Murray D. Levine

Upper-Ocean Inertial Currents Forced by a Strong Storm. Part I: Data and Comparisons with Linear Theory

Abstract A strong, isolated October storm generated 0.35–0.7 m s−1 inertia] frequency currents in the 40-m deep mixed layer of a 300 km×300 km region of the northeast Pacific Ocean. The authors describe the evolution of these currents and the background flow in which they evolve for nearly a month following the storm. Instruments included CTD profilers, 36 surface drifters, an array of 7 moorings, and air-deployed velocity profilers. The authors then test whether the theory of linear internal waves propagating in a homogeneous ocean can explain the observed evolution of the inertial frequency currents. The subinertial frequency flow is weak, with typical currents of 5 cm s−1, and steady over the period of interest. The storm generates inertial frequency currents in and somewhat below the mixed layer with a horizontal scale much larger than the Rossby radius of deformation, reflecting the large-scale and rapid translation speed of the storm. This scale is too large for significant linear propagation of the...

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 ...

The advective flux of heat by mean geostrophic motions in the Southern Ocean

A method that is independent of choice of reference level is advanced for calculating, from standard hydrographic data, advective oceanic heat flux by mean geostrophic motions. The method depends on the ability to choose a path of constant vertically averaged potential temperature (reference temperature). It was applied to a collection of historical hydrographic data from the Southern Ocean. A circumpolar path with reference temperature 1.3°C that closely follows the mean position of the Antarctic Polar Front was chosen. The advective geostrophic heat flux across this path was calculated to be 0±23×1013 W. The standard error (and bias, which was found to be small) of this calculation was estimated from a statistical model of correlation of ocean variability that seems appropriate to the way the data are sampled in space and time. The wind-driven Ekman heat flux was calculated at −15×1013 W, that is, equatorward. To balance Gordons estimate of sea to air heat transfer of 30×1013 W south of the Polar Front, a compensating poleward flux of +45±30×1013 W is postulated. Eddy heat flux seems a prime candidate for accomplishing this flux.

Internal Waves in the Arctic Ocean: Comparison with Lower-Latitude Observations

Abstract A thermistor chain was moored below the pack ice from 50–150 m in the Arctic Ocean for five days in 1981. Oscillations in temperature are attributed to the vertical dispalcement of internal waves. The spectral shape of isotherm dispalcement is consistent with the Garrett-Munk model and other internal wave observations, but the spectral level is significantly lower. Other observations from the Arctic Ocean also exhibit lower internal-wave energy when compared with historical data from lower latitudes. The lower energy may be related to the unique generation and dissipation mechanisms present in the ice-covered Arctic Ocean. Significant peaks in vertical coherence occur at 0.81 and 2.6 cph. The peak at 2.6 cph coincides approximately with the high-frequency spectral cutoff near the local buoyancy frequency; this feature has been observed in many other internal wave experiments. The coherent oscillations at 0.81 cph exhibit a node in vertical dispalcement at 75–100 m. This is consistent with either ...

The Application of Internal-Wave Dissipation Models to a Region of Strong Mixing

Abstract Several models now exist for predicting the dissipation rate of turbulent kinetic energy, ϵ, in the oceanic thermocline as a function of the large-scale properties of the internal gravity wave field. These models are based on the transfer of energy toward smaller vertical scales by wave-wave interactions, and their predictions are typically evaluated for a canonical internal wave field as described by Garrett and Munk. Much of the total oceanic dissipation may occur, however, in regions where the wave field deviates in some way from the canonical form. In this paper simultaneous measurements of the internal wave field and ϵ from a drifting ice camp in the eastern Arctic Ocean are used to evaluate the efficacy of existing models in a region with an anomalous wave field and energetic mixing. By explicitly retaining the vertical wavenumber bandwidth parameter, β*, models can still provide reasonable estimates of the dissipation rate. The amount of data required to estimate β*, is, however, substanti...

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.

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 ...

Altimeter-derived variability of surface velocities in the California Current System: 1. Evaluation of TOPEX altimeter velocity resolution

In this paper, we evaluate the temporal and horizontal resolution of geostrophic surface velocities calculated from TOPEX satellite altimeter heights. Moored velocities (from vector-averaging current meters and an acoustic Doppler current profiler) at depths below the Ekman layer are used to estimate the temporal evolution and accuracy of altimeter geostrophic surface velocities at a point. Surface temperature gradients from satellite fields are used to determine the altimeters horizontal resolution of features in the velocity field. The results indicate that the altimeter resolves horizontal scales of 50–80 km in the along-track direction. The rms differences between the altimeter and current meters are 7–8 cm s−1, much of which comes from small-scale variability in the oceanic currents. We estimate the error in the altimeter velocities to have an rms magnitude of 3–5 cm s−1 or less. Uncertainties in the eddy momentum fluxes at crossovers are more difficult to evaluate and may be affected by aliasing of fluctuations with frequencies higher than the altimeters Nyquist frequency of 0.05 cycles d−1, as indicated by spectra from subsampled current meter data. The eddy statistics that are in best agreement are the velocity variances, eddy kinetic energy and the major axis of the variance ellipses. Spatial averaging of the current meter velocities produces greater agreement with all altimeter statistics and increases our confidence that the altimeters momentum fluxes and the orientation of its variance ellipses (the statistics differing the most with single moorings) represent well the statistics of spatially averaged currents (scales of 50–100 km) in the ocean. Besides evaluating altimeter performance, the study reveals several properties of the circulation in the California Current System: (1) velocity components are not isotropic but are polarized, strongly so at some locations, (2) there are instances of strong and persistent small-scale variability in the velocity, and (3) the energetic region of the California Current is isolated and surrounded by a region of lower energy starting 500–700 km offshore. This suggests that the source of the high eddy energy within 500 km of the coast is the seasonal jet that develops each spring and moves offshore to the central region of the California Current, rather than a deep-ocean eddy field approaching the coast from farther offshore.

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.