Nature 448, 680-683 (9 August 2007) | doi:10.1038/nature06043; Received 9 February 2007; Accepted 21 June 2007

Intense mixing of lower thermocline water on the crest of the Mid-Atlantic Ridge

Louis C. St Laurent1 & Andreas M. Thurnherr2

  1. Department of Oceanography, Florida State University, Tallahassee, Florida 32306, USA
  2. Division of Ocean and Climate Physics, Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA

Correspondence to: Louis C. St Laurent1 Correspondence and requests for materials should be addressed to to L.C.StL. (Email: stlaurent@ocean.fsu.edu).

Buoyancy exchange between the deep and the upper ocean, which is essential for maintaining global ocean circulation, mainly occurs through turbulent mixing1, 2. This mixing is thought to result primarily from instability of the oceanic internal wave field3, but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence4 and the resulting distribution of turbulent mixing remains unknown. Another, more direct, mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography, but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements. Here we present observations from the crest of the Mid-Atlantic Ridge in the subtropical North Atlantic Ocean that suggest that passages in rift valleys and ridge-flank canyons provide the most energetic sites for oceanic turbulence. Our measurements show that diffusivities as large as 0.03 m2 s-1 characterize the mixing downstream of a sill in a well-stratified boundary layer, with mixing levels remaining of the order of 10-4 m2 s-1 at the base of the main thermocline. These mixing rates are significantly higher than the diffusivities of the order of 10-5 m2 s-1 that characterize much of the global thermocline and the abyssal ocean5. Our estimates suggest that overflows associated with narrow passages on the Mid-Atlantic Ridge in the North Atlantic Ocean produce as much buoyancy flux as has previously been estimated for the entire Romanche fracture zone6, 7, a large strait in the Mid-Atlantic Ridge that connects the North and South Atlantic basins. This flux is equivalent to the interior mixing that occurs in the entire North Atlantic basin at the depth of the passages, suggesting that turbulence generated in narrow passages on mid-ocean ridges may be important for buoyancy flux at the global scale.