A new study published in Climate Dynamics by Meirong Wang, Anmin Duan, and Jun Wang offers the most detailed answer yet. Using the Japanese 55-year Reanalysis dataset, known as JRA-55, together with a moist static energy budget analysis and barotropic energy diagnostics, the researchers traced the life histories of QBWO events that propagate outward from the plateau in both meridional directions. What they found is a striking asymmetry that rewrites how scientists should think about the plateau’s role in Asian weather: the northward branch of the oscillation dies quickly inside the plateau itself, surviving only about six days, while the southward branch lives roughly twice as long, crossing the Himalayas and marching all the way to the edge of the tropics before fading near 5°N.

The asymmetry matters because the two branches influence entirely different populations. The short-lived northward disturbances remain trapped within the plateau interior, where they modulate local convection and the generation of plateau vortices—shallow low-pressure systems that can later drift eastward and trigger heavy rain in China. The long-lived southward disturbances, by contrast, descend the southern slopes of the Himalayas and imprint themselves directly onto South Asian monsoon precipitation, modulating the life-giving rains that more than a billion people depend on. Understanding why one branch flourishes and the other withers therefore has direct consequences for forecasting monsoon variability on the critical two-to-three-week timescale, a window that sits squarely within the emerging frontier of subseasonal-to-seasonal prediction.

The researchers’ energy diagnostics revealed that the two branches are fueled in fundamentally different ways. The northward branch, they found, is dynamically dominated. Its principal energy source is not moisture or convection but a process known as upscale kinetic energy extraction: the disturbance essentially siphons kinetic energy from the horizontal shear of the Subtropical Westerly Jet, the powerful ribbon of winds that flows around the plateau’s northern flank. This barotropic energy conversion can sustain the disturbance for a while, but the supply is limited and the environment hostile. Within roughly six days, the northward branch dissipates, unable to maintain itself once it exhausts the jet’s energy reservoir or moves beyond the region of strong shear.

The southward branch tells a completely different story, one built on a spatially coordinated chain of thermodynamic and dynamic processes. The chain begins with low-level anomalous southerly winds, which the authors identify as the primary pioneer of the equatorward journey. As these winds impinge on the steep background meridional moisture gradient along the southern slopes of the plateau—one of the sharpest humidity transitions anywhere on Earth—they drive strong positive horizontal advection of moist static energy, the thermodynamic quantity that combines sensible heat, latent heat, and potential energy of an air column. In plain terms, the winds systematically push moist, energized air into regions that are primed to convert that energy into rainfall.

This thermodynamic pre-conditioning is the key to the southward branch’s remarkable longevity. The advection of moist static energy extends well into lower latitudes, between 15° and 25°N, creating a precursory zone of moisture instability that leads the oscillation’s convective core by five to ten degrees of latitude. In other words, before the storm’s heaviest rainfall arrives at any given location, the atmosphere there has already been moistened and destabilized by the leading edge of the disturbance. This leading corridor of instability acts like a paved road, allowing the convective center to follow along a path of least resistance as it travels equatorward. The mechanism echoes ideas developed for the northward-propagating monsoon intraseasonal oscillation, but here it operates in reverse, guiding energy from a high-altitude continental heat source down toward the tropical ocean.

Crucially, the study identifies a second, synergistic ingredient that reinforces this corridor: radiation from high clouds. As convection develops on the southern flank of the plateau, it produces extensive decks of high cirriform cloud. These clouds trap outgoing longwave radiation, warming the atmospheric column below and adding a radiative heating term to the moist static energy budget. The authors show that this cloud-induced longwave heating works hand in hand with the low-level moisture import, jointly reinforcing and sustaining the leading instability corridor. Neither process alone would suffice; it is their spatial coordination—moisture advection at low levels, radiative warming aloft—that locks the southward branch into a self-maintaining mode of propagation.

The energetic bookkeeping confirms the contrast. Barotropic energy conversions, which dominate the northward branch’s budget, play a secondary role in the southward one. Instead, the southward branch’s persistence is written in the thermodynamic terms of the moist static energy equation: horizontal advection supplies energy ahead of the convection, longwave heating sustains it beneath the cloud shield, and the coupling between the two allows the disturbance to survive the formidable topographic barrier of the Himalayas—a transition that would ordinarily shred a weather system. The result is a disturbance that can modulate South Asian monsoon rainfall for nearly two weeks, roughly double the lifetime of its northern twin.

