New findings from an international research team based in the Netherlands, USA, China and UK suggest that this relationship is more complicated than previously assumed. Their results indicate that changes in Agulhas Leakage do not always produce the expected response in deep water formation. The study offers a clearer picture of how the AMOC works and how its behavior may vary under different climate conditions.

Testing a Textbook Explanation of the AMOC

The Atlantic Meridional Overturning Circulation (AMOC) transports warm surface water toward the north while carrying colder, denser water back toward the south at greater depths. Through this exchange, it influences temperatures across the North Atlantic and much of Europe, making it a central part of the global climate system.

For decades, researchers have proposed that Agulhas Leakage helps maintain or strengthen the AMOC. This leakage carries warm, salty Indian Ocean water around South Africa and into the Atlantic. Its strength is influenced by shifts in the position of the subtropical front. According to the traditional explanation, the added salt encourages the formation of North Atlantic Deep Water, which helps sustain Atlantic overturning.

"This was basically the first textbook concept, that I learnt when I was a bachelor student. It was surprising to find geological evidence showing that it isn't universally true. The AMOC can remain strong even when Agulhas Leakage weakens," explains lead author Dr. Suning Hou from Utrecht University.

Hou and his colleagues examined conditions during the late Pliocene (3.6-2.6 million years ago). This period included a short but significant glacial event followed by the mid-Piacenzian Warm Period, when global conditions were warmer than they are today. The transition provided researchers with a useful opportunity to study how ocean circulation near South Africa and within the Atlantic responded as the climate shifted from cooler to warmer conditions.

Ancient Sediments Reveal Shifting Ocean Boundaries

The team studied a marine sediment core collected from International Ocean Discovery Program Site U1475 on the Agulhas Plateau, about 500 kilometers south of South Africa.

Within the core, researchers examined fossilized microplankton known as dinocysts, along with organic lipid biomarkers. These materials allowed them to estimate past ocean temperatures and track movements of the Southern Ocean subtropical front toward the north or south.

Hou: "If you find a change in the dinocyst assemblage in the sediment, this means that the front shifted. For instance, if you find more of the warmer species and less of the colder ones, the front has moved south. A more southerly front generally opens a wider pathway for Indian Ocean water to leak into the Atlantic, and vice versa."

Using these indicators, the researchers produced a detailed reconstruction of possible changes in Agulhas Leakage throughout the late Pliocene.

To determine how the Atlantic responded, they also developed temperature records from Ocean Drilling Program Site 625 in the northern Gulf of Mexico. They combined those findings with previously published evidence from the equatorial Atlantic, the North Atlantic and the Caribbean Sea.

The team then compared the geological records with numerical climate model simulations covering the late Pliocene glacial event and the warmer period that followed. Together, the evidence from several ocean basins connected movements of the Southern Ocean front with changes in Atlantic temperature layers and overturning circulation.

Agulhas Leakage Declined While Overturning Intensified

The reconstruction from waters south of Africa showed that the subtropical front began moving northward around 3.4 million years ago and continued doing so during the glacial event.

At the same time, temperatures in the Agulhas region fell by about 3 degrees Celsius. Site U1475 also developed subpolar conditions. These changes indicate that Agulhas Leakage weakened dramatically and may have come close to stopping.

According to the conventional theory, a reduction in the transport of salty water should have weakened the AMOC. Instead, both the geological evidence and the computer simulations revealed the opposite pattern in important parts of the circulation system.

During the glacial interval, the North Atlantic Current did not extend as far into the high northern latitudes. Even so, the formation of North Atlantic Deep Water became stronger, as did overturning at lower latitudes. This intensified circulation caused the thermocline, the boundary between warmer surface water and colder deep water, to become shallower across the Atlantic.

Co-author Carolien van der Weijst, a previous PhD student with Utrecht University, first detected this unusual signal years earlier in a single sediment record. The finding suggested that something unexpected had occurred, but it was not yet clear whether the pattern reflected a local change or something much larger.

"When the same pattern was discovered in the Agulhas Plateau, we realized we were looking at a basin-wide reorganization of the ocean thermocline rather than a local anomaly. Then we confirmed this with climate model simulations," says Prof. Francien Peterse from Utrecht University. "The whole story suddenly made sense."

Rethinking What Controls Atlantic Circulation

The results challenge the idea that Agulhas Leakage directly controls the AMOC by delivering salt to the Atlantic. Instead, they show that the relationships among major parts of the global ocean circulation system can change depending on the climate and the physical boundaries of the ocean.

Future assessments of AMOC behavior may therefore need to consider more than the supply of salty water from the south. Local processes that control deep water formation in the North Atlantic may be just as important, or even more important, under certain conditions.

The researchers emphasize that their conclusions are based on the geography and climate of the late Pliocene. The findings should not be treated as a direct forecast of how the AMOC will respond to current or future global warming.

A future southward movement of the subtropical front could increase the amount of salty water entering the Atlantic. However, the modern AMOC is also affected by saltwater and freshwater arriving from the Arctic, and those conditions differ from the geography of the late Pliocene.

The broader lesson is that the main forces governing Atlantic overturning may not remain constant throughout Earth's history. Different climate states can change which mechanisms have the greatest influence on the circulation.

This research was part of a larger research project called "OceaNice," funded by the European Research Council.