Ellyn T. Montgomery

Evidence for enhanced mixing over rough topography in the abyssal ocean

The overturning circulation of the ocean plays an important role in modulating the Earths climate. But whereas the mechanisms for the vertical transport of water into the deep ocean—deep water formation at high latitudes—and horizontal transport in ocean currents have been largely identified, it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished. Turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise. However, measurements of the internal wave field, the main source of energy for mixing, and of turbulent dissipation rates, have typically implied diffusivities across surfaces of equal density of only ∼0.1u2009cm2u2009s-1, too small to account for the return flow. Here we report measurements of tracer dispersion and turbulent energy dissipation in the Brazil basin that reveal diffusivities of 2–4u2009cm2u2009s-1 at a depth of 500u2009m above abyssal hills on the flank of the Mid-Atlantic Ridge, and approximately 10u2009cm2u2009s-1 nearer the bottom. This amount of mixing, probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry, may be largexa0enough to close the buoyancy budget for the Brazil basin andxa0suggests a mechanism for closing the global overturning circulation.

Report on Sediment Transport Events on Shelf and Slope (STRESS) field season 1 : Winter 1988-1989 Benthic Acoustic Stress Sensor (BASS)

Abstract : Data on the effects of winter conditions on the transport of sediment on the continental shelf off Northern California were collected during the first year of the Sediment TRansport Events on Shelves and Slopes (STRESS) Experiment. This experiment was done in conjunction with Shelf Mixed Layer Experiment (SMILE) and Biological Effects on Coastal Ocean Sediment Transport (BECOST) to provide a complete suite of measurements of nearshore dynamics, sediment transport, and biological interactions. This report includes a general description of the work accomplished during the STRESS field season, carried out in the winter of 1988-1989 off the Northern California coast. Three cruises were completed, one each for deployment, turnaround, and recovery of the instruments. This created two back-to-back sections of data, one from December 4, 1988 to January 23, 1989, and the other from January 29 to March 17, 1989. This report also documents the use of the Benthic Acoustic Stress Sensor (BASS), and the associated acoustic data telemetry link in STRESS. BASS had been used in different configurations previously, but the acoustic telemetry system is new. (edc)

Turbulence and Waves Over Irregularly Sloping Topography: Cruise Report - Oceanus 324

Abstract : This report documents the work of R/V Oceanus cruise 324, which occurred during May of 1998. This cruise was the field component of the Turbulence and Waves in Irregularly Sloping Topography (TWIST) program. TWIST was part of the Littoral Internal Wave Initiative (LIWI) supported by the Office of Naval Research. The objective of TWIST was to sample the background, internal wave and turbulence properties on the Continental Slope in the Mid-Atlantic Bight. Previous investigations have revealed strongly enhanced finescale internal wavefields and much more energetic turbulence due to internal wave breaking above topographic roughness associated with the Mid-Atlantic Ridge. So, an area of steeply sloping ridges and troughs running perpendicular to the continental slope near 36 34N, 74 39W was chosen as the site of the observational program due to its topographic similarity to the Mid-Atlantic Ridge. Five instrument systems were employed to make observations during this cruise: the High Resolution Profiler (HRP), three Moored Profiler (MP) moorings, a Lowered Acoustic Doppler Current Profiler/Conductivity, Temperature, Depth (LADCP/CTD) rosette, eXpendable Current Profilers/eXpendable CTD (XCP/XCTD), and finally, the shipboard ADCP. The data from these instruments (more than 1100 full depth profiles) provide adequate spatial and temporal resolution to describe the finescale and turbulent processes observed.

Use of the High Resolution Profiler (HRP) in the Brazil Basin Tracer Release Experiment.

Abstract : On two recent cruises (January 1996 and February 1997) aboard the R/V Seward Johnson, scientists from the Woods Hole Oceanographic Institution studied the deep mixing processes in the Brazil Basin. Two instrument systems were used in this experiment: the tracer injection and sampling system, and the High resolution Profiler (HRP). The HRP measurements complement those obtained by the tracer sampling system, providing independent estimates of the turbulent and diffusive mixing occurring in the study area. During the cruises, the HRP was used to make two zonal sections across the basin, provide data used to select the tracer injection site, and explore the jagged terrain near the Mid-Atlantic Ridge. The HRP component of the work at sea, an instrument description, data return and some preliminary results are presented in this report.

