Timothy J. Boyd

Energetics of M2 Barotropic-to-Baroclinic Tidal Conversion at the Hawaiian Islands

Abstract A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (M2) over a region of the Hawaiian Ridge from Niihau to Maui. This region includes the Kaena Ridge, one of the three main internal tide generation sites along the Hawaiian Ridge and the main study site of the Hawaii Ocean Mixing Experiment. The 0.01°–horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations, moored ADCP currents, and notably reasonable agreement with microstructure data. Barotropic and baroclinic energy equations are derived from the model’s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The M2 barotropic tide loses 2.7 GW of energy over the study region. Of this, 163 MW (6%) is dissipated by bottom friction and 2.3 GW (85%) is converted into internal tides. Internal tide generation primarily occurs along the flanks of the Kaena Ridge and ...

A modified law-of-the-wall applied to oceanic bottom boundary layers

[1] Near the bottom, the velocity profile in the bottom boundary layer over the continental shelf exhibits a characteristic law-of-the-wall that is consistent with local estimates of friction velocity from near-bottom turbulence measurements. Farther from the bottom, the velocity profile exhibits a deviation from the law-of-the-wall. Here the velocity gradient continues to decrease with height but at a rate greater than that predicted by the law-of-thewall with the local friction velocity. We argue that the shape of the velocity profile is made consistent with the local friction velocity by the introduction of a new length scale that, near the boundary, asymptotes to a value that varies linearly from the bottom. Farther from the boundary, this length scale is consistent with the suppression of velocity fluctuations either by stratification in the upper part of the boundary layer or by proximity to the free surface. The resultant modified law-of-the-wall provides a good representation of velocity profiles observed over the continental shelf when a local estimate of the friction velocity from coincident turbulence observations is used. The modified law-ofthe-wall is then tested on two very different sets of observations, from a shallow tidal channel and from the bottom of the Mediterranean outflow plume. In both cases it is argued that the observed velocity profile is consistent with the modified law-of-the-wall. Implicit in the modified law-of-the-wall is a new scaling for turbulent kinetic energy dissipation rate. This new scaling diverges from the law-of-the-wall prediction above 0.2D (where D is the thickness of the bottom boundary layer) and agrees with observed profiles to 0.6D.

Circulation features in the central Arctic Ocean revealed by nuclear fuel reprocessing tracers from Scientific Ice Expeditions 1995 and 1996

Measurements of the tracer radionuclides 129 I and 137 Cs were conducted on seawater samples collected during the Scientific Ice Expedition cruises to the Arctic Ocean of the U.S. Navy nuclear submarines, USS Cavalla and USS Pogy in 1995 and 1996, respectively. These radionuclides are derived mainly from discharges from the Sellafield (England, United Kingdom) and La Hague (France) nuclear fuel reprocessing plants and are subsequently transported with Atlantic water into the Arctic Ocean through Fram Strait and the Barents Sea. Iodine 129 results from halocline waters (water depths of 59 and 134 m) collected virtually synoptically throughout the central Arctic Ocean during the USS Cavalla cruise clearly show the front between Atlantic origin water having high 129 I levels (>100 × 10 7 atoms L -1 ) and Pacific origin water labeled mainly by fallout ( 30 × 10 7 atoms L -1 ) measured in central regions of the Canada Basin indicate that the interior is more efficiently ventilated than previously thought, possibly by separation from boundary currents flowing over the continental margin north of the Chukchi Plateau. The 129 I and 137 Cs data were interpreted using a transit time model that provided estimates of 6-7 years (±0.5 years) for the passage of halocline and Atlantic Water from the Norwegian Coastal Current (60°N) to the continental slope of the Makarov Basin and a lower limit of 8 years for transport to interior regions of the Makarov and Amundsen Basins.

Incoherent Nature of M2 Internal Tides at the Hawaiian Ridge

AbstractMoored current, temperature, and conductivity measurements are used to study the temporal variability of M2 internal tide generation above the Kaena Ridge, between the Hawaiian islands of Oahu and Kauai. The energy conversion from the barotropic to baroclinic tide measured near the ridge crest varies by a factor of 2 over the 6-month mooring deployment (0.5–1.1 W m−2). The energy flux measured just off the ridge undergoes a similar modulation as the ridge conversion. The energy conversion varies largely because of changes in the phase of the perturbation pressure, suggesting variable work done on remotely generated internal tides. During the mooring deployment, low-frequency current and stratification fluctuations occur on and off the ridge. Model simulations suggest that these variations are due to two mesoscale eddies that passed through the region. The impact of these eddies on low-mode internal tide propagation over the ridge crest is considered. It appears that eddy-related changes in stratif...

