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

The Indian Ocean Dipole, or IOD, is a naturally occurring climate pattern in which sea surface temperatures swing in a seesaw between the western and eastern tropical Indian Ocean. During a positive IOD event, the western basin near Africa becomes unusually warm while waters off Sumatra turn cooler, shifting rainfall and winds across the entire region. During a negative IOD, the pattern reverses. Scientists have long known that these events leave fingerprints on sea level in the northern Indian Ocean, but the relative importance of equatorial wind forcing versus local wind forcing has been difficult to separate, particularly because each IOD event unfolds with its own personality and intensity.

Nikitha Syam of the Department of Physical Oceanography at Cochin University of Science and Technology and Arnab Mukherjee of the National Centre for Polar and Ocean Research in Goa tackled this problem by combining two complementary lines of evidence. The first came from satellite altimeter observations of sea level anomaly, obtained from the Copernicus Climate Data Store, which track how far the ocean surface departs from its long-term average. The second was a linear continuously stratified ocean model, a numerical tool that simulates how the ocean responds to wind stress while faithfully representing the ocean’s continuous density layering with depth. The model was forced with ERA5 reanalysis winds from the European Centre for Medium-Range Weather Forecasts, allowing the researchers to isolate the wind-driven component of sea level variability.

The technical power of the approach lies in a set of damping-based experiments. By selectively switching off either the remote equatorial winds or the local basin-scale winds in the model, and by measuring how the damped ocean response decays, the researchers could quantify exactly how much each forcing agent contributes to the observed autumn sea level anomalies in each basin. This kind of forcing decomposition is essential because the ocean integrates wind information over time: winds along the equator excite eastward-travelling Kelvin waves that bounce off the eastern boundary and propagate as coastally trapped waves around India and Sri Lanka, while local winds stir up their own responses through Ekman pumping driven by wind stress curl and direct wind setup.

The results confirm a broad picture that oceanographers had suspected, but with important new nuance. Positive IOD events are generally associated with negative sea level anomalies across the Bay of Bengal, and the analysis attributes this to a negative contribution from remote equatorial wind forcing. The mechanism is well understood in dynamical terms: during a positive IOD, anomalous easterly winds along the equator drive upwelling Kelvin waves that travel eastward, lower the thermocline in the east, and send reflected, upwelling-favourable signals poleward along the coastal waveguide into the Bay of Bengal, where they raise the thermocline and depress sea level. Negative IOD events generally produce the mirror image, with positive sea level anomalies in the bay as downwelling signals propagate through the same waveguide.

The Arabian Sea tells a more complicated story. Its response to the IOD is systematically weaker and spatially more variable than that of the Bay of Bengal, and the decomposition experiments show why: local atmospheric forcing carries proportionally greater weight in the Arabian Sea than remote equatorial winds do. The geometry of the region helps explain the difference. The Bay of Bengal sits directly downstream of the equatorial waveguide along a coastal boundary that efficiently channels equatorial Kelvin wave energy northward, whereas the Arabian Sea receives a more attenuated share of that energy after it rounds Sri Lanka and sweeps up the west coast of India. Meanwhile, the Arabian Sea’s own monsoonal wind regime, with its strong seasonal reversals and pronounced wind stress curl patterns, imprints a local signal that can either reinforce or fight against the remote one.

Two individual events, 2015 and 2019, illustrate just how sharply the balance can differ from one IOD year to the next. In 2015, the observed sea level anomaly in the Bay of Bengal was actually positive even though the equatorial wind forcing contribution was negative. The decomposition indicates that local forcing over the Bay of Bengal partly offset the remote signal, overriding the expected dipole response. In 2019, the pattern split between basins: over the Arabian Sea, positive local wind forcing partially cancelled a negative equatorial contribution, leaving the basin-mean modelled response only weakly negative, while over the Bay of Bengal the negative equatorial wind contribution dominated outright and the modelled response remained strongly negative. The same climate event, acting on the same season, produced fundamentally different outcomes in two neighbouring basins because the internal balance of forces tipped in different directions.

The model itself performed credibly against observations, capturing the large-scale interannual sea level variability seen by the altimeters, although with reduced amplitude, a common characteristic of linear models that omit nonlinear processes, thermocline feedbacks and steric effects tied to heat fluxes. Regional time-series statistics were used to evaluate how well the model reproduced the observed variability across the different IOD categories. The agreement gives confidence that the wind-driven dynamics isolated in the experiments, rather than other processes such as freshwater fluxes from river discharge and monsoon rainfall, dominate the autumn interannual signal at the basin scale, even if the full observed amplitude requires additional contributions that a linear model cannot generate.

The practical implications extend well beyond academic dynamical bookkeeping. Sea level anomalies in the Bay of Bengal and the Arabian Sea modulate coastal flooding, saltwater intrusion, fishery productivity and the safety of low-lying delta communities that are home to hundreds of millions of people. If the IOD phase provides a seasonal lead indicator of likely sea level conditions, and if the remote equatorial wind signal can be anticipated from emerging dipole forecasts, then the new decomposition framework offers a way to translate climate prediction into more useful regional sea level outlooks. The study’s message is carefully hedged, however: the IOD phase is an important framework for understanding autumn variability, but the magnitude and spatial structure of the response in any given year depend on the race between remote equatorial waves and local wind forcing. As the researchers put it through their experiments, knowing the dipole phase tells you which way the wind is expected to blow, but the ocean’s answer depends on how hard each wind blows, and where.

The work also adds to a growing body of research showing that the northern Indian Ocean is a region where climate teleconnections and local processes are tightly intertwined. Earlier studies have traced Kelvin wave propagation along the equatorial waveguide, documented the role of the East India Coastal Current in transmitting equatorial signals, and examined how ENSO and the IOD together shape sea level along the Indian subcontinent. By quantifying the damping-based contributions of remote and local forcing across twenty-four years of dipole events, positive and negative alike, the new analysis provides a benchmark against which future event-by-event predictions can be tested. As ocean warming continues to raise baseline sea levels and potentially amplify interannual variability, understanding which lever, equatorial winds or local winds, moves the ocean surface in any given autumn becomes an increasingly urgent question for the densely populated coasts of South Asia.

Subject of Research: Autumn sea level variability in the Bay of Bengal and Arabian Sea driven by remote equatorial and local wind forcing during Indian Ocean Dipole years

Article Title: Role of local and remote equatorial wind forcing in autumn sea level variability in the Bay of Bengal and Arabian Sea during IOD years between 1999 and 2022

Article References: Syam, N., & Mukherjee, A. (2026). Role of local and remote equatorial wind forcing in autumn sea level variability in the Bay of Bengal and Arabian Sea during IOD years between 1999 and 2022. Ocean Dynamics, 76(10), Article 101. https://doi.org/10.1007/s10236-026-01857-w

Image Credits: AI Generated

DOI: 10.1007/s10236-026-01857-w

Keywords: Indian Ocean Dipole, sea level anomaly, Bay of Bengal, Arabian Sea, equatorial Kelvin waves, Rossby waves, wind stress curl, linear continuously stratified model, satellite altimetry, coastal waveguide, interannual variability, monsoon winds