Ocean Currents Set for a Global Split as Warming Reshapes the Seas, CMIP6 Models Show

The great ocean currents that sculpt climates, steer fisheries, and move heat across the planet are heading toward a deeply divided future, according to a new analysis of the latest generation of global climate simulations. By the final decade of this century, under a scenario in which global warming of two degrees Celsius relative to the beginning of the century is extremely likely to be exceeded, the Northern Hemisphere’s powerful western boundary currents are projected to lose strength, while several of their Southern Hemisphere counterparts are set to intensify. The study, published in the journal Climate Dynamics, is among the first to carry out a truly inter-basin assessment of how the volume of water carried by the world’s major currents will change, and to trace those changes back to specific, basin-dependent mechanisms.

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