The argument, laid out by Benjamin Petrick of University College London in a commentary accompanying new research by Hou and colleagues, forces scientists to confront an uncomfortable possibility: the modern relationship between Indian Ocean salt export and Atlantic overturning cannot simply be projected backwards into warmer climates, nor assumed to persist unchanged as the planet heats up in the future. If the Agulhas Leakage did not help drive Atlantic overturning during the Pliocene, then the mechanisms linking the two basins must have shifted at some point in the more recent geological past, and identifying when and why that shift occurred has become one of palaeoceanography’s most pressing questions.

To appreciate why the stakes are so high, it helps to understand the physics of the overturning circulation itself. The Atlantic Meridional Overturning Circulation, often abbreviated as AMOC, operates as a planetary-scale conveyor. Warm, salty surface waters flow northward from the South Atlantic across the equator and into the subpolar North Atlantic. There, in the Labrador and Nordic Seas, they lose heat to the frigid atmosphere, become denser, and sink to great depths, forming North Atlantic Deep Water that spreads southward through the deep ocean and eventually feeds the global overturning cell. The entire engine depends on a delicate balance of temperature and salinity. Saltier surface water is denser at a given temperature, so the salt delivered by the Agulhas Leakage into the South Atlantic is thought to precondition the system, helping surface waters in the north reach the density threshold needed for deep convection. In the modern ocean, model studies and observations alike suggest that leakage variations ripple through the overturning on decadal to millennial timescales, making the Agulhas system a potential lever on abrupt climate change.

The new research challenges that picture for the Pliocene by assembling evidence from marine sediment cores that record the history of the leakage over millions of years. Multiple independent proxies converge on a consistent story. Records of sea surface temperature and salinity from the Agulhas corridor, the region south of Africa where Indian Ocean water transits into the Atlantic, indicate that during the Pliocene the leakage pathway behaved very differently than it does today. Studies of the Benguela upwelling system off southwestern Africa, where nutrient-rich waters rise to the surface, and analyses of Agulhas sediment sequences published over the past decade have painted a picture of a region whose dynamics were governed by different boundary conditions, including higher global sea levels, different wind regimes and a configuration of ocean gateways that has since evolved.

One of the most striking lines of evidence concerns the closure history of the Indonesian Seaway and the tectonic evolution of the region connecting the Pacific and Indian Oceans. As documented in foundational work by Haug and colleagues and refined in subsequent studies, the progressive restriction of this gateway during the Pliocene redirected the flow of warm, low-salinity Pacific water, fundamentally altering the salt budget of the Indian Ocean and the properties of the water available for export around southern Africa. In other words, the very substance of the leakage, its temperature and salinity signature, was different in the Pliocene. Water that leaked into the Atlantic during that epoch may have carried a buoyancy anomaly that hindered rather than helped deep water formation, or at minimum played a far more ambiguous role in the overturning than the modern salt conveyor narrative implies.

The implications extend beyond academic curiosity into the heart of climate projection. State-of-the-art climate models used by the Intergovernmental Panel on Climate Change represent the Agulhas Leakage and its influence on the AMOC with varying degrees of fidelity, and the future trajectory of the overturning circulation remains one of the largest sources of uncertainty in regional climate forecasts. A weakening AMOC would cool northwestern Europe, disrupt the West African and Indian monsoons, shift the Intertropical Convergence Zone and potentially raise sea levels along the North American Atlantic coast. If the leakage-overturning coupling is contingent on specific boundary conditions rather than a permanent feature of the climate system, then models calibrated against the modern ocean may misjudge how the AMOC responds to the rapidly warming, freshening conditions of the twenty-first century.

The Pliocene offers a particularly valuable analogue for that future because atmospheric carbon dioxide concentrations then stood near levels comparable to those projected for the coming decades, and global mean temperatures were elevated by roughly two to three degrees Celsius relative to the preindustrial baseline. Yet the Pliocene was not simply a warmer version of the present. Ice sheets were smaller, sea levels were higher, and the ocean’s circulation operated under a different set of constraints. The new findings suggest that palaeoceanographers must be far more cautious when using Pliocene records to validate models of future AMOC behavior. If the leakage did not drive overturning then, agreement or disagreement between Pliocene data and model simulations cannot be interpreted through the same lens applied to the modern ocean.

