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

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 HB as well as the Ellison LE and Thorpe LTh scales in the abyss were evaluated in relation to a roughness parameter zr, 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 HB was perfectly lognormal with the median 46.1 m, irrespective of the latitude. On the average, HB 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 HB/D is as high as 10%. The same tendency of increasing HB/D with D was observed for D>5.5 km, in the abyss. The reduced Ellison scale (LE−LEo) correlated with the Thorpe scale, LE−LEo=0.5LTh, and LE at two post-WOCE transects showed a dependence on zr, LE−LEo=Azr, where A=O(10−2). Above the regions of enhanced zr, 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 zr. The correlations between (LE−LEo), LTh, and zr as well as HB and zr can be used to evaluate turbulent lengthscales ltr in the abyssal boundary layer. An example of the calculation of ltr is given for a post-WOCE transatlantic equatorial transect.