Observations of the vertical distribution of low frequency kinetic energy in the western North Atlantic

Abstract

Eddy or low frequency (periods greater than a day) kinetic energy per unit mass (KB) in the western North Atlantic is observed to increase approaching the Gulf Stream system along 55W longitude from the interior of the subtropical gyre, by a factor of 30 in the thermocline and by over two orders of magnitude at 4000 m depth. KE at 4000 m depth decreases moving up the Continental Rise from the Gulf Stream. The relative vertical distribution of KE is found to be less depth dependent near the Gulf Stream (with increasing KE?) than in the interior of the gyre. The vertical structure in the MODE-I region is an intermediate case. The shape of the frequency spectrum for KE varies with depth and with geographical position; spectral estimates in the thermocline from the MODE-I area (28N, 70W) are dominated by longer time scales, but at a site near the Gulf Stream (37.5N, 55W), where spectral shape does not vary strongly in the vertical, the thermocline spectrum is more weighted toward the temporal mesoscale (~50-100 days) that dominates at 4000 m depth in the MODE-I area. Spectral estimates at depth appear to be weighted toward shorter time scales moving from the Gulf Stream up the Continental Rise, Slope, and Shelf. If the frequency spectrum for KB is divided into two pieces with periods less than 100 days in one and greater than 100 days in the second, then: (1) The relative vertical KB distribution near the Gulf Stream does not differ significantly from band to band, whereas in the MODE-I region the lower frequencies are much more depth dependent than the higher. (2) The relative vertical KE distribution for periods less than 100 days is not appreciably dissimilar at the two locations, although KE level and total vertical structure are distinct. (3) The relative vertical KE distribution at the lower frequencies in the MODE-I region does not differ significantly from the vertical distribution of total KB at a site farther into the interior of the gyre. Time averaged currents at all depths where data are available show a relatively narrow (~200 km) and comparatively depth-independent return flow for the Gulf Stream system along 55W; transporting roughly 70 X 106 m3 s-1 westward, with about 25 X 106 m3 s-1 relative to the bottom and approximately 45 X 106 m3 s-1 associated with the "bottom transport."