Researchers led by Raquel Toste of the Federal University of Rio de Janeiro analyzed monthly output from 24 climate models participating in the Coupled Model Intercomparison Project Phase 6, or CMIP6, under the SSP2-4.5 scenario. This pathway represents an update of the RCP4.5 scenario used in earlier IPCC assessments, and corresponds to a world in which a two-degree warming relative to the start of the century is extremely likely. To give every model equal weight in the ensemble, the team retained a single realization from each model, a standard practice that avoids over-representing institutions that contributed large ensembles. The variables examined included surface winds, wind stress, sea-level pressure, sea surface height, and the horizontal velocity of seawater at every depth.

The physical heart of the analysis lies in volume transport, a measure of how much water a current carries through a fixed cross-section of the ocean, expressed in Sverdrups, where one Sverdrup equals one million cubic meters per second. Rather than relying on surface speed alone, the researchers computed transport by numerically integrating the velocity field cell by cell across carefully defined staircase sections on each model’s native grid, capturing the full vertical structure of every current. Sections were placed across the Kuroshio and Oyashio Currents in the North Pacific, the Alaska and California Currents in the eastern North Pacific, the East Australian and Humboldt Currents in the South Pacific, the Gulf Stream in the North Atlantic, the Brazil, North Brazil, and Benguela Currents in the South Atlantic, and the Agulhas Current in the Indian Ocean.

The headline finding is a stark hemispheric asymmetry. Northern Hemisphere western boundary currents exhibit an overall reduction in volume transport by 2090-2100. The Gulf Stream is projected to weaken by 15.89 percent, a loss of 7.36 Sverdrups, with all 24 models agreeing on the sign of the change. The Agulhas Current is projected to decline by 10.40 percent, or 7.03 Sverdrups, also with unanimous model agreement. In the equatorial Atlantic, the North Brazil Current loses 10.42 percent of its transport, about 3.08 Sverdrups. In sharp contrast, the Southern Hemisphere is projected to experience enhancements in the volume transport of the Brazil Current, which strengthens by 26.56 percent, and the East Australian Current, which gains 3.27 percent. In the subpolar North Pacific, the Oyashio intensifies by 13.49 percent, driven by the strengthening and shifting of the Aleutian Low, which appears to spin up the subpolar gyre.

The study goes beyond transport to examine where currents sit on the globe. The northern branches of the subtropical gyres generally shift northward, a movement consistent with the poleward expansion of the Hadley circulation that both CMIP5 and CMIP6 models project under rising greenhouse gases. The most dramatic displacement belongs to the North Atlantic Current Drift, which on average is expected to sit 2.54 degrees north of its present latitude by the century’s end. The Atlantic South Equatorial Current is the lone exception, drifting slightly southward. These migrations matter because the position of gyre branches partly determines how much water western boundary currents can collect and carry.

Crucially, the research tested whether these changes can be understood within a single dynamical framework, and the answer is a resounding no. Using multiple linear regression and variance decomposition across the model ensemble, the team found that future transport changes are associated with fundamentally different mechanisms in different basins. In the North Atlantic, Gulf Stream transport is jointly linked to changes in the North Atlantic Current Drift position and to wind stress curl anomalies, with the combined regression explaining 44.5 percent of the inter-model variance. Wind stress curl, the rotation of the wind field that drives the interior flow of subtropical gyres through Sverdrup dynamics, independently accounts for roughly a quarter of the explained spread, while gyre migration explains about 17 percent. This joint response is broadly consistent with the projected weakening of the Atlantic Meridional Overturning Circulation, which modulates atmospheric temperature gradients and generates surface wind anomalies that reshape the wind-driven circulation.

Elsewhere the picture inverts. In the South Pacific, changes in the East Australian Current are mostly related to the migration of the South Pacific Equatorial Current, which emerges as the dominant predictor in the regression and explains part of the spread among model projections. In the Indian Ocean, the Agulhas slowdown is primarily associated with wind stress curl anomalies rather than the latitude of the Indian South Equatorial Current, suggesting that large-scale wind forcing exerts stronger control over this system than gyre repositioning. In the South Atlantic and North Pacific, by contrast, the Brazil and Kuroshio Currents showed no significant relationships with either of the evaluated mechanisms, indicating that additional regional dynamical processes dominate their long-term evolution at the studied sections. The Kuroshio is particularly revealing: its regression explains a mere 2.2 percent of the inter-model variance, pointing to complex, topographically constrained behavior in which regional thermal gradients and localized atmospheric forcing compete with the large-scale poleward shift of the westerlies.

One of the most striking technical discoveries concerns the vertical structure of these changes. When the team analyzed transport restricted to the upper 200 meters alongside full-depth transport, they found decoupled behavior in several systems. The Kuroshio’s total transport declines slightly by 1.37 percent, yet its upper-200-meter transport intensifies significantly by 1.08 Sverdrups, with positive velocity anomalies confined to the upper layers and negative anomalies below. This vertical shear enhancement aligns with the mechanism proposed by earlier work showing that greenhouse-gas-induced surface warming amplifies upper-ocean density stratification, trapping momentum in the uppermost layers and altering vertical velocity profiles. The Brazil Current shows a similar surface concentration, with about 72 percent of its transport increase packed into the upper 200 meters, reflecting the shallow dynamic nature of this flow, which is vertically constrained by the opposing northward Intermediate Western Boundary Current directly beneath it. Deep-penetrating currents such as the Gulf Stream and Agulhas behave differently, with their deceleration extending below 1,400 meters, a signature that modifies how these currents interact with bottom topography and could trigger path instabilities.

The broader implications are considerable. Ocean currents redistribute heat, carbon, and nutrients, so hemispheric shifts in their strength and position will reverberate through regional climates, marine ecosystems, and coastal economies on both sides of the equator. The finding that no single framework, whether classical Sverdrup dynamics or simple gyre reorganization, can universally explain future changes in western boundary current transport underscores the sophistication of the ocean’s response to warming. The authors point to atmospheric reorganization as the deeper driver: subtropical high-pressure systems in the Southern Hemisphere expand, the North Pacific Subtropical High contracts, and the North Atlantic Subtropical High migrates poleward, with sea-level pressure anomalies and wind stress curl changes reshaping the wind fields that ultimately spin the great gyres. Future studies combining overturning diagnostics, stratification changes, wind-driven circulation, and momentum budget analyses, the team suggests, would help quantify the relative importance of these mechanisms and further constrain the physical drivers of projected ocean circulation change. What is already clear is that the ocean’s engine room is being rewired, and that each basin is rewiring in its own way.

Subject of Research: Projected changes in ocean current volume transport and positioning under the CMIP6 SSP2-4.5 warming scenario

Article Title: Projections for ocean currents’ transport and positioning from CMIP6 models

Article References: Projections for ocean currents’ transport and positioning from CMIP6 models. (n.d.). https://doi.org/10.1007/s00382-026-08385-9

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08385-9

Keywords: ocean currents, CMIP6, climate change, ocean circulation, western boundary currents, Gulf Stream, Agulhas Current, Brazil Current, subtropical gyres, wind stress curl, volume transport, Sverdrup 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