These findings carry practical weight for forecasters. Intraseasonal oscillations are widely regarded as the primary source of predictable signal on the subseasonal horizon, offering windows of opportunity for skillful forecasts of extreme rainfall beyond the conventional weather-forecast range. Knowing that the plateau’s QBWO sends a long-lived, moisture-driven messenger southward into the monsoon zone—while its northward branch is a transient, jet-fed phenomenon confined to the plateau—gives model developers and forecasters a clearer physical template. It suggests that improving the representation of low-level moisture gradients, cloud-radiation interactions, and the plateau’s southern slope dynamics should directly improve two-to-three-week rainfall predictions across South Asia.

The study also reframes the Tibetan Plateau itself. Long celebrated as the world’s water tower and as an atmospheric heat pump that drives the Asian summer monsoon, the plateau now emerges as an asymmetric oscillator, dispatching weather signals that are shaped—and selected—by the very different energetics of its northern and southern flanks. The authors describe their result as a novel thermodynamic-dynamic coupling framework for understanding the bidirectional transmission of plateau weather systems. As warming continues to alter the plateau’s snow cover, surface heating, and moisture supply, the delicate balance that sustains the southward branch may itself shift, with consequences for the rhythm of the monsoon. For now, the work stands as a vivid reminder that even the planet’s highest terrain beats to a two-week drum, and that the echoes travel very unevenly across Asia.

Subject of Research: Mechanisms and energetics of the asymmetric meridional propagation of the summertime Tibetan Plateau quasi-biweekly oscillation

Article Title: Asymmetric meridional outward propagation of the summertime Tibetan Plateau quasi-biweekly oscillation: mechanisms and energetics

Article References: Wang, M., Duan, A., & Wang, J. (2026). Asymmetric meridional outward propagation of the summertime Tibetan Plateau quasi-biweekly oscillation: mechanisms and energetics. Climate Dynamics, 64(11), Article 456. https://doi.org/10.1007/s00382-026-08410-x

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08410-x

Keywords: Tibetan Plateau, quasi-biweekly oscillation, intraseasonal variability, Asian summer monsoon, moist static energy budget, barotropic energy conversion, Subtropical Westerly Jet, Himalayas, cloud-radiation interaction, subseasonal prediction, atmospheric dynamics, Climate Dynamics

Atmospheric blocking occurs when a persistent anticyclone, or high-pressure ridge, parks itself over a region and deflects the usual eastward march of weather systems. Blocks are among the most consequential features of the extratropical circulation, yet predicting them more than two weeks ahead has long been considered near the edge of what is possible. The challenge is especially acute during transitional seasons, when the large-scale circulation is shifting rapidly between winter and summer regimes and the background flow is unusually volatile. The May–June 2023 sequence offered the researchers a rare natural experiment: three successive blocking episodes over Canada, the Ural Mountains, and Europe, each tied to a distinct surface disaster.

To untangle the dynamics, the team combined ERA5 reanalysis data, the Tibaldi–Molteni blocking index for identifying large-scale blocks, and a relatively new diagnostic technique based on the Hilbert transform. This method allows scientists to compute local wave parameters—phase speed, amplitude, and zonal wavenumber—at every grid point and every moment, rather than relying on hemispheric averages that wash out regional detail. They supplemented this with the Takaya–Nakamura wave activity flux, which traces the horizontal propagation of quasi-stationary Rossby wave energy, and with the stationary wavenumber, a theoretical reference scale at which waves tend to become quasi-stationary in a given background flow.

The diagnostics revealed a clear chain of causation in the wave energy budget. During the first episode, from 1 to 19 May, wave activity flux originating over the North Pacific converged over Canada, sustaining a strong quasi-stationary ridge there. When that Canadian blocking collapsed in mid-May, its energy did not simply dissipate; instead, it dispersed downstream toward the Ural Mountains and Europe, promoting the establishment of the ridges that dominated the second episode from 20 May to 5 June. In the third episode, from 6 to 20 June, wave energy propagating across the North America–North Atlantic sector split into two branches, one heading southeast toward the northwestern Atlantic and the other northeast via Greenland, and both converged over Europe, where a new blocking event formed between 11 and 16 June.

