For decades, oceanographers have treated the Agulhas Leakage as a crucial salt-delivery system for the Atlantic Ocean. Warm, salty water from the Indian Ocean flows around South Africa and enters the Atlantic, where its salt content is thought to help maintain the density differences that drive the Atlantic Meridional Overturning Circulation, or AMOC. A new study in Nature Geoscience now challenges that long-standing picture, presenting geological evidence that the AMOC can remain strong—even intensify—when Agulhas Leakage weakens dramatically.
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
