The central puzzle the team confronted is one that has long frustrated the subseasonal-to-seasonal, or S2S, forecasting community. The skill of forecasts targeting East Asian winter cold surges does not vary smoothly or randomly; instead, it clusters. Some initialization dates yield forecasts that capture the timing and intensity of a surge with impressive accuracy, while others—initialized only days apart—miss the event entirely. Historically, the only way to know whether a forecast fell into a high-skill window was to wait several days after issuance, when verifying observations began to arrive. By then, of course, the forecast had already been delivered to energy planners, agricultural agencies, and disaster-preparedness officials, who had no way of knowing whether to lean on it or discount it. The new study asks a deceptively simple question: can those windows be recognized in advance, before the forecast is made, using precursor signals in the ocean?

To answer it, the researchers mined ECMWF hindcast archives covering the winters of 1997 through 2021, a quarter-century record that spans numerous El Niño and La Niña events and a wide range of Arctic sea-ice conditions. From this archive they systematically identified fourteen high-skill windows, encompassing fifty-six individual forecast cases in which week-three cold surge predictions performed exceptionally well. The first, and in some ways most sobering, finding was that the pre-forecast atmospheric circulation offered almost no help. When the team examined the state of the atmosphere in the days leading up to initialization, they could not reliably distinguish upcoming high-skill windows from ordinary periods, leaving a false alarm ratio of roughly nineteen percent. The atmosphere, in other words, does not announce when it is about to become predictable. The answer, it turned out, lay beneath the surface—in the sea.

Regardless of whether the tropical Pacific was in an El Niño or La Niña phase, every one of the fourteen high-skill windows was preceded by the same sea surface temperature pattern in the mid-latitude North Pacific: a warm anomaly over the western basin paired with a cold anomaly over the eastern basin, a configuration the authors describe as a warm-west/cold-east dipole. This dipole, averaged over the seven days before forecast initialization, emerged as a consistent precursor signal across all ENSO backgrounds. Its physical significance is considerable, because mid-latitude SST anomalies of this kind can reshape the baroclinic environment of the North Pacific storm track and excite Rossby wave trains—vast, undulating disturbances in the atmospheric flow that propagate downstream and can lock the wintertime circulation into persistent, high-impact patterns such as the Siberian high and Ural blocking.

Yet the study’s most important conclusion is that the North Pacific dipole, on its own, is not sufficient. A high-skill window materializes only when the dipole is joined by a matching sea surface temperature configuration in the other key ocean basins—and, remarkably, the state of those ancillary regions also determines which of two distinct dynamical pathways the atmosphere will follow. When warm SST anomalies occupy the Barents-Kara Seas, a condition widely regarded as an indicator of regional sea-ice loss, the Rossby wave train excited by the North Pacific dipole is favored to maintain itself and then to sustain the Ural blocking high, the anticyclonic anomaly over western Russia that acts as a gateway for Arctic air to spill into East Asia. In this pathway, the oceanic signal reinforces the blocking ridge, and the cold surge follows a well-teleconnected route from the polar reservoir southward.

The alternative pathway unfolds when cold anomalies instead dominate the Indian Ocean. In that case, the wave energy excited by the North Pacific dipole is confined to the North Pacific–polar sector rather than propagating through the Ural sector, and the circulation response takes the form of a meridional dipole that steers polar air directly southward into East Asia. Two different oceanic backgrounds, two different wave-guide behaviors, two different routes to the same destructive outcome. The practical implication is that forecasters cannot simply check one index or one basin; they must verify that the sea surface temperature anomalies in all the key regions—the mid-latitude North Pacific, the Barents-Kara Seas, the Indian Ocean, and the tropical Pacific—meet their respective thresholds at the same time. Only this simultaneous, basin-wide alignment marks a genuine high-skill window.

When the team applied this multi-region screening criterion to the hindcast archive, the results were dramatic. Forecasts selected by requiring all key-region SST thresholds to be satisfied simultaneously achieved hit rates of one hundred percent under La Niña-A conditions, ninety-five percent under La Niña-B conditions, and ninety percent under El Niño conditions. Those figures stand in stark contrast to the roughly nineteen percent false alarm ratio that prevailed when no such screening was applied. In effect, the oceanic precursor pattern functions as a reliability certificate: if the prescribed SST configuration is present at initialization, the week-three cold surge forecast can be issued with a degree of confidence that S2S prediction has rarely been able to claim, and if it is absent, users know to treat the forecast with caution.

Beyond its operational value, the study carries a deeper scientific message: forecast skill windows are not random accidents of chaos but physically traceable events, anchored in slow, predictable components of the climate system. The ocean evolves on timescales of weeks to months, far slower than the atmosphere, and its anomalies act as a kind of memory that conditions how atmospheric disturbances will amplify, propagate, and persist. By mapping which oceanic configurations render the atmosphere more predictable, the researchers have effectively converted an abstract question about ensemble spread and verification statistics into a concrete, observable checklist. The work also underscores the growing recognition that mid-latitude predictability is jointly governed by the tropics, the Arctic, and the mid-latitudes themselves, with the Barents-Kara Seas linking the story of Arctic sea-ice decline directly to the practical skill of winter forecasts thousands of kilometers away.

For the agencies that must act on winter forecasts—grid operators deciding when to pre-position fuel reserves, transportation authorities planning for ice and snow, and farmers protecting overwintering crops—the ability to know in advance that the coming two to three weeks are likely to be a high-skill period is of immediate practical value. It allows limited confidence to be spent where it is justified and withheld where it is not, sharpening decisions in energy dispatch and agricultural disaster prevention alike. As subseasonal prediction matures from a research frontier into an operational mainstay, studies of this kind suggest that the future of reliable week-three forecasting may depend less on faster supercomputers than on learning to read the ocean’s slow, patient signals before the atmosphere ever makes its move.

Subject of Research: Identifying sea surface temperature precursors that reveal high-skill windows in subseasonal forecasts of East Asian winter cold surges

Article Title: A "warm-west/cold-east" North Pacific sea surface temperature dipole, working in concert with anomalies in other key ocean regions, reveals when subseasonal forecasts of East Asian winter cold surges can be trusted

Article References: A "warm-west/cold-east" North Pacific sea surface temperature dipole, working in concert with anomalies in other key ocean regions, reveals when subseasonal forecasts of East Asian winter cold surges can be trusted. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: subseasonal forecasting, East Asian winter monsoon, cold surges, sea surface temperature, North Pacific dipole, El Niño, La Niña, Barents-Kara Seas, Ural blocking, Rossby wave train, ECMWF hindcasts, forecast skill windows

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.