Abstract
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.
Highlights
This studywas undertakenfor two purposes.The firstwas to examine an 8-month near-bottom velocity and temperature record from the Eastward Scarp of the Bermuda Rise at 32ø52.5'N, 57ø29.0'W in water depth of 4620 m, and to compare these observationswith a numerical bottom boundary layer (BBL) model describedby Weatherly et al [1980] and Weatherly and Martin [1978]
It has been observed in flows composed of a mean flow and a small oscillatory flow that these componentsare damped unequally as the bottom is approached in the BBL [Weatherly and Van Leer, 1977; Weatherly and Wimbush, 1980]
U and V are the x andy componentsof velocity relative to the geostrophicvelocity components,U and V, in the interior above the boundary layer, and z is the vertical coordinate, positive upward from the bottom; a' is the bottom slope and a is the isotherm slope relative to the bottom, both assumed to be uniform and small and tilted in the x direction only; f is the Coriolis parameter and X =/3 g is a buoyancy parameter, where /3 is the coefficient of thermal expansionfor seawater; 0 is the potential temperature relative to the interior, or free-stream temperature, 0t linearly extrapolated into the BBL
Summary
This studywas undertakenfor two purposes.The firstwas to examine an 8-month near-bottom velocity and temperature record from the Eastward Scarp of the Bermuda Rise at 32ø52.5'N, 57ø29.0'W in water depth of 4620 m, and to compare these observationswith a numerical bottom boundary layer (BBL) model describedby Weatherly et al [1980] and Weatherly and Martin [1978]. The secondpurposewas to extend the study begun in Weatherly et al [1980] of the combined effects of tidal and mean currents on the structure of the BBL. It has been observed in flows composed of a mean flow and a small oscillatory flow that these componentsare damped unequally as the bottom is approached in the BBL [Weatherly and Van Leer, 1977; Weatherly and Wimbush, 1980]. O(O)/Ozis taken to be constantand set to match the BruntVaisala frequency in the interior; -u'w' and -v'w' are turbulent Reynoldsstressesdividedby density,and -O'w' is the turbulent heat flux divided by density and heat capacity
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