The local wave parameters told an equally striking story. All three blocks were dominated by planetary-scale Rossby waves with zonal wavenumbers of roughly 2 to 4, about one wavenumber lower than the climatological average. Because larger-scale disturbances feel the scale effect—broad waves propagate westward relative to the mean flow more readily than narrow ones—these anomalously low wavenumbers translated into exceptionally slow phase speeds, sometimes even westward motion. The result was a circulation that barely moved for weeks, baking Canada, funneling cold air southward into East Asia alongside the Ural ridge and the East Asian trough, and holding Europe under sustained subsidence and clear skies that marine heatwave feedbacks further amplified.

Perhaps most remarkable were the abrupt regime transitions the diagnostics exposed. At the onset of each block, the wave field flipped within just a few days from eastward-propagating, small-amplitude synoptic-scale waves—wavenumbers above six—to quasi-stationary or westward-propagating, large-amplitude planetary-scale waves. Phase speed dropped sharply, in the Canadian case from more than 3 meters per second to negative values, while amplitude surged past one standard deviation and the wavenumber fell below the stationary value. During decay, the sequence ran in reverse. Comparing the diagnosed wavenumber with the stationary wavenumber provided a useful rule of thumb: blocks stayed put when the two were similar, moved westward when the wavenumber was smaller, and resumed rapid eastward progression once it exceeded that threshold.

The second half of the study turned to forecast skill, evaluating the 50-member real-time ensemble of the ECMWF subseasonal-to-seasonal prediction system. The headline finding is genuinely encouraging: for all three episodes, forecasts initialized 15 to 19 days ahead showed high predictability, with 40 to 65 percent of members correctly predicting 500 hPa geopotential height anomalies exceeding one standard deviation and more than 80 percent predicting above-normal heights. Predictability dipped for forecasts made during the following week before recovering strongly at lead times of 2 to 5 days. The Ural episode proved the most skillful, with the best ensemble members reproducing both the intensity and duration of the ridge.

What separated successful forecasts from failures was not luck but wave dynamics. Comparing the ten strongest and ten weakest members for each episode, the researchers found that good forecasts captured the upstream quasi-stationary troughs—a trough over the North Pacific for the Canadian block, and one over the North Atlantic for the Ural and European blocks—along with the subsequent downstream energy dispersion that built the planetary-scale trough-ridge patterns. Poor forecasts, by contrast, let synoptic-scale waves run eastward along the subtropical or mid-latitude jet without ever amplifying into a blocking pattern, and their predictions drifted back toward climatology. Even the best members underestimated the amplification of wave amplitude and spatial scale, particularly for the Canadian and European blocks, a systematic shortfall that points to a concrete target for model improvement.

The study also uncovered a window of opportunity with real operational implications. Because the three blocks were dynamically chained, getting the first one right paid dividends downstream. In forecasts initialized on 17 April, members that correctly predicted the strong Canadian ridge went on to anticipate a stronger Ural ridge in late May, at a lead time exceeding 33 days, while members that missed the Canadian block also missed the Ural block. In other words, the collapse of the Canadian blocking and the associated shift toward a North Atlantic Oscillation-like regime acted as a predictable precursor, extending the horizon of useful guidance well beyond the conventional two-week barrier.

The authors are careful to note the limits of their analysis. The role of vertical planetary wave propagation, stratospheric processes, and lower-boundary forcing remains unexplored, and three case studies cannot establish how generally the wave-parameter transitions apply across seasons and regions. Future work, they suggest, should extend the Hilbert-transform diagnostics to larger blocking samples and probe why models consistently fail to amplify wave amplitude and scale to observed levels. Still, the central message is clear and actionable: the key to subseasonal prediction of persistent, high-impact blocking lies in correctly capturing upstream Rossby wave precursors and the quasi-stationary energy they dispatch downstream. For forecasters and disaster planners alike, the atmosphere’s worst weeks may be legible weeks in advance—if one knows where to look for the wave train.

