Mean velocity, eddy kinetic energy (EKE), and eddy potential energy (EPE) Horizontal diffusivity estimates are presented in section 3.Ģ. Obviously these assumptions are rarely met and care must be exercised to apply Taylor's concepts in realistic geophysical situations. Following Taylor's 1921 seminal paper, horizontal diffusivities can also be obtained directly from the statistics of particle trajectories under the assumptions of homogeneous and stationary turbulence. In this paper (Part II), float data are analyzed statistically in an Eulerian fashion.īox statistics of Eulerian means and variances of float velocities and temperatures are presented in section 2. Float dispersion from their initial clusters (situated on either side of the MAR axis: one near 36°N, 40°W, the other near 33°N, 33°W) favors zonal over meridional directions over a long time (after one or two years). Basically, float trajectories paint a very turbulent ocean circulation near the base of the subtropical thermocline, with evidently little communications between both sides of the Mid-Atlantic Ridge (MAR). The experimental context and setup and a description of the mesoscale motions as revealed by the individual quasi-Lagrangian trajectories has been given in a companion paper ( Ollitrault and Colin de Verdière 2002, which is Part I of this paper). Lateral diffusivity increases westward (1.5 10 3 m 2 s −1 in the Canary Basin, 3.5 10 3 m 2 s −1 in Newfoundland Basin, 4.1 10 3 m 2 s −1 near Corner Rise Seamounts) and scales approximately as eddy velocity times the first baroclinic Rossby radius of deformation.īetween July 1983 and June 1989, 53.4 float years were collected near 700 m in the central North Atlantic, with 26 SOFAR floats, as part of the TOPOGULF experiment. Eddy kinetic energy and eddy potential energy (the latter inferred from temperature measurements) are equipartitioned on the scale of the eddy field and show a tenfold increase from 33°N, 33°W to 38°N, 50°W. In between, east of the ridge, a tongue of high eddy energy indicates stronger eddy activity and local instabilities of the Azores Current. A mean Azores Current is observed both west of the ridge near 33°N and east of 30°W near 34°N. West of the ridge and north of 38°N, the general northeastward flow of the Gulf Stream system is recovered while, south of 38°N the westward recirculation observed from historical float data between 70° and 55°W is shown to extend as far as 40°W. The mean circulation lacks a significant southward Sverdrupian flow and shows instead zonal bands of alternating westward and eastward currents (except in the Canary Basin). Here an Eulerian analysis of the 53.4 collected float years is performed by grouping data in 2° latitude × 4° longitude boxes. Part I of this paper has given a descriptive view of the trajectories of 26 SOFAR floats drifting near 700-m depth in the central North Atlantic during the mid-1980s, as part of the TOPOGULF experiment.
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