Abyssal ocean overturning shaped by seafloor distribution

• Published in: Nature (London) 2017-11, Vol.551 (7679), p.181-186
• Main Authors: de Lavergne, C., Madec, G., Roquet, F., Holmes, R. M., McDougall, T. J.
• Format: Article
• Language: English
• Subjects: 704/106/2738; 704/829/2737; Abyssal circulation; Abyssal zone; Basins; Humanities and Social Sciences; Meridional flow; multidisciplinary; Natural history; Ocean basins; Ocean bottom; Ocean circulation; Ocean floor; Oceanography; Oceans; Palaeoceanography; Physical oceanography; Science; Sciences of the Universe; Thermohaline circulation
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature24472

The abyssal ocean is broadly characterized by northward flow of the densest waters and southward flow of less-dense waters above them. Understanding what controls the strength and structure of these interhemispheric flows—referred to as the abyssal overturning circulation—is key to quantifying the ocean’s ability to store carbon and heat on timescales exceeding a century. Here we show that, north of 32° S, the depth distribution of the seafloor compels dense southern-origin waters to flow northward below a depth of about 4 kilometres and to return southward predominantly at depths greater than 2.5 kilometres. Unless ventilated from the north, the overlying mid-depths (1 to 2.5 kilometres deep) host comparatively weak mean meridional flow. Backed by analysis of historical radiocarbon measurements, the findings imply that the geometry of the Pacific, Indian and Atlantic basins places a major external constraint on the overturning structure. The geometry of the ocean floor sets key regime transitions in the circulation of deep ocean waters. Motion in the deep ocean 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.