Subject of Research: Subseasonal predictability and Rossby wave dynamics of atmospheric blocking events in May–June 2023

Article Title: Subseasonal predictability and Rossby wave dynamics of blocking high during transitional seasons: insights from three successive events in May–June 2023

Article References: Li, Z., Lu, J., & Liu, Y. (2026). Subseasonal predictability and Rossby wave dynamics of blocking high during transitional seasons: insights from three successive events in May–June 2023. Weather and Climate Dynamics, 7(3), 1821-1836. https://doi.org/10.5194/wcd-7-1821-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1821-2026

Keywords: atmospheric blocking, Rossby waves, subseasonal prediction, wave activity flux, Canadian wildfires, European heatwave, East Asian precipitation, ECMWF S2S, ERA5 reanalysis, phase speed, planetary waves, transitional seasons

A new study published in Climate Dynamics by Haonan Guo, Jiehong Xie, Ziqian Wang, Haolin Luo, Xiaomin Cai, and Song Yang, researchers at Sun Yat-sen University and Chengdu University of Information Technology, dissects exactly how this happened. Their analysis, built on the high-resolution ERA5 reanalysis dataset from the European Centre for Medium-Range Weather Forecasts and the ERSSTv5 sea surface temperature record from NOAA, reveals that the delayed onset was not a failure of the ocean’s seasonal memory but a consequence of fast-moving atmospheric waves arriving from both the tropics and the extratropics in a nearly perfectly timed sequence. The finding reinforces a growing suspicion in the monsoon community: the predictive utility of the El Niño–Southern Oscillation, or ENSO, for pinning down the South China Sea monsoon onset date has weakened in recent years, and the day-to-day choreography of intraseasonal oscillations now matters more than the slow drift of equatorial sea temperatures.

To understand why the 2025 onset was so late, it helps to start with the physical yardstick that monsoon scientists use to define the event itself. The South China Sea summer monsoon is considered to have begun when the low-level winds over the basin flip from winter’s northeasterlies to summer’s southwesterlies, sustained by a reversal in the meridional temperature gradient—the difference in temperature between the land and ocean to the north and the tropical ocean to the south. When the southern side of the basin becomes warmer than the northern side in the mid-to-upper troposphere, the resulting pressure gradients and thermal winds support the establishment of deep convection and the monsoon circulation. As long as the meridional temperature gradient remains unfavorable, the monsoon cannot switch on, no matter what the equatorial Pacific is doing. The 2025 case, the authors show, was essentially a story about how a series of transient atmospheric disturbances kept resetting that gradient in the wrong direction during the critical weeks of May.

The first blow came from high latitudes. In mid-May, a wave train with a characteristic period of 30 to 50 days propagated southeastward from the extratropical atmosphere toward the South China Sea. This high-latitude intraseasonal oscillation carried cold anomalies into the region, cooling the mid-to-upper troposphere directly over the northern part of the basin. Because the monsoon onset depends on the northern side of the domain warming relative to the south, this injected pool of cold air sharply reduced the meridional temperature gradient over the South China Sea. In effect, the atmosphere over southern China was briefly transported back into a pre-monsoon thermal state, and the switch that should have flipped stayed off. The wave train acted like an invisible hand pressing down on the thermostat precisely when the seasonal background was trying to push it up.

Then the tropics weighed in. Between May 16 and May 24, the boreal summer intraseasonal oscillation—known to specialists as BSISO1, the leading mode of 30-to-50-day convective variability during the Asian summer monsoon—underwent a marked intensification of deep convection over the Indian Ocean. Deep convection of this kind does not stay local. The latent heat released by towering cumulonimbus clouds forces a response in the surrounding circulation, and in this case the intensified Indian Ocean convection excited a northeastward-propagating wave train in the upper troposphere, in the manner classically described by Gill-type solutions for heat-induced tropical circulation. This tropical wave train traveled toward East Asia and, crucially, arrived in phase with the extratropical wave train already cooling the northern South China Sea. The two disturbances reinforced one another, further suppressing the meridional temperature gradient and extending the pre-monsoon deadlock.

The tropical convection also launched a second, faster messenger. The BSISO1-related heating over the Indian Ocean triggered an atmospheric Kelvin wave—an eastward-propagating pulse of pressure and wind anomalies that travels along the equator at speeds of roughly 15 to 20 meters per second. As the Kelvin wave swept across the Maritime Continent and into the western Pacific, it produced easterly wind anomalies over the South China Sea at low levels. Easterlies are the signature of the winter regime; the monsoon onset requires their replacement by southwesterlies. By maintaining easterly anomalies over the basin during the second half of May, the Kelvin wave postponed the onset from a purely dynamical standpoint, independent of the thermal argument. In other words, the tropical oscillation sabotaged the monsoon’s arrival through two channels at once: it cooled and confused the thermal gradient via its upper-level wave train, and it held the low-level winds in the wrong direction via its equatorial Kelvin wave.

