Deep-ocean mixing on the basin scale: Inference from North Atlantic transects

Abstract

The measurements of thermohaline fine structure in the North Atlantic along four WOCE zonal transect and two post-WOCE sections were analyzed. The statistics of the near-bottom mixed-layer height H B as well as the Ellison L E and Thorpe L Th scales in the abyss were evaluated in relation to a roughness parameter z r, defined in terms of the rms of bathymetric irregularities of horizontal lengthscales less than ∼20–25 km. Within the near-bottom mixed layer, the variations of potential temperature did not exceed 0.005 °C. A combined probability distribution of H B was perfectly lognormal with the median 46.1 m, irrespective of the latitude. On the average, H B is about 1% of the ocean depth D for the depth range ∼1.5–5.5 km. In shallower regions over the continental slopes ( D<1.5–2 km), the ratio H B/ D is as high as 10%. The same tendency of increasing H B/ D with D was observed for D>5.5 km, in the abyss. The reduced Ellison scale ( L E− L Eo) correlated with the Thorpe scale, L E− L Eo=0.5 L Th, and L E at two post-WOCE transects showed a dependence on z r, L E− L Eo= Az r, where A=O(10 −2). Above the regions of enhanced z r, larger values of Ellison and Thorpe scales were found at larger distances ζ from the ocean floor. The thermohaline fine structure and mixing in abyssal waters are influenced by the topography up to hundreds of meters above the bottom, the latter increasing with z r. The correlations between ( L E− L Eo), L Th, and z r as well as H B and z r can be used to evaluate turbulent lengthscales l tr in the abyssal boundary layer. An example of the calculation of l tr is given for a post-WOCE transatlantic equatorial transect.