Tidally Forced Internal Waves and Overturns Observed on a Slope: Results from HOME

Abstract Tidal mixing over a slope was explored using moored time series observations on Kaena Ridge extending northwest from Oahu, Hawaii, during the Survey component of the Hawaii Ocean Mixing Experiment (HOME). A mooring was instrumented to sample the velocity and density field of the lower 500 m of the water column to look for indirect evidence of tidally induced mixing and was deployed on a slope in 1453-m water depth for 2 months beginning in November 2000. The semidiurnal barotropic tidal currents at this site have a significant cross-ridge component, favorable for exciting an internal tidal response. A large-amplitude response is expected, given that the slope of the topography (4.5°) is nearly the same as the slope of the internal wave group velocity at semidiurnal frequency. Density overturns were inferred from temperature profiles measured every 2 min. The number and strength of the overturns are greater in the 200 m nearest the bottom, with overturns exceeding 24 m present at any depth nearly ...

Oceanic heat delivery via Kangerdlugssuaq Fjord to the south‐east Greenland ice sheet

Acceleration of the Greenland Ice Sheet (GrIS) tidewater outlet glaciers has increased the ice sheets contribution to global sea level rise over the last two decades. Coincident increases in atmospheric temperatures around Greenland explain some of the increased ice loss, but warm Atlantic-origin water (AW) is increasingly recognized as contributing to the accelerating ice-mass loss, particularly, via the outlet glaciers of south-east (SE) Greenland. However, there remains a lack of understanding of the variability in heat content of the water masses found to the east of Greenland and how this heat is communicated to the outlet glaciers of the GrIS. Here a new analysis is presented of ocean/GrIS interaction in which the oceanic heat flux toward the ice sheet in Kangerdlugssuaq Fjord (0.26 TW) is an order-of-magnitude greater than that reported for the other major outlet glacier of SE Greenland (Helheim). Heat delivered by AW to the calving front of Kangerdlugssuaq is equivalent to ∼10 m d−1 melt (i.e., 30–60% of the ice flow speed), and thus is highly significant. During the observational campaign in September 2010 warm Polar Surface Water (PSWw) melted a substantial volume of ice within the fjord; equivalent to 25% of the volume melted by AW alone. Satellite-derived sea surface temperatures show large interannual variability in PSWw over the 20 year period 1991–2011. Anomalously warm PSWw was observed within the fjord prior to the well-documented major ice front retreats of May 2004 and November 2010.

Convectively Driven Mixing in the Bottom Boundary Layer

Closely spaced vertical profiles through the bottom boundary layer over a sloping continental shelf during relaxation from coastal upwelling reveal structure that is consistent with convectively driven mixing. Parcels of fluid were observed adjacent to the bottom that were warm (by several millikelvin) relative to fluid immediately above. On average, the vertical gradient of potential temperature in the superadiabatic (statically unstable) bottom layer was found to be 21.7 3 1024 Km 21, or 6.0 3 1025 kg m24 in potential density. Turbulent dissipation rates («) increased toward the bottom but were relatively constant over the dimensionless depth range 0.4‐1.0z/D (where D is the mixed layer height). The Rayleigh number Ra associated with buoyancy anomalies in the bottom mixed layer is estimated to be approximately 1011, much larger than the value of approximately 10 3 required to initiate convection in simple laboratory or numerical experiments. An evaluation of the data in which the bottom boundary layer was unstably stratified indicates that the greater the buoyancy anomaly is, the greater the turbulent dissipation rate in the neutral layer away from the bottom will be. The vertical structures of averaged profiles of potential density, potential temperature, and turbulent dissipation rate versus nondimensional depth are similar to their distinctive structure in the upper ocean during convection. Nearby moored observations indicate that periods of static instability near the bottom follow events of northward flow and local fluid warming by lateral advection. The rate of local fluid warming is consistent with several estimates of offshore buoyancy transport near the bottom. It is suggested that the concentration of offshore Ekman transport near the bottom of the Ekman layer when the flow atop the layer is northward can provide the differential transport of buoyant bottom fluid when the density in the bottom boundary layer decreases up the slope.

