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

The two oscillations in question are easily distinguished by their periods and their pathways. The first is a quasi-biweekly mode, an oscillation with a period of roughly 10 to 20 days that approaches the South China Sea while propagating westward across the western Pacific. The second is the longer 30-to-60-day oscillation, closely related to the Madden-Julian Oscillation, the planet’s most prominent intraseasonal disturbance, which reaches the region while propagating northward from the tropical Indian Ocean. Both carry enhanced convective activity and low-level wind anomalies, and when either one arrives over the South China Sea at the right moment in the seasonal cycle, it can tip the atmosphere past the threshold for monsoon onset. Forecasters have long known that both waves matter, but predicting which one will deliver the decisive push in a given year has remained a stubborn challenge.

The research team, led by Xue Han of the National Marine Environmental Forecasting Center in Beijing together with Kuiping Li of the First Institute of Oceanography and colleagues, approached the problem using the ERA5 global reanalysis, a state-of-the-art reconstruction of past atmospheric and oceanic conditions produced by the European Centre for Medium-Range Weather Forecasts. Rather than simply cataloguing which wave arrived in which year, the team asked a more fundamental question: what physical processes allow each oscillation to moisten the atmosphere over the South China Sea in the critical period before onset? Their focus settled on the planetary boundary layer, the lowest kilometer or so of the atmosphere, where moisture accumulates ahead of the oscillation’s main convective center and sets the stage for the deep thunderstorms that define the monsoon.

Using a formal moisture budget diagnosis, a mathematical accounting of every term that adds or removes water vapor from the boundary layer, the researchers found that the key to pre-onset moistening is not the oscillation acting alone. Instead, it is the interaction between the intraseasonal perturbation and the slowly evolving background state of the atmosphere and ocean, the anomalies that persist on interannual timescales of a few years. When the products of these two components are evaluated in the moisture budget, the cross-terms reveal how the background flow can either amplify or suppress the wave’s ability to build moisture in the boundary layer. In other words, the same intraseasonal wave can arrive over a South China Sea that is primed to receive it or one that resists it, depending on the larger climate setting of that particular year.

The team then separated onset years according to which oscillation played the leading role and compared the interannual background anomalies between the two groups. The differences were striking. In years when the 10-to-20-day quasi-biweekly oscillation triggers the onset, the subtropical high strengthens over the northern South China Sea and easterly vertical wind shear becomes pronounced over the western Pacific. The sea itself tends toward dryness, with suppressed convection and a low-level anticyclonic circulation sitting over the region. On its face, this seems like an inhospitable environment for a monsoon trigger, yet the analysis shows precisely the opposite: against this particular background, the interaction between the anomalies and the 10-to-20-day perturbations enhances boundary-layer moistening through two specific channels, the horizontal advection of moisture by the combined flow and a moisture source term associated with the anomalies. The dry, anticyclonic setting thus acts as an amplifier for the faster wave rather than a barrier.

The picture reverses almost completely for years governed by the 30-to-60-day oscillation. In those years, easterly wind shear dominates the Indian Ocean while westerly shear prevails over the western Pacific, a configuration that shapes how the longer wave develops as it migrates northward. The South China Sea in these years is warmer than normal and rich in moisture, with low-level convergence drawing air upward and stronger convection already under way. Here, the critical moistening pathway is meridional moisture advection, the north-south transport of water vapor, which strengthens the boundary-layer moistening through the interaction between the interannual anomalies and the 30-to-60-day perturbations. This process preconditions the basin for the slower, northward-propagating wave, allowing it to complete the monsoon onset when it arrives.

These findings matter because the two oscillations carry very different implications for prediction. The quasi-biweekly mode evolves rapidly, giving forecasters perhaps a week or two of lead time, while the 30-to-60-day mode, being an extension of the Madden-Julian Oscillation, offers the potential for sub-seasonal forecasts issued several weeks in advance. Knowing which interannual background state favors which trigger means that seasonal outlooks can, in principle, identify the more likely onset pathway before the intraseasonal wave even forms. If the background anomalies resemble the dry, anticyclonic, easterly-shear configuration, attention can shift to monitoring the western Pacific for quasi-biweekly disturbances; if the warm, moist, convergent configuration appears, the Indian Ocean’s slow-moving envelope of convection becomes the object to watch. Such conditional forecasting strategies could sharpen the lead times for one of the most consequential seasonal transitions in Asia.

The stakes are considerable. The South China Sea summer monsoon onset marks the beginning of the rainy season for southern China and influences rainfall across much of East and Southeast Asia, affecting agriculture, water resources and flood risk for hundreds of millions of people. The onset date varies substantially from year to year, and errors in anticipating it cascade into errors in seasonal rainfall prediction across the entire East Asian monsoon domain. Previous work has linked onset variability to sea surface temperature anomalies associated with El Niño and La Niña events, to the thermal state of the tropical western Pacific, and even to tropical cyclones that occasionally deliver the final nudge. The new study adds a crucial mechanistic layer to this picture by showing that these large-scale influences do not merely shift the mean conditions; they selectively modulate which intraseasonal vehicle carries the monsoon into being.

The research also speaks to a broader theoretical development in tropical meteorology, the growing recognition that intraseasonal oscillations behave as moisture modes, disturbances whose propagation and amplification depend fundamentally on how they reshuffle water vapor within the circulation. By demonstrating that the boundary-layer moistening ahead of the convective center, the same mechanism emphasized in modern theories of the Madden-Julian Oscillation, is central to monsoon onset over the South China Sea, the study ties a regionally specific forecasting problem to a unifying physical framework. The finding that interannual anomalies enter through specific budget terms, horizontal advection and the moisture source in one regime and meridional advection in the other, provides a quantitative handle that model developers can use to evaluate whether their simulations capture the right physics.

There remain open questions. The analysis rests on reanalysis data, which blend observations with model physics, and the regional distinctions the team identified will need to be tested in prediction systems and climate models to confirm their practical value. Still, the central message is clear and potentially transformative for monsoon forecasting: the annual contest between two atmospheric rhythms over the South China Sea is not a coin flip but a choreographed outcome, decided years’ worth of climate memory in advance. By reading the background state of the basin each spring, scientists may now be able to anticipate not just when the monsoon will begin, but which of two very different atmospheric clocks will ring in its arrival.

Subject of Research: Interannual modulation of intraseasonal oscillations triggering the South China Sea summer monsoon onset

Article Title: Interannual modulation of two dominant intraseasonal oscillations triggering the South China sea monsoon onset

Article References: Han, X., Li, K., Shi, Z., Han, Y., Wang, L., Yuan, J., Feng, L., & Chen, X. (2026). Interannual modulation of two dominant intraseasonal oscillations triggering the South China sea monsoon onset. Climate Dynamics, 64(10), Article 433. https://doi.org/10.1007/s00382-026-08388-6

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08388-6

Keywords: South China Sea summer monsoon, monsoon onset, intraseasonal oscillations, quasi-biweekly oscillation, Madden-Julian Oscillation, planetary boundary layer moistening, moisture budget, interannual variability, wind shear, subtropical high, ERA5 reanalysis, Climate Dynamics