Short-circuiting of the overturning circulation in the Antarctic Circumpolar Current

Stirring the oceans Ocean mixing in the current that flows around Antarctica plays a key role in global ocean circulation, as it influences the rate at which water sinking to the deep ocean at high latitudes returns to the surface in the Southern Ocean. But the rates of mixing in the Antarctic Circumpolar Current, and the extent of upwelling induced, remain poorly understood due to a lack of direct observations. A natural phenomenon, the release of helium from submarine volcanoes into the current near Drake Passage, provided an opportunity to fill in that observational gap. This natural tracer release experiment made it possible to measure both mixing and upwelling in the southwest Atlantic sector of the current, and the results indicate that the rough topography of the ocean floor there leads to rapid mixing across density surfaces and rapid upwelling along density surfaces. This creates a previously unrecognized 'short circuit' in the global oceanic overturning circulation, allowing cold waters sinking to the ocean abyss to return to the surface more rapidly than was expected. A natural tracer-release experiment (an injection of helium by submarine volcanoes into the current that flows around Antarctica) is used to measure the rates of mixing and upwelling in the southwest Atlantic sector of the current. Results indicate that the rough topography of the ocean floor in this region leads to both rapid mixing across density surfaces and rapid upwelling along density surfaces, which together create a 'short circuit' in the global oceanic overturning circulation. The oceanic overturning circulation has a central role in the Earth’s climate system and in biogeochemical cycling 1 , 2 , as it transports heat, carbon and nutrients around the globe and regulates their storage in the deep ocean. Mixing processes in the Antarctic Circumpolar Current are key to this circulation, because they control the rate at which water sinking at high latitudes returns to the surface in the Southern Ocean 3 , 4 , 5 , 6 , 7 , 8 . Yet estimates of the rates of these processes and of the upwelling that they induce are poorly constrained by observations. Here we take advantage of a natural tracer-release experiment—an injection of mantle helium from hydrothermal vents into the Circumpolar Current near Drake Passage 9 —to measure the rates of mixing and upwelling in the current’s intermediate layers over a sector that spans nearly one-tenth of its circumpolar path. Dispersion of the t