According to a new study published in Climate Dynamics, the rhythm of winds high above the equator, known as the quasi-biennial oscillation, determines where the cold air unleashed by Ural blocking will go, how long it will linger, and how deeply it will reach into the stratosphere. The research, based on more than six decades of reanalysis data, offers a fresh dynamical framework that could sharpen subseasonal-to-seasonal forecasts of winter cold across a continent that is home to hundreds of millions of people.

Ural blocking is a quasi-stationary anticyclone that routinely forms over the Ural Mountains, typically persisting for five to ten days at a time. When it intensifies, it reinforces the Siberian High and drives frigid air masses far from their source region, shaping temperature extremes from Central Asia to Europe and the Arctic. The blocking also pumps planetary waves upward into the stratosphere, where they can weaken the polar vortex and nudge the Arctic and North Atlantic Oscillations toward their negative phases—states classically associated with cold outbreaks in the midlatitudes. Yet despite decades of research on both the blocking itself and on the quasi-biennial oscillation, the interaction between the two had remained largely unexplored.

The quasi-biennial oscillation, or QBO, is a natural stratospheric cycle in which equatorial zonal winds alternate between westerly and easterly phases with a period of roughly 24 to 30 months. First documented in the 1960s, it is the most regular stratospheric cycle known, and its reach extends well beyond the tropics. Since the classic Holton–Tan mechanism was proposed in 1980, scientists have understood that the QBO can modulate the polar vortex and thereby influence midlatitude winter weather. What the new study by Ramin Ahmadi of the University of Tehran and Omid Alizadeh of Humboldt-Universität zu Berlin demonstrates is that this tropical oscillation leaves a distinctive fingerprint on the very anatomy of Ural blocking events.

The researchers compositing daily fields from the NCEP/NCAR reanalysis covering the winters of 1960 through 2023, linearly detrended to remove the signature of long-term warming and standardized so that different variables and regions could be compared on equal footing. They classified each winter month as westerly QBO (WQBO) or easterly QBO (EQBO) using the zonal-mean zonal wind at 70 hPa averaged between 10 degrees south and 10 degrees north, retaining only months whose standardized index exceeded plus or minus 0.9 standard deviations. This deliberately strict threshold yielded 37 westerly-phase months and 45 easterly-phase months, large enough samples to ensure statistical robustness. Blocking events themselves were identified with a two-dimensional diagnostic based on reversals of the meridional gradient of 500 hPa geopotential height, requiring affected areas of at least 500,000 square kilometers persisting for at least five consecutive days.

The results reveal two strikingly different regimes. During the westerly QBO phase, Ural blocking anomalies are stronger, more zonally elongated, and largely confined to the troposphere. The blocking high generates powerful easterly and northeasterly wind anomalies along its southern flank that sweep cold Siberian air westward into eastern and central Europe and the western Mediterranean. A companion cyclonic anomaly develops over the Mediterranean, strengthening northerly winds and reinforcing the cold there. During the easterly QBO phase, by contrast, the blocking becomes more meridionally oriented and vertically extended, with geopotential height anomalies reaching all the way up to 10 hPa in the stratosphere. Cold anomalies instead stream southwestward from Central Asia toward the Middle East and the Mediterranean, with the strongest cooling exceeding half a standard deviation over northwestern China, western Mongolia, and eastern Kazakhstan.

The life cycles differ as well. In the westerly phase, negative temperature anomalies emerge almost immediately after blocking onset and peak around day four, with minima over Eastern Europe and near Lake Baikal, before gradually weakening. In the easterly phase, the most intense cooling arrives surprisingly late—six to seven days after onset, well into the decay stage of the blocking—with cold anomalies stretching from the Mediterranean and Middle East across Central Asia to Mongolia and Siberia. Crucially, blocking events last longer under EQBO conditions: positive height anomalies persist for up to nine days, whereas under WQBO conditions they typically dissipate within seven. Longer-lived blocking means more prolonged cold, an insight with direct implications for extended-range forecasting.

The mechanism underlying these contrasts lies in the behavior of planetary waves, which the authors diagnosed using the Eliassen–Palm flux, a standard tool that tracks wave propagation and wave–mean flow interaction in the meridional plane. During the westerly QBO, planetary wave propagation is enhanced poleward but weakened upward; during the easterly QBO the pattern reverses. The WQBO is associated with a strengthened subtropical jet and weaker stratosphere–troposphere coupling, whereas the EQBO produces a weakened, poleward-shifted jet and stronger coupling, consistent with the Holton–Tan framework in which the easterly phase narrows the midlatitude waveguide and directs more wave activity toward the polar stratosphere, weakening the polar vortex.

The vertical structure of the Eliassen–Palm flux divergence proved particularly revealing. Under WQBO conditions, divergence of wave activity in the lower-to-middle troposphere and convergence in the upper troposphere are both intensified, especially during blocking events. This configuration promotes conversion between eddy kinetic energy and zonal kinetic energy in ways that are unfavorable for maintaining the blocking, explaining its shorter lifetime. Under EQBO conditions, these divergence and convergence features are weaker—likely because a larger fraction of upward-propagating waves is reflected downward, so divergence associated with upward propagation is offset by convergence from downward-propagating waves. The net effect is more sustained eddy kinetic energy and longer-lived blocking.

The study also documents a distinctive barotropic structure during the easterly phase: a broad cyclonic circulation in the lower stratosphere sits directly above the tropospheric cyclone downstream of the blocking, from the Middle East to East Asia, indicating vertically aligned anomalies that favor persistence. During the westerly phase, this downstream trough weakens markedly in the lower stratosphere. Sea-level pressure composites reinforce the picture: WQBO blocking features a strong but localized high-pressure system over the Urals and Scandinavia, while EQBO blocking shows a weaker center whose pressure anomalies extend farther south toward southern Europe and the Middle East, carrying cold air with them.

The findings carry practical weight for forecasters. Subseasonal-to-seasonal prediction systems increasingly exploit stratospheric signals, and the QBO is already recognized as a source of predictable skill at lead times of weeks to months. Knowing that the QBO phase modulates not only the probability of cold extremes but also their geographic footprint—which regions of Eurasia will freeze and for how long—offers forecasters a physically grounded way to condition their outlooks. The authors note that the QBO’s influence likely operates through both the classical stratospheric pathway and an independent tropospheric pathway involving wave trains emanating from tropical convection and propagating through the upper troposphere–lower stratosphere, a route recently highlighted in separate work on QBO teleconnections with Eurasian and North American climate.

The study also situates Ural blocking within the broader family of QBO–extratropical interactions. Previous research has shown that a weakened polar vortex favors blocking over Greenland and shifts blocking frequency across the Atlantic storm track, and that a negative North Atlantic Oscillation is associated with a westward displacement of Ural blocking. The new analysis is consistent with these results: during EQBO winters, when the NAO tends toward its negative phase, blocking stagnates and drifts slightly westward, whereas during WQBO winters it shifts eastward. Whether the QBO similarly modulates blocking in other sectors of the Northern Hemisphere remains an open question that the authors flag for future research.

For a phenomenon as consequential as Ural blocking—one implicated in warm Arctic–cold Siberia patterns, sea-ice loss in the Barents–Kara Seas, and some of Eurasia’s most severe cold waves—identifying a tropical pacemaker represents a meaningful advance. The message of this work is that the same blocking event can paint very different weather maps depending on which way equatorial stratospheric winds happen to be blowing, and that those winds, cycling with a quiet regularity of their own, may hold one of the keys to anticipating Eurasia’s harshest winters weeks in advance.