Observations of the Bottom Boundary Layer in the Deep Western Boundary Current

Despite the relative abundance of Atlantic data, scientists still have a very poor understanding of the physical processes of the Deep Western Boundary Current (DWBC), from formation by deep convection, to flow along the continental rise, and to distribution throughout the world's oceans. Scientific understanding of the kinetics of the DWBC is no better than scientific knowledge of its volume transport. But the dynamics of an intense abyssal current flowing over thousands of kilometers of sea-floor cannot be ignored. Knowledge of this flow's force balance, energetics, and internal dissipation is minimal. Transport, topographic stress, entrainment, and mixing all need to be quantified. In particular, the bottom boundary layer (BBL) of this current has not been well measured; this layer may he important in both the dynamics and the mixing of the deep currents. Direct observations of the physical properties of the flow are needed to improve scientific understanding of the kinetics of the DWBC and to constrain general circulation numerical models. An improved understanding of DWBC dynamics will provide an opportunity to begin monitoring DWBC transport. The data described in this report were gathered during an expedition in August 1992 to survey the Deep Western Boundary Current (DWBC) in the North Atlantic ocean. The authors chose the Blake-Bahamas Outer Ridge (BBOR) as the site for a study of the transport and kinetics of the DWBC because the flow is particularly strong, narrow, and steady. The objectives of the experiment were to measure the transport of the DWBC, to determine whether the BBOR might be a suitable site for monitoring the transport, and to study the dynamics of the DWBC as it flows around the BBOR. In particular, the authors sought evidence for the existence of postulated slippery bottom boundary layers. The thesis focuses on a description of the observations of the frictional bottom boundary layer.