Direct measurement of the cross-density overturning circulation in the Southern Ocean

The global ocean overturning circulation plays a central role in climate by transporting heat, freshwater and carbon around the Earth. Cold water that sinks at high latitudes is upwelled along surfaces of constant density connecting the deep ocean to the sea surface in the Southern Hemisphere. Cross-density circulation can however shortcut the along-density circulation, thereby impacting large-scale water mass pathways, overturning time scales, and the abyssal ocean’s ability to act as a carbon reservoir. Despite its crucial role for climate, the cross-density circulation is poorly known since our capacity to measure it has historically been lacking. In this study, we present the first measurements of the slow multi-year cross-density circulation in the Southern Ocean interior. We show that 10–25% of the Southern Ocean overturning circulation is redirected through cross-density shortcuts. We find strong regional variations in the intensity of cross-density circulation, with hotspots where cross-density velocities are enhanced by 5 to 14-fold. Our results suggest a disproportionate contribution of those regions to the Southern Ocean overturning circulation, and highlight the importance of reliably representing them in our conceptual and forecast models of the ocean’s role in climate.