Two Wave Trains and a Kelvin Wave: Why Asia’s 2025 Monsoon Arrived Late

In the spring of 2025, the atmosphere over the tropical western Pacific staged a quiet rebellion against one of seasonal forecasting’s most trusted rules of thumb. When a La Niña event precedes the boreal summer, the South China Sea summer monsoon—the great atmospheric engine that switches on the rainy season across Southeast Asia and southern China—almost always arrives early. The cool equatorial Pacific sea surface temperatures left behind by La Niña warm the surrounding maritime continent, sharpen the thermal contrast that drives monsoon flow, and nudge the onset date earlier than average. Yet in 2025 the monsoon did not arrive early. It arrived on May 29, one of the latest onsets on record, defying the canonical expectation and leaving forecasters scrambling to explain the mismatch between the seasonal background and the observed reality.

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