Fine- and Microstructure Observations at Fieberling Guyot R/V New Horizon Cruise Report

Abstract : This report describes fine- and microstructure profile data taken on a cruise to Fieberling Guyot, a seamount in the northeast subtropical Pacific Ocean. The work performed at sea, instruments used, data return and processing procedures will be summarized here. This cruise took place between March 4 and March 28, 1991 on the R/V New Horizon, and was part of the interdisciplinary Accelerated Research Initiative (ARI) for Abrupt Topography sponsored by the Office of Naval Research. An overall goal of the ARI was to understand the physical, biological, and geological processes occurring near a seamount. The scientific objective of the Seamount Mixing Cruise was to collect data describing the oceanic fine-scale velocity and density fields, as well as the related turbulence and mixing in the vicinity of the seamount. The High Resolution Profiler (HRP) was deployed 95 times above and around the seamount. As well, two test dives were conducted on the way to the site, and eight deployments completed in deep basins off the southern California coast before returning to port. Three near-synoptic surveys of the seamount were completed with the deployment of 128 Expendable Current Profilers (XCPs). The temperature field of the upper 760 meters of water within a 50 kilometer radius of the seamount was mapped using 144 Expendable Bathythermographs (XBTs). Oceanography at seamounts, Internal and inertial waves, Turbulence, Diffusion & mixing processes.

A new vehicle for measuring deep ocean mixing

The High Resolution Profiler II (HRP-II) is a unique free vehicle developed at WHOI to measure a range of scales of ocean mixing, from turbulence to advection. On-board mission control software allows autonomous operation during data acquisition profiles to depths of up to 6000 meters. The new profiler was exercised in deep water during a successful test cruise in January 2004.

Water samples in support of oceanographic and water-quality measurements in Chincoteague Bay, Maryland and Virginia, 2014-15, U.S. Geological Survey Field Activity 2014-048-FA

U.S. Geological Survey scientist and technical support staff measured oceanographic, water quality, seabed elevation change, and meteorological parameters in Chincoteague Bay, Maryland and Virginia, during the period of August 13, 2014 to July 14, 2015 as part of the Estuarine Physical Response (EPR) to Storms project (GS2-2D) supported by the Department of the Interior Hurricane Sandy Recovery program. These measurements provide time-series and supplemental data to quantify the response and understand the resilience of this back-barrier estuarine system to storm events. The Assateague Island National Seashore (U.S. National Park Service) and the Chincoteague National Wildlife Refuge (U.S. Fish and Wildlife Service) are both situated on the east side of Chincoteague Bay, and serve as the barrier between Chincoteague Bay and the Atlantic Ocean as well as providing globally important migratory bird habitat and a popular recreation destination.

Summary of oceanographic measurements for characterizing light attenuation and sediment resuspension in the Barnegat Bay-Little Egg Harbor Estuary, New Jersey, 2013

The U.S. Geological Survey, in cooperation with the New Jersey Department of Environmental Protection, measured suspended-sediment concentrations, currents, waves, light attenuation, and a variety of other water-quality parameters in the summer of 2013 in Barnegat Bay-Little Egg Harbor, New Jersey. These measurements quantified light attenuation and sediment resuspension in three seagrass meadows. Data were acquired sequentially at three paired channel-shoal sites, as the equipment was moved from south to north in the estuary. Data were collected for approximately 3 weeks at each site.

Studying seafloor bedforms using autonomous stationary imaging and profiling sonars

The Sediment Transport Group at the U.S. Geological Survey, Woods Hole Coastal and Marine Science Center uses downward looking sonars deployed on seafloor tripods to assess and measure the formation and migration of bedforms. The sonars have been used in three resolution-testing experiments, and deployed autonomously to observe changes in the seafloor for up to two months in seven field experiments since 2002. The sonar data are recorded concurrently with measurements of waves and currents to: a) relate bedform geometry to sediment and flow characteristics; b) assess hydrodynamic drag caused by bedforms; and c) estimate bedform sediment transport rates, all with the goal of evaluating and improving numerical models of these processes. Our hardware, data processing methods, and test and validation procedures have evolved since 2001. We now employ a standard sonar configuration that provides reliable data for correlating flow conditions with bedform morphology. Plans for the future are to sample more rapidly and improve the precision of our tripod orientation measurements.

Use of the High Resolution Profiler (HRP) in the Salt Finger Tracer Release Experiment (SFTRE)

Abstract : The Salt Finger Tracer Release Experiment (SFTRE) was conducted in the tropical North Atlantic in 2001. The experimental area was east of Barbados and is characterized by thermohaline staircase features prevalent in the depth range of 200-600 meters. The goal of this experiment was to quantify the distribution and intensity of vertical mixing in a region of thermohaline staircases. Two cruises were required to accomplish this goal: one to survey with the High Resolution Profiler (HRP) and inject sulfur hexalluoride (SF6) tracer, and another ten months later to map the spatial distribution of tracer and obtain additional estimates of diffusive and turbulent mixing rates using the HRP. The first cruise of the SFTRE experiment took place between January 15 and February 12, 2001 on the R/V Oceanus, leg 365-2 (OC365). An XBT survey identified an area of robust staircases that became the injection site. Then 175 kg of SF6 tracer was injected in nine streaks in a layer with temperature of about 10 deg C. When the injection mechanism was being replenished, HRP profiles were made in the area of the tracer patch. The profiles yielded estimates of the mixing rates at the start of the experiment. Near the end of the cruise, water samples from the patch were used to map the actual tracer distribution immediately after deployment.