The Southern Ocean's massive conveyor belt is shifting and splitting apart

Countries & Cities · Published 29 September 2026 Swipe more like this →

In short: A critical component of the global ocean conveyor belt is undergoing a profound reshaping. While the upper circulation has surged in strength, the deep, lower cell is weakening. This divergence, driven by human-induced changes, threatens to disrupt how our planet redistributes heat, carbon, and nutrients.

The Southern Ocean Meridional Overturning Circulation (MOC) is a vital engine of the global climate, acting as a bridge between the deep abyss and the surface. It functions through two distinct components: a time-mean 'Eulerian-mean' component driven by westerly winds, and a time-varying 'eddy-induced' component created by transient mesoscale eddies. Together, these create a residual circulation that regulates the transport of heat, salt, and carbon across the globe.

Since the mid-1970s, this system has undergone significant structural changes. The upper cell, which is highly sensitive to surface winds, has seen its flow increase by 50–60%. Conversely, the larger lower cell—governed by the temperature and salinity of Antarctic bottom water—has weakened by 10–20%. This shift is driven by a combination of factors: anthropogenic CO2 emissions and stratospheric ozone depletion have shifted and intensified the Southern Hemisphere westerly winds, while melting Antarctic ice sheets release fresh meltwater that dilutes salty bottom water.

This dilution increases ocean stratification, making it harder for surface waters to sink and mix with deeper layers. While eddy-induced circulation can sometimes act as a compensatory mechanism to offset wind-driven changes, this balance is precarious. Under high-emission scenarios like SSP5-8.5, the complex interactions between these components remain uncertain. Some studies suggest that under the worst climate change scenarios, the circulation could lose half its strength by 2050, potentially fundamentally altering the ocean's capacity to act as a global heat and carbon sink.

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