Cooling of the West Spitsbergen Current: Wintertime Observations West of Svalbard

The West Spitsbergen Current (WSC) is the major source of heat and salt for the Arctic Ocean and the areas of deep convection in the Greenland Sea. The WSC current cools dramatically downstream. Hydrographic and velocity data from a 3-week, midwinter cruise off Spitsbergen are used to investigate the heat budget of the WSC and the mechanisms of cooling. The downstream divergence of mean heat flux in the WSC produces a heat loss of at least 1000±400 Wm−2 averaged over the width of the current. Approximately 350 Wm−2 is lost to the atmosphere and 200 Wm−2 is lost to melting ice over a region somewhat wider than the current. Cooling of the WSC to the atmosphere converts the inflowing Atlantic Water (AW) to Lower Arctic Intermediate Water, which is sufficiently salty to convect. Cooling by ice converts the AW to much fresher Arctic Surface Water, which is too light to convect. The relative importance of these two conversions is primarily controlled by the rate at which the wind advects ice from the Barents Sea over the WSC. The warmest water of the WSC is often observed 100–200 m below the surface. Despite the lack of direct contact with the surface, this warm core cools at about 800 Wm−2 in our observations. This rate is too large to be caused by diapycnal diffusion. We suggest that the energetic eddy field in this area diffuses heat along the steeply sloping isopycnal surfaces that connect the warm core to the surface, renewing the surface layer several times per day. This is consistent with the very shallow surface mixed layers and high level of intrusions observed. We conclude that the surface layer of the WSC is cooled by the atmosphere and by ice from the Barents Sea and that isopycnal diffusion by mesoscale eddies continually renews this surface, thus cooling the interior of the WSC. The relative magnitude of these processes determines whether the inflowing warm, salty AW is converted to light, fresh surface water or salty, cold intermediate water.

Atmospheric forcing of the Oregon coastal ocean during the 2001 upwelling season

larger-scale and longer-term conditions. Southward wind stresses of 0.05� 0.1 N m � 2 occurred roughly 75% of the time, with a sustained period of dominantly southward stress from mid-June through July. Wind variations were correlated with variations in the largescale Aleutian Low and North Pacific High pressure centers; correlations with the continental Thermal Low were small. Intraseasonal oscillations in alongshore wind stress (periods near 20 days) correlated with the north-south position of the jet stream. These stress oscillations drove 20 day oscillations in upper ocean temperature, with a lag of roughly 5 days for maximum correlation and amplitudes near 4� C. The sum of sensible and latent air-sea heat fluxes was generally into the atmosphere through June, then weakly into the ocean thereafter, with fluctuations on synoptic timescales. Semidiurnal fluctuations in surface air temperature were observed at two northern moorings, apparently forced indirectly by nonlinear internal ocean tides. The diurnal cycle of wind stress was similar for both southward and northward wind conditions, with the diurnal alongshore fluctuation southward in the evening and northward in the morning. During southward winds the marine atmospheric boundary layer (MABL) was typically defined clearly by a strong temperature inversion, and a shallow stable internal boundary layer often formed within the MABL over cool upwelled waters, with surface air temperature roughly 1� C lower inshore than offshore. During northward winds, essentially no low-level temperature stratification was observed.

Organization of stratification, turbulence, and veering in bottom Ekman layers

[1] Detailed observations of the Ekman spiral in the stratified bottom boundary layer during a 3-month period in an upwelling season over the Oregon shelf suggest a systematic organization. Counter-clockwise veering in the bottom boundary layer is constrained to the weakly stratified layer below the pycnocline, and its height is nearly identical to the turbulent boundary layer height. Veering reaches 13+/� 4 degrees near the bottom and exhibits a very weak dependence on the speed and direction of the interior flow and the thickness of the veering layer. A simple Ekman balance model with turbulent viscosity consistent with the law-of-the-wall parameterization modified to account for stratification at the top of the mixed layer is used to demonstrate the importance of stratification on the Ekman veering. The model agrees reasonably well with observations in the lower 60–70% of the bottom mixed layer, above which it diverges from the data due to the unaccounted physics in the interior. Neglect of stratification in an otherwise identical model results in far worse agreement with the data yielding veering in the bottom Ekman layer which is much smaller than measured, but distributed over a much thicker layer.