Salty Indian Ocean Waters May Not Have Always Powered the Atlantic’s Great Conveyor

Deep beneath the surface of the South Atlantic, one of the planet’s most consequential water movements begins with a dramatic act of oceanic theft. Off the southern tip of Africa, the Agulhas Current, a swift and powerful western boundary current of the Indian Ocean, retroflects back on itself in a vast loop, pinching off enormous swirling eddies that drift westward into the Atlantic. This process, known as the Agulhas Leakage, transfers warm, salty Indian Ocean water into the Atlantic basin, and for decades oceanographers have regarded it as a critical gear in the machinery of the Atlantic Meridional Overturning Circulation, the great conveyor belt of currents that regulates climate across Europe, Africa and the Americas. Now a new analysis published in Nature Geoscience argues that this intimate coupling between leakage and overturning may be a surprisingly recent feature of Earth’s climate system, one that was apparently absent during the Pliocene epoch, a warm period spanning roughly 5.3 to 2.6 million years ago.

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