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Upwelling in the world's strongest current, the Antarctic Circumpolar Current, is thought to be driven by wind stress, surface buoyancy flux, and mixing generated from the interaction between bottom currents and rough topography. However, the impact of localized injection of heat by hydrothermal vents where the Antarctic Circumpolar Current interacts with mid-ocean ridges remains poorly understood. Here a circumpolar compilation of helium and physical measurements are used to show that while geothermal heat is transferred to the ocean over a broad area by conduction, heat transfer by convection dominates near hydrothermal vents. Buoyant hydrothermal plumes decrease stratification above the vent source and increase stratification to the south, altering the local vertical diffusivity and diapycnal upwelling within 500 m of the sea floor by an order of magnitude. Both the helium tracer and stratification signals induced by hydrothermal input are advected by the flow and influence properties downstream.

Plain Language Summary Oceans soak up over 90% of the energy from global warming and regulate the Earth's climate. Along the ocean floor, more than 630 hydrothermal vents are spewing superhot plumes of water out of cracks in the Earth's crust. At the same time, the ocean floor is being gently warmed by magma under the Earth's crust, known as geothermal heating. But few research studies have measured and compared the effect of both hydrothermal and geothermal heat sources on major ocean currents. In this study, we analyzed over 3 million temperature, salinity and helium data points across the Southern Ocean that houses the world's strongest current, the Antarctic Circumpolar Current. The aim of the study was to determine how hydrothermal heat and geothermal heat affect the already-turbulent circulation of this current. The study finds that the circulation within a few hundred meters of hydrothermal vents in the Antarctic Circumpolar Current increases by tenfold, compared to circulation around it. The authors show, for the first time, that hydrothermal vents play a major role in ocean currents at a local scale (more than geothermal heat), and this role cannot be ignored, as has previously been done in climate modeling and ocean circulation research.