Mixed up at the sea floor
Circulation of the ocean's densest waters modulates millennial-scale shifts in climate. Contrary to conventional wisdom, a study finds that the shape of the sea floor constrains where these waters rise towards the surface. See Article p.181 Deep ocean circulation, below about 2.5 kilometres (km), is a key driver of oceanic uptake or release of heat and carbon. A few regions, such as the North Atlantic, are known sources of deep water formation, but the processes that control the movement of deep waters remain as opaque as the abyss itself. Now, Casimir de Lavergne and colleagues show that the characteristic flow patterns—northward below 4 km depth and southward between 2.5 km and 4 km—are tied to the distribution of the ocean floor at certain depths. Essentially, the bathymetry of the ocean, rather than extrinsic properties such as climate gradients, drives key features of deep ocean circulation. The authors incorporate an analysis of radiocarbon into their model that reveals a shadow layer between the surface and deep ocean, with stagnant properties and high potential for long-term carbon storage.