Observational evidence suggests that a significant portion of the diapycnal buoyancy fluxes in the abyss take place in sub- marine valleys, such as the thousands of canyons that corrugate the flanks of slow-spreading mid-ocean ridges, as well as the axial rift valleys. Hydrographically and dynamically, many of these deep valleys are similar, with strong tidal velocities superimposed on low-frequency flows down along-valley density gradients. The effects of diapycnal mixing, leading to a quasi-steady mean state balance the corresponding along-valley advection of dense water. However, in order to parameterize the effects of mechanical mixing on the water properties and dynamics, the underlying processes must be known. Two different processes have been proposed to account for the observations of elevated mixing near rough topography: breaking tidally forced internal waves, and overflows across sills. However, the only existing microstructure (mixing) data set from an abyssal valley is of insufficient horizontal resolution to determine the relative contributions of the two processes to the mixing budget; in particular, there are no microstructure profiles available in the vicinity of sills. In this study, researchers at Columbia University and Florida State University will perform a hydrographic, velocity and mixing survey in the rift valley of the Lucky Strike segment (37 N, Mid-Atlantic Ridge), using CTD, LADCP and microstructure profilers. Two deep basins connected by two parallel sills characterize the rift valley of the Lucky Strike segment. Both sills and deep basins will be sampled and the near-sill velocity field will be monitored throughout the survey using a bottom-mounted ADCP. Currently, available data indicate strong tidal velocities, as well as low-frequency flows across the sills. The results gathered from this work will elucidate and quantify the dominant patterns of the hydrography, circulation and mixing in the Lucky Strike segment and determine the relative contributions of different processes that have been proposed to account for the association between strong mixing and rough topography. This information will be of value for making progress toward realistic mixing parameterizations for inclusion in numerical models, e.g. those used for climate research. In addition to the involvement of a graduate student, the broader impacts also include the potential use of the data to investigate advective and eddy diffusive dispersal near the Lucky Strike hydrothermal vent field, which is the main focus of an on-going inter- national monitoring effort on the Mid-Atlantic Ridge (the MoMAR project).
