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 IWMO workshops are managed by an international steering committee of scientists drawn from ten different countries, with local volunteers from the hosting institute handling organization and support. A defining tradition of the series is the publication of a special issue, or Topical Collection, of peer-reviewed papers in the journal Ocean Dynamics after each meeting. More than 200 papers have now appeared in these collections, making the workshop series one of the most consistently productive venues for ocean modeling research. To promote international collaboration and diverse participation, organizers deliberately rotate the meeting across continents: IWMO-2023 was held in Hamburg, Germany, and IWMO-2024 took place in Sapporo, Japan. Since the organization’s inauguration in Taiwan in 2009, six meetings have been held in Asia, four in Europe, four in North America, one in South America and one in Australia. Only two previous meetings were held in the United States, in Virginia in 2010 and in Ann Arbor, Michigan in 2022. Following the Stanford meeting, IWMO-2026 was held in Palma de Mallorca, Spain, and IWMO-2027 is planned for June 2027 in Hangzhou, China.

The scientific program at Stanford featured 32 oral presentations, including four keynote invited presentations by prominent scientists, along with 10 posters. Sessions spanned the breadth of modern ocean modeling: internal and surface waves, coupled physical-biological interactions, global and basin-scale circulation, mixing and sub-mesoscale processes, coastal and estuarine modeling, and numerical methods. Continuing the IWMO tradition of encouraging early-career participation, 13 graduate students and postdoctoral researchers presented their work as part of the Outstanding Young Scientist Award competition, with the top winners hailing from the United States, Japan and Germany. The accompanying Topical Collection in Ocean Dynamics includes eight peer-reviewed papers from workshop participants, each subjected to rigorous review by IWMO members and external experts under the stewardship of a team of guest editors. Together the papers showcase a variety of numerical ocean and wave models, diverse analysis methods, and study sites ranging from the German Bight and the coasts of the United States and Canada to seas along the Asian Pacific coasts.

Among the collection’s contributions, Zhang and colleagues developed an idealized coupled Computational Fluid Dynamics and Discrete Element Method model, known as CFD-DEM, to study how cross-flow velocities affect particle transport near a subsea pipeline. The simulations identified distinct critical velocity thresholds for different particle sizes, offering a practical engineering tool for subsea pipeline design and backfilling projects, where the loss of backfill material is a persistent operational concern. The work illustrates how idealized numerical experiments can distill complex sediment-structure interactions into design guidance that engineers can apply directly to offshore infrastructure.

Wave dynamics featured prominently in the collection as well. Imamura and Yoshikawa tackled the long-standing challenge of parameterizing surface-wave breaking in wave-resolving simulations using a nonhydrostatic numerical model. Rather than relying on the empirical criteria that many existing models employ, they introduced a fourth-order diffusivity into the surface elevation equation to explicitly represent wave breaking. The approach was validated successfully against laboratory experiments and theory, demonstrating a physically grounded alternative to heuristic breaking schemes. In a related vein of coastal prediction, Ezer used the Princeton Ocean Model to explore how high-frequency oscillations in the Gulf Stream, with periods ranging from one week to two months, can induce coastal sea level variability. Comparisons with observations revealed a statistically significant anticorrelation between Gulf Stream strength and coastal sea level, but the relationship depends nonlinearly on location along the coast and on the frequency of the current’s variability, complicating the prediction of coastal flooding.

Nature-based coastal protection emerged as another theme. Jacob, Pein and Staneva used the unstructured-grid SCHISM hydrodynamic model, extended with sediment dynamics and vegetation, to evaluate seagrass as a nature-based solution for coastal protection in the German Wadden Sea. Their simulations, which incorporated end-of-the-century sea level rise projections, showed that despite rising seas, seagrass meadows retain their damping capacity and can reduce wave heights by as much as 30 percent in shallow areas. This finding carries significant implications for coastal management, suggesting that restored ecosystems could complement or even substitute for hard engineering defenses in some settings. In the Pacific Northwest, Khangaonkar and colleagues tested a sigma-coordinates formulation with shaved cells in the SCHISM model to simulate the Salish Sea, a fjord-like deep estuary. The refined grid and higher resolution compared with a past FVCOM configuration of the region eliminated the need for bathymetric smoothing and improved predictions of currents and temperatures, resolving near-shore intertidal and deep estuarine circulation simultaneously.

Aquaculture’s footprint on coastal hydrodynamics was examined by Fu and colleagues, who applied the FVCOM numerical model with an enhanced dual-drag parameterization to simulate the impact of large-scale floating-raft aquaculture structures in the open waters near Zhangzi Island in the northern Yellow Sea of China. The simulations showed that floating rafts can significantly reduce tidal current velocities, especially near the surface, with consequences for regional circulation and the transport of material. On the neighboring Pacific coast of North America, Lin and Dunphy deployed the NEMO model with AGRIF two-way nesting to resolve a fjord tidal jet in Quatsino Sound, British Columbia, Canada. The nested model reproduced tidal mixing in good agreement with observations and generated internal waves during each flood phase, with barotropic-to-baroclinic energy conversion for the M2 tide identified along the tidal-jet slope and in shallow waters.

Artificial intelligence also made its mark on the collection. Hu and colleagues proposed a geospatially encoded dual-channel network with attention mechanisms and physics constraints for reconstructing complex seafloor topography. Bathymetric prediction based on gravity anomalies remains the dominant approach for mapping the seafloor, but the new study integrates traditional physical models with neural networks to enhance the model’s fitting capacity, pointing toward a hybrid future in which machine learning and process-based oceanography reinforce one another rather than compete.

Taken together, the IWMO-2025 proceedings capture a field in vigorous transition, one in which classical process models such as the Princeton Ocean Model and FVCOM share the stage with next-generation frameworks like SCHISM and NEMO, and in which data-driven methods are beginning to reshape how the seafloor and the flows above it are mapped. The breadth of applications, from pipeline engineering and aquaculture siting to flood prediction and ecosystem-based coastal defense, underscores why ocean modeling has become indispensable infrastructure for climate adaptation worldwide. The organizers extended thanks to the local organizing committee at Stanford and to all participants for sustaining the collaborative spirit of the IWMO tradition, and the community now looks ahead to Palma de Mallorca and Hangzhou, where the next chapters of this sixteen-year-old scientific conversation will unfold.

Subject of Research: The 15th International Workshop on Modeling the Ocean (IWMO-2025) and its associated peer-reviewed papers on numerical ocean modeling

Article Title: The 15th international workshop on modeling the ocean (IWMO-2025) in Stanford, California, USA, June 30th – July 1st, 2025

Article References: Fringer, O., de Camargo, R., Chang, Y.-L., Xue, H., & Ezer, T. (2026). The 15th international workshop on modeling the ocean (IWMO-2025) in Stanford, California, USA, June 30th – July 1st, 2025. Ocean Dynamics, 76(9), Article 95. https://doi.org/10.1007/s10236-026-01853-0

Image Credits: AI Generated

DOI: 10.1007/s10236-026-01853-0

Keywords: IWMO-2025, ocean modeling, Ocean Dynamics, Stanford University, coastal sea level, seagrass coastal protection, SCHISM, NEMO, FVCOM, Gulf Stream, wave breaking parameterization, seafloor topography reconstruction