An estimate of bottom frictional dissipation by Gulf Stream fluctuations

Using published values of the kinetic energy per unit mass of fluctuating motions for the deep western North Atlantic an estimate is made of the energy dissipation rate from fluctuations by bottom friction for the Gulf Stream System. It appears that bottom friction may account for all the energy input by the wind, and that this dissipation occurs in only about 20% of the areal extent of this gyre

A Numerical Study of the Interaction Between a Deep Cold Jet and the Bottom Boundary Layer of the Ocean

Abstract A two-dimensional (x-z) primitive equation model is used to study the interaction between a deep cold jet on a sloping bottom and the bottom boundary layer (BBL) of the deep ocean. Two closure schemes are used: a standard second order turbulence closure (SOTC) scheme (the level 2 1/2 model of Mellor and Yamada), and a new eddy viscosity closure scheme (K-model). The latter is a computationally simple model that produces very similar eddy viscosity and velocity fields as the more complicated SOTC-model while saving about 20% of the computational time. The results of the numerical simulations compare favorably to observations from the base of the North Atlantic continental rise where the cold jet known as the Cold Filament (CF) is found. The interaction between the CF and the BBL is found to be dominated by cross-isotherm Ekman flow, resulting in an asymmetry effect with different dynamics at each one of the fronts associated with the CF. Some of the unusual characteristics of this region are explained with the aid of the numerical experiments. These are: velocity profiles significantly different from those obtained by classical Ekman dynamics, unstable BBLs and detachment of bottom layers. Spatial variations in the characteristics of the BBL which are often neglected in deep-ocean studies are found to be significant in this region

Energetics of the Kuroshio south of Japan

Historical GEK data are used to estimate the flow of energy from the mean current to the fluctuations for the surface of the Kuroshio in a region south of Japan. The horizontal transfer components of kinetic energy and temperature variance are calculated and are found to be similar to estimates for the Florida Current...

Near bottom speed and temperature observations on the Blake-Bahama outer ridge

Speed and temperature measurement made in the bottom boundary layer (BBL) in the region of the Western Boundary Undercurrent at 28°22′N, 74° 13′W over an ∼11 day period are presented. The observations suggest that the BBL structure is consistent with that of a turbulent Ekman layer formed in an initially stably stratified fluid over a uniform surface even though they were obtained in and above an abyssal furrow. The inferred friction velocities u*(ū* = 0.66 cm/s) generally are larger than those inferred by Weatherly (1972) under the Florida Current and at times sufficiently large to result in erosion of some of the finer cohesive sediments if the criterion for their erosion summarized in McCave (1978) is assumed to apply at the site of the observations

Too cold\u27 bottom layers at the base of the Scotian Rise

CTD data indicate that the bottom boundary layer in a region of the western North Atlantic near 40N 62W is a distinct body of relatively cold (θ \u3c 1.82°C), fresh (S \u3c 34.894‰) water. This region is a strip ~ 100 km wide aligned approximately along isobaths centered near the 4900 m isobath on the continental margin near its base...

A study of the bottom boundary layer over the Eastward Scarp of the Bermuda Rise

Velocity and temperature measurements from the bottom boundary layer (BBL) over the Eastward Scarp of the Bermuda Rise in water depth of 4620 m show little variability over an 8‐month period. The free‐stream flow 62 m above the bottom was south‐southeasterly following the isobaths in the region with an average speed of 22 cm/s. The current vector in the BBL rotated an average of 5° in a counterclockwise sense between 62 and 0.8 m above the bottom. The thickness of the BBL was ∼40 m and the average magnitude of the bottom stress was ∼0.7 dynes/cm2. Mean speed profiles, height of the BBL, and the magnitude of the bottom stress predicted by a model compare favorably with the observations, but the model predicts a rotation of the current vector between 62 and 0.8 m more than twice that measured. The time‐dependent nature of the flow field is also reproduced by the model. The Bermuda Rise data and speed profile measurements at the base of the Scotian Rise show that the M2 clockwise polarized tide is damped more than the mean current as the bottom is approached in the BBL. This phenomenon is reproduced by the model and can be explained by differing effective Ekman layer thicknesses associated with tidal and steady components of the flow

Shelf harpacticoid copepods do not escape into the seabed during winter storms

Winter storms on temperate shelves frequently rework bottom sediments. When the sediment is put in motion, sediment-dwelling harpacticoid copepods risk being suspended. We tested for evidence that adult harpacticoids move below the layer of reworked sediment to avoid suspension. To do so, we determined the rate at which a moderate storm at a site at 18 m depth in the northern Gulf of Mexico (29° 40.63′N, 84° 22.80′W) exposed subsurface sediment during bed-form development and then subjected intact cores from that site to a similar rate of exposure in a laboratory flume. We found no significant difference in vertical position of the population median for adult males of most species and adult females of all species tested between the eroded and control cores. Even the adult males that moved down did not move far enough and were eroded. We conclude that adult harpacticoids do not shelter from winter storms in the seabed. As they are capable of such behavior, being suspended must be more advantageous than living temporarily at depth in the sediment