Resolving the timing and mechanism of the coupling’s emergence will require a concerted effort across several fronts. Sediment cores from the Agulhas corridor and the South Atlantic must be analyzed at higher temporal resolution, using proxies for temperature, salinity and current strength that can be cross-validated against one another. Paired measurements of foraminiferal geochemistry, including magnesium-to-calcium ratios for temperature and oxygen isotope or trace metal approaches for salinity, allow researchers to disentangle the competing signals of warming and freshening. Radiogenic isotope signatures such as neodymium can trace the provenance of water masses, revealing whether Indian Ocean water actually penetrated the Atlantic interior during key intervals. Meanwhile, high-resolution numerical models of the Pliocene ocean, constrained by these data, can test hypotheses about how eddy shedding, wind stress curl and the retroflection dynamics of the Agulhas Current responded to altered boundary conditions.

The commentary also highlights a broader lesson about the fragility of teleconnections in the Earth system. The Agulhas Leakage is not a fixed pipeline but a chaotic, eddy-mediated process sensitive to the winds of the Southern Hemisphere, the position of the subtropical front and the bathymetry of the Agulhas Bank. Small changes in any of these factors can alter the volume and properties of the water transferred between basins. The Pliocene evidence implies that the leakage can operate in a regime where its influence on the overturning is negligible or even reversed, a reminder that seemingly robust components of the climate machine can be reconfigured as boundary conditions drift. For a planet now pushing the climate system toward conditions unseen in millions of years, that is a sobering thought.

What emerges from this rethinking is a picture of the Atlantic conveyor as a system with a history, one whose modern wiring was assembled over the course of the Pliocene and Pleistocene as gateways closed, ice sheets grew and wind belts migrated. The Agulhas Leakage may have been gradually recruited into its present role as a salt supplier to the overturning, a transition whose timing remains to be pinned down precisely. Until that transition is mapped in detail, scientists will need to treat the deep past not as a simple mirror of the present but as a laboratory of alternative climate configurations. The salty waters spinning off the tip of Africa have long been counted among the AMOC’s most reliable allies. The new evidence suggests that this alliance, like so much else in the climate system, has a beginning, and understanding when it formed may prove essential to anticipating how the conveyor will behave in the warming century ahead.

Subject of Research: The role of the Agulhas Leakage in Atlantic Meridional Overturning Circulation during the Pliocene

Article Title: Rethinking Agulhas Leakage impacts on Atlantic circulation

Article References: Petrick, B. (2026). Rethinking Agulhas Leakage impacts on Atlantic circulation. Nature Geoscience, 19(10), 1142-1143. https://doi.org/10.1038/s41561-026-02111-0

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02111-0

Keywords: Agulhas Leakage, Atlantic Meridional Overturning Circulation, Pliocene, palaeoceanography, AMOC, Indian Ocean, South Atlantic, salinity, ocean circulation, climate change, sediment cores, Nature Geoscience

The AMOC is one of Earth’s largest climate-regulating systems. It transports warm surface water northward, releases heat to the atmosphere in the North Atlantic, and returns colder, denser water toward the deep ocean. This circulation influences temperatures and precipitation patterns across the North Atlantic region and Europe. Because seawater becomes denser when it is colder and saltier, scientists have proposed that the salt carried from the Indian Ocean through the Agulhas system helps sustain the formation of North Atlantic Deep Water, a key component of the AMOC’s returning deep branch.

The new research, led by Suning Hou of Utrecht University with collaborators in the Netherlands, the United States, China and the United Kingdom, examined a major climate transition during the late Pliocene, between approximately 3.6 and 2.6 million years ago. This interval included a pronounced glacial event followed by the mid-Piacenzian Warm Period, when global temperatures exceeded modern preindustrial conditions. The ancient climate offered the researchers a natural experiment: they could investigate how ocean circulation responded when temperatures, ocean fronts and the exchange of water between basins changed substantially.

