For decades, scientific consensus held that the transfer of warm, saline Indian Ocean water around South Africa into the Atlantic Ocean—known as Agulhas Leakage—played a fundamental role in sustaining or strengthening the Atlantic Meridional Overturning Circulation (AMOC). According to conventional theory, this interocean exchange supplies essential salt that supports North Atlantic Deep Water formation. However, new research led by an international team from institutions across the Netherlands, the USA, China, and the UK has challenged this foundational textbook concept, demonstrating that the AMOC can remain strong even when Agulhas Leakage weakens significantly, as reported by miragenews.com.
New Geological Evidence Challenges Longstanding Textbook Theories
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,
said lead author Dr Suning Hou from Utrecht University, according to EurekAlert. Hou added that the AMOC can remain robust despite a reduction in the leakage from the Indian Ocean.
Examining Late Pliocene Climate Transitions
To test ocean-circulation dynamics, researchers investigated the late Pliocene spanning 3.6 to 2.6 million years ago. This specific interval encompassed a brief but pronounced glacial event followed by the mid-Piacenzian Warm Period, which featured warmer climate conditions than today. New reconstructions from offshore South Africa indicate that the subtropical front shifted northward beginning around 3.4 million years ago as the glacial event developed.
During this cooler timeframe, the Agulhas region experienced a temperature drop of roughly 3 degrees Celsius, and Site U1475 encountered subpolar conditions. These changes suggested that Agulhas Leakage weakened substantially and may have approached a shutdown. Under the traditional hypothesis, this reduction in salt transport should have triggered a weakening of the AMOC. Instead, geological evidence and numerical climate-model simulations revealed a contrasting pattern.
Basin-Wide Reorganization of the Ocean Thermocline
To track how Atlantic oceanography responded to shifts in Agulhas Leakage, the research team generated temperature records from Ocean Drilling Program Site 625 in the northern Gulf of Mexico. They combined these findings with published evidence gathered from the equatorial Atlantic, the North Atlantic, and the Caribbean Sea. Comparing these observational results with numerical climate simulations allowed the team to connect Southern Ocean frontal movement with Atlantic water-column structure and overturning circulation.
Although the North Atlantic Current reached less far into high northern latitudes during the glacial event, North Atlantic Deep Water formation and lower-latitude overturning intensified, leading to a shoaling thermocline across the Atlantic basin.
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,
explained Prof. Francien Peterse from Utrecht University, as noted by miragenews.com. Ultimately, the new findings alter the prevailing scientific understanding by indicating that Agulhas Leakage does not directly dictate the behavior of the AMOC through salt transport alone.
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