Abstract

Recent observational, theoretical, and modeling studies all suggest that the upper part of the downwelling limb of the thermohaline circulation is concentrated in strong currents subject to buoyancy loss near lateral boundaries. This is fundamentally different from the traditional view that downwelling takes place in regions of deep convection. Even when resolving the buoyant boundary currents, coarse resolution global circulation and climate models rely on parameterizations of poorly known turbulent mixing processes. In this study, the first direct measurements of downwelling occurring within a basin subject to buoyancy loss are obtained. Downwelling is observed near the basin’s vertical wall within the buoyant boundary current flowing cyclonically around the basin. Although the entire basin is cooled, downwelling is absent in the basin interior. Laboratory rotating experiments are conducted to explicitly resolve the turbulent mixing due to convective plumes, the baroclinic eddies generated by the boundary current, and identify where downwelling takes place. Hence, in the laboratory small vertical velocities can be measured more reliably than in many numerical calculations, while the measurement of these small vertical velocities is still a challenge for field experiments. Downwelling is observed to occur near the vertical wall within a boundary layer with a thickness that scales with the Rossby radius of deformation, consistent with the dynamical balance proposed by a previous numerical study. Hence, downwelling in the Labrador Sea and Lofoten Basin cyclonic boundary currents may be concentrated in a Rossby deformation thick boundary layer in regions with large eddy generation.