Researchers have combined satellite measurements with underwater robot profiles to map vertical ocean currents plunging more than half a mile deep in the waters surrounding Antarctica, revealing fine-scale motions that climate models have long failed to capture. The work, published in Communications Earth and Environment, pairs wide-swath sea-surface height data from NASA’s SWOT satellite with hydrographic readings from ocean gliders to diagnose vertical velocities across the Antarctic Circumpolar Current. The findings carry direct consequences for projections of how much heat and carbon dioxide the Southern Ocean absorbs, a question that shapes global climate forecasts at a time when polar ice loss is accelerating.
Why satellite-mapped deep currents off Antarctica matter right now
The Antarctic Circumpolar Current is the largest ocean current on Earth, circling the continent and connecting every major ocean basin. It acts as a gateway for heat moving toward Antarctic ice shelves and for carbon dioxide drawn from the atmosphere into the deep ocean. Yet the vertical motions inside this current, the upwelling and downwelling driven by swirling eddies, have been nearly impossible to observe at the scales where they actually operate. Eddies near Antarctica can be as small as a few kilometers across because the Rossby deformation radius in polar waters shrinks to roughly 2 to 5 kilometers, far below the resolution of older satellite altimeters.
That gap between what satellites could see and what the ocean was actually doing meant that global climate models, including those in the CMIP family used by the Intergovernmental Panel on Climate Change, had to estimate eddy-driven vertical transport through simplified assumptions. If those assumptions are wrong, the models may be systematically misrepresenting how much carbon the Southern Ocean pulls down during different seasons. The long-standing hypothesis that eddy-driven upwelling in the Antarctic Circumpolar Current could be significantly stronger during austral winter than summer, potentially producing a seasonal bias in carbon uptake, has not yet been tested with direct observational evidence at the right spatial scales. The new study provides the first satellite-plus-glider framework that could make such a seasonal test possible by tying observed sea-surface height patterns to vertical motions hundreds of meters below.
How SWOT and ocean gliders revealed half-mile-deep motion
The study published in Communications Earth and Environment used SWOT’s wide-swath sea-surface height measurements together with interior hydrographic profiles collected by autonomous ocean gliders to reconstruct the three-dimensional structure of fine-scale vertical velocities in the Southern Ocean. SWOT, a joint mission between NASA and the French space agency CNES, measures the height of the ocean surface across a broad swath rather than along a single track, giving it the ability to detect the surface signatures of eddies and jets at scales previous altimeters missed entirely.
The SWOT mission documentation notes that the satellite’s observations can resolve small-scale sea-surface height variations and retrieve surface currents, including motions that extend beyond standard geostrophic balance. By pairing those surface readings with vertical temperature and salinity profiles gathered by gliders diving through the water column, the researchers could infer how water moves up and down at depth, not just sideways at the surface. The result is a diagnosis of vertical velocities reaching more than half a mile below the surface, a dimension of ocean circulation that had previously been inferred only from sparse ship-based surveys or coarse numerical simulations.
In practice, the team first used SWOT to map the fine-scale bumps and dips in sea level associated with eddies and filaments in the Antarctic Circumpolar Current. They then deployed gliders to repeatedly dive through these features, measuring how density, temperature and salinity changed with depth. By combining the glider-derived stratification with the satellite-derived horizontal pressure gradients, they could apply dynamical constraints to estimate vertical velocities. This approach allowed them to resolve motions on the order of a few kilometers across, matching the natural scales set by the Rossby deformation radius in polar waters.
CSIRO’s research vessel RV Investigator has supported in situ measurement campaigns in the Southern Ocean that feed into this kind of analysis. Voyage records from CSIRO’s Data Trawler catalog the ship’s operations and provide metadata for the glider deployments and oceanographic surveys that complement satellite observations. These coordinated efforts ensure that the satellite snapshots of surface structure are tied to well-documented profiles through the water column, improving confidence in the inferred vertical motions.
The study shows that vertical velocities associated with fine-scale eddies can reach values large enough to substantially influence how heat and carbon are exchanged between the surface and the deep ocean. In regions where eddies drive strong downwelling, warm, carbon-rich surface waters can be pushed downward, effectively sequestering heat and carbon dioxide away from the atmosphere. Conversely, eddy-driven upwelling can bring carbon-poor deep waters back to the surface, where they can take up additional CO2. Capturing the balance between these opposing motions is critical for understanding the net role of the Southern Ocean in moderating climate change.
Parallel advances in reading ocean currents from orbit
The SWOT-based study sits within a broader push to observe ocean currents from space with increasing precision. A separate technique called GOFLOW, published in Nature Geoscience, uses deep learning applied to thermal imagery from geostationary satellites to derive surface currents and their gradients at fine scales. Researchers at the Scripps Institution of Oceanography developed the approach, which works by training neural networks on sequences of sea-surface temperature patterns captured by satellites that stare continuously at the same patch of ocean. The method can infer surface currents from geostationary satellite imagery at resolutions that complement what SWOT provides from its polar orbit.
These two approaches solve different parts of the same problem. SWOT excels at measuring sea-surface height with enough precision to detect small eddies, especially near Antarctica where those eddies are tiny. GOFLOW excels at continuous temporal coverage, watching how surface patterns evolve hour by hour over fixed regions. SWOT’s polar orbit provides global coverage over repeated cycles, while geostationary platforms focus on specific latitude bands but deliver near-constant monitoring.
Neither method alone can see below the surface, which is why the combination of satellite data with glider profiles in the new study represents a distinct step forward. By fusing SWOT’s fine-scale surface height maps with in situ measurements, researchers can tie the remotely sensed patterns to actual vertical motions in the ocean interior. In the future, similar data-assimilation strategies could merge GOFLOW-derived surface currents with glider and float observations, yielding even more complete pictures of how eddies pump heat and carbon up and down.
Implications for climate models and future observing systems
The emerging picture from these efforts is that fine-scale vertical motions in the Antarctic Circumpolar Current are both stronger and more structured than many models have assumed. If eddies are more efficient at subducting heat and carbon than previously thought, then the Southern Ocean may be playing a larger short-term buffering role in climate change, while also storing more heat at depth that could affect ice shelves and sea level over longer timescales. Conversely, if certain regions experience persistent upwelling tied to eddy activity, they could act as hotspots where stored carbon is more readily returned to the atmosphere.
For climate modelers, the new satellite–glider framework offers a way to directly evaluate and tune parameterizations of eddy-driven vertical transport. Rather than relying solely on coarse-resolution simulations and sparse ship-based measurements, models can now be confronted with detailed maps of vertical velocity structures at the scales where eddies actually operate. That, in turn, should sharpen projections of how the Southern Ocean’s role as a carbon and heat sink will evolve as winds, stratification and sea-ice cover change.
The work also underscores the importance of sustained, multi-platform observing systems in the Southern Ocean. SWOT, geostationary imagers, autonomous gliders, profiling floats and research vessels each provide a different piece of the puzzle. Together, they are beginning to reveal how the world’s most remote current moves heat and carbon between the atmosphere and the deep ocean, and how those exchanges may shift as the climate warms. By resolving vertical motions more than half a mile beneath the surface, scientists are gaining a clearer view of the hidden machinery that helps regulate Earth’s climate, and a more robust foundation for forecasting its future trajectory.
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*This article was researched with the help of AI, with human editors creating the final content.