Just when it seemed the sequence could not become more intricate, a third actor entered from the North Atlantic. From May 25 to May 28, two distinct wave trains with periods of 8 to 20 days—the quasi-biweekly range that dominates midlatitude weather regimes—originated over the North Atlantic and propagated eastward toward Asia. One traveled along the subtropical westerly jet, the ribbon of strong upper-level winds that guides disturbances across Eurasia; the other followed the polar front jet farther north. Over East Asia, the two wave trains merged, combining their energy into a single, more potent disturbance. This merged system triggered one final, sharp reduction in the meridional temperature gradient over the South China Sea, delivering the decisive blow that pushed the onset date to May 29. The monsoon, in the end, was not merely delayed once but repeatedly, by a cascade of disturbances on three different timescales arriving in a conspiratorial sequence.

The scientific significance of this case study extends well beyond a single unusual spring. For decades, the statistical relationship between ENSO and the South China Sea monsoon onset has been a cornerstone of seasonal prediction for the Asian summer monsoon: La Niña years favor early onsets, El Niño years favor late ones, and forecasters have leaned on this link to anticipate the start of the rainy season that sustains agriculture for hundreds of millions of people. But recent research, including work by some of the same authors, has documented a weakening of this relationship in recent decades, and the 2025 event is perhaps the most vivid demonstration yet. A favorable La Niña background was completely overridden by intraseasonal variability, suggesting that seasonal-mean predictors alone are increasingly insufficient and that subseasonal-to-seasonal forecasting systems must capture the phase and amplitude of the BSISO, extratropical wave trains, and equatorial Kelvin waves to get the onset date right.

The case also highlights the growing recognition that the monsoon is not a purely tropical phenomenon. The North Atlantic has emerged as an increasingly influential upstream driver of South China Sea monsoon onset since the early 2000s, with sea surface temperature anomalies there modulating the waveguides along which disturbances travel toward East Asia. The 2025 episode shows that midlatitude jets can serve as conveyor belts delivering precisely timed perturbations into the tropical monsoon domain, and that the interaction between tropical and extratropical intraseasonal oscillations—rather than either alone—can determine the outcome of the onset process. This kind of cross-latitudinal synergy has been implicated in other extreme events, from persistent Yangtze floods to record-breaking North China rainfall, and the 2025 monsoon delay now joins that roster as a textbook example.

For the people of southern China and Southeast Asia, the practical stakes are considerable. The onset date of the South China Sea summer monsoon marks the transition from the dry pre-flood season to the season of torrential rains, tropical cyclone activity, and the water resources that fill reservoirs and irrigate rice paddies. A delayed onset can prolong drought conditions, stress water management systems, and shift the timing of the most hazardous flood-producing rainfall. A delayed onset can also concentrate the subsequent transition, producing abrupt and intense rainfall once the monsoon finally breaks through. By tracing the 2025 delay to identifiable, physically coherent wave dynamics—each stage documented in the reanalysis record with wave-activity diagnostics and filtering techniques—the study offers forecasters a concrete set of monitoring targets. If the phases of the BSISO, high-latitude wave trains, and quasi-biweekly disturbances from the Atlantic can be tracked in real time, the pieces now exist to anticipate precisely the kind of onset delay that caught the seasonal outlooks off guard in the spring of 2025.

Subject of Research: Mechanisms of the extremely delayed 2025 South China Sea summer monsoon onset involving tropical and extratropical intraseasonal oscillations

Article Title: Tropical and extratropical intraseasonal oscillations jointly delayed the 2025 South China Sea summer monsoon onset

Article References: Guo, H., Xie, J., Wang, Z., Luo, H., Cai, X., & Yang, S. (2026). Tropical and extratropical intraseasonal oscillations jointly delayed the 2025 South China Sea summer monsoon onset. Climate Dynamics, 64(10), Article 416. https://doi.org/10.1007/s00382-026-08371-1

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08371-1

Keywords: South China Sea summer monsoon, monsoon onset, intraseasonal oscillation, BSISO, Kelvin wave, ENSO, La Niña, meridional temperature gradient, wave train, North Atlantic, Climate Dynamics, subseasonal prediction