Two Rhythmic Climate Waves Take Turns Igniting the South China Sea Monsoon

Every spring, one of the most dramatic switch-flips in Earth’s climate system plays out over a stretch of warm ocean between Vietnam, the Philippines and southern China. For months the South China Sea sits under dry, subsiding air, and then, almost abruptly, deep convection erupts, winds reverse direction, and the South China Sea summer monsoon begins. This onset does not happen at random. It is typically triggered by the arrival of one of two great atmospheric rhythms known as intraseasonal oscillations, weather patterns that wax and wane on timescales of weeks rather than days or seasons. A new study published in Climate Dynamics now explains why, in some years, one of these rhythms dominates the onset while in others the other takes over, and the answer lies in how year-to-year climate anomalies quietly prepare the atmosphere for the trigger.

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