To reconstruct conditions south of Africa, the team analysed a sediment core collected from International Ocean Discovery Program Site U1475 on the Agulhas Plateau, roughly 500 kilometres south of the South African coast. The sediment preserved microscopic biological and chemical signals from the overlying ocean. Fossil dinocysts—planktonic organisms whose species distributions reflect water temperature—were used to track movement of the Southern Ocean subtropical front. The researchers also analysed organic lipid biomarkers, molecules that can record past sea-surface temperatures and provide an independent indication of changing ocean conditions.

The evidence showed that the subtropical front shifted northward from about 3.4 million years ago into the glacial interval. As the front moved north, the region around the Agulhas Plateau cooled by approximately 3 degrees Celsius and developed subpolar conditions. Because the position of this front helps determine how easily Indian Ocean water can flow around southern Africa and enter the Atlantic, the reconstruction indicates that Agulhas Leakage weakened sharply and may have nearly shut down. Under the traditional salt-transport hypothesis, such a reduction should have weakened the AMOC.

The Atlantic records told a different story. To determine how the circulation responded, the researchers produced temperature reconstructions from Ocean Drilling Program Site 625 in the northern Gulf of Mexico and compared them with published records from the equatorial and North Atlantic and the Caribbean Sea. They also tested the geological evidence against numerical climate-model simulations of the same late Pliocene transition. Together, the data and simulations indicated that North Atlantic Deep Water formation and lower-latitude overturning intensified even as the Agulhas contribution declined.

The circulation did not simply become stronger everywhere. The simulations showed that the North Atlantic Current reached less far into the high northern latitudes during the glacial event, while overturning intensified farther south. This reorganization produced a shallower thermocline across much of the Atlantic. The thermocline is the layer separating relatively warm surface water from colder deep water, and its depth strongly influences heat storage, nutrient distribution and the exchange of energy between the ocean and atmosphere. The result suggests that a basin-wide adjustment of the Atlantic water column compensated for the reduced southern salt input.

The pattern initially appeared in an isolated sediment record and was difficult to interpret. When a similar signal was found in sediments from the Agulhas Plateau, the researchers recognized that it was unlikely to be a local anomaly. Instead, the records pointed to a broad restructuring of the Atlantic thermocline and overturning system. Climate-model experiments supported that interpretation, showing that changes in high-latitude deep-water formation and the internal structure of the Atlantic could override the direct influence expected from Agulhas salt transport.

The findings do not mean that Agulhas Leakage is irrelevant to the AMOC, nor do they provide a simple forecast of how the modern circulation will respond to global warming. The late Pliocene had different continental configurations, ice sheets, atmospheric conditions and freshwater pathways from those of today. In particular, Arctic freshwater input—an important influence on modern deep-water formation—was not identical to the present-day system. The study instead shows that the controls on ocean overturning are not fixed: the same ocean connection can have very different consequences under different climate and boundary conditions.

That conclusion has major implications for climate research. Models and observations often treat the exchange of salt between ocean basins as a direct influence on the AMOC, but the new evidence emphasizes the importance of local processes in the North Atlantic, including cooling, mixing, water-column structure and the precise location of deep-water formation. A future southward shift of the subtropical front could increase salt transport into the Atlantic, while increased Arctic freshwater could counteract that influence. Understanding which effect dominates will require combining geological records, modern observations and climate simulations rather than relying on a single controlling mechanism. The research was conducted as part of the OceaNice project, funded by the European Research Council.

Subject of Research: Ocean circulation and paleoclimate

Article Title: Disconnection of the late Pliocene Agulhas Leakage from Atlantic Meridional Overturning Circulation

News Publication Date: 3-Aug-2026

Web References: https://doi.org/10.1038/s41561-026-02055-5

References: Nature Geoscience, DOI: 10.1038/s41561-026-02055-5

Image Credits: NASA’s Goddard Space Flight Center

Keywords: Agulhas Leakage, Atlantic Meridional Overturning Circulation, AMOC, ocean circulation, climate change, paleoclimate, late Pliocene, North Atlantic Deep Water, Southern Ocean subtropical front, Utrecht University