The vertical structure of the bottom boundary layer on the southern flank of the George Bank during late winter

Abstract

The bottom boundary layer structure on the southern flank of Georges Bank was investigated for the period February 19 to March 20, 1995. During this time, vertical stratification was weak and the near bottom waters were almost homogeneous. Georges Bank is a region of strong tidal currents, with the dominant constituent, the M2, carrying most of the tidal energy. A clockwise around-bank circulation is present throughout the year, with flow directions being to the southwest along the southern flank, and to the northeast along the northern flank. Main generation mechanisms of the mean circulation are tidal rectification over the sloping bottom, horizontal stratification, and, along the continental shelf, flow associated with upstream sources including the Labrador Current. Due to the effects of winter cooling, cross-bank density gradients are weak during winter, and the clockwise mean flow is smallest during the cold seasons. Velocity, temperature, and salinity data were taken at Stratification Site 1 (40°51'N, 67°33'W) located on the 76 m isobath 30 km upslope of the shelf slope front. The M2 pressure gradient was almost rectilinear and across-bank, forcing a current ellipse with eccentricity e ≈ 0.62 and depth-averaged current amplitudes of ≈ 40 cm/s. The magnitude of the depth-averaged mean flow is 9 ± 5 cm/s for the period investigated and small compared to the tidal currents. Estimates of friction velocities and bottom roughness were obtained from best-fit logarithmic profiles to velocity measurements taken between 0.28 and 2.5 m above the sea floor. Boundary layer heights for the tidal and subtidal flows were derived from profiles of the M2 and low-pass filtered currents. Meaningful parameterizations of these heights were found from the equations of motion assuming a logarithmic velocity region in the vicinity of the sea floor as suggested by the observations. Scaling arguments show that the mean and tidal flows interact through the effects of bottom friction, with the largest part of the turbulent fluctuations being set by the M2 tide. Scaling arguments for the tidal boundary layer height were discussed with respect to earlier results by Soulsby (1990). Integrated velocity defects were computed for the M2 and low-pass filtered flow, and compared to boundary layer transports predicted by the estimated bottom stress. Closer investigation of subtidal velocity defects reveals the importance of advective terms in the time-averaged along-bank momentum equation, in agreement with previous studies (Zimmermann, 1980; Loder, 1980; Huthnance, 1981) discussing the nonlinear character of tidal rectification. In a numerical modeling study, the performance of a simple one-dimensional, two-layer model was examined. Numerical mixing coefficients were parameterized according to K = K-u*z in the sublayer z ≤ l, and K = K-u*l in the rest of the water column, where -u* is the mean friction velocity during one tidal cycle. Based on the comparison of model solutions to observations, a characteristic parameterization for the sublayer thickness was derived, suggesting optimal values for l to be similar to the observed logarithmic layer height l ≈ 5 m. Numerical predictions using the Mellor-Yamada level 2.5 turbulence closure were also investigated. The performance of this advanced closure scheme was found to be less convincing than results from the much simpler two-layer model.