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Tom Sanford

Senior Principal Oceanographer Emeritus

Professor Emeritus, Oceanography

Email

sanford@apl.washington.edu

Phone

206-543-1365

Research Interests

Physical Oceanography; Instrumentation; Structure and Dynamics of Currents, Eddies and Waves; Propagation and Dissipation of Internal Waves

Biosketch

Dr. Sanford conducts innovative, high-quality basic and applied oceanographic research, teaches graduate students, mentors postdoctoral researchers, and fosters collaborations with national and international investigators and organizations. Broadly, his research exploits motional induction theory — the motion of seawater through the Earth's magnetic field that produces electric currents and magnetic fields — to infer important aspects of ocean properties and kinetic structure. These methods have been applied to a range of studies in the open ocean and within channels. In nearly five decades as an experimental physical oceanographer, Sanford has participated in many dozen cruises and research projects, provided the oceanographic community with important results, and developed several instruments and new observational methods. Recent efforts include the development of two ocean velocity sensors, one an autonomous vertical profiler (EM-APEX) and the other a bottom lander (HPIES). Prior to these, he led the development of the XCP, an expendable current profile. These are being used to study upper-ocean mixing and convective processes, interactions between internal waves and steady currents, momentum flux into the ocean (such as from hurricanes), as well as the structure and variability of oceanic boundary and estuarine currents.

As Professor in the UW School of Oceanography, Dr. Sanford has taught courses and advised advanced research for about two-dozen graduate students and postdocs. Dr. Sanford is a Fellow of the American Geophysical Union and American Meteorological Society, received the IEEE/Ocean Engineering Society 2008 Distinguished Technical Achievement Award and in 2010 the AMS Henry Stommel Research Award. Since 2008 he has served as an ONR Secretary of the Navy/Chief of Naval Operations Chair of Oceanographic Sciences.

Department Affiliation

Ocean Physics

Education

A.B. Physics, Oberlin College, 1962

Ph.D. Physical Oceanography, Massachusetts Institute of Technology, 1967

Projects

Origins of the Kuroshio and Mindanao Currents

The boundary currents off the east coast of the Philippines are of critical importance to the general circulation of the Pacific Ocean. The westward flowing North Equatorial Current (NEC) runs into the Philippine coast and bifurcates into the northward Kuroshio and the southward Mindanao Current. Quantifying these flows and understanding bifurcation dynamics are essential to improving predictions of regional circulation and Pacific Ocean climate. We have deployed five HPIES off NE Luzon Island under the Kuroshio and nine EM-APEX floats in the NEC as it flows westward toward the Philippine Islands.

8 May 2013

Lateral Mixing

Small scale eddies and internal waves in the ocean mix water masses laterally, as well as vertically. This multi-investigator project aims to study the physics of this mixing by combining dye dispersion studies with detailed measurements of the velocity, temperature and salinity field during field experiments in 2011 and 2012.

1 Sep 2012

Publications

2000-present and while at APL-UW

Autonomous microstructure EM-APEX floats

Lien, R.-C., T.B. Sanford, J.A. Carlson, and J.H. Dunlap, "Autonomous microstructure EM-APEX floats," Methods Oceanogr., 17, 282-295, doi:10.1016/j.mio.2016.09.003, 2016.

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1 Dec 2016

Highlights

• The EM-APEXX float measures U, V, T, S, and turbulence.
• First deployment of synchronized autonomous vertical profilers in a swarm.
• Slow profiling speed captures entire turbulence temperature spectrum.
• Turbulent temperature variance dissipation rate and diffusivity are estimated.
• Provides observations to relate turbulence mixing to N, shear, and Ri.

Eddy-Kuroshio interaction processes revealed by mooring observations off Taiwan and Luzon

Tsai, C.-J., M. Andres, S. Jan, V. Mensah, T.B. Sanford, R.-C. Lien, and C.M. Lee, "Eddy-Kuroshio interaction processes revealed by mooring observations off Taiwan and Luzon," Geophys. Res. Letts., 42, 8090-8105, doi:10.1002/2015GL065814, 2015.

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16 Oct 2015

The influence and fate of westward propagating eddies that impinge on the Kuroshio were observed with pressure sensor-equipped inverted echo sounders (PIESs) deployed east of Taiwan and northeast of Luzon. Zero lag correlations between PIES-measured acoustic travel times and satellite-measured sea surface height anomalies (SSHa), which are normally negative, have lower magnitude toward the west, suggesting the eddy-influence is weakened across the Kuroshio. The observational data reveal that impinging eddies lead to seesaw-like SSHa and pycnocline depth changes across the Kuroshio east of Taiwan, whereas analogous responses are not found in the Kuroshio northeast of Luzon. Anticyclones intensify sea surface and pycnocline slopes across the Kuroshio, while cyclones weaken these slopes, particularly east of Taiwan. During the 6%u2009month period of overlap between the two PIES arrays, only one anticyclone affected the pycnocline depth first at the array northeast of Luzon and 21%u2009days later in the downstream Kuroshio east of Taiwan.

The LatMix summer campaign: Submesoscale stirring in the upper ocean

Shcherbina, A.Y., et al., including E. D'Asaro, R.R. Harcourt, C.M. Lee, R.-C. Lien, and T.B. Sanford, "The LatMix summer campaign: Submesoscale stirring in the upper ocean," Bull. Am. Meteor. Soc., 96, 1257-1279, doi:10.1175/BAMS-D-14-00015.1, 2015.

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1 Aug 2015

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s-1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.

More Publications

Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate

Waterhouse, A.F., et al., including M.H. Alford, E. Kunze, T.B. Sanford, and C.M. Lee, "Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate," J. Phys. Oceanogr., 44, 1854-1872, doi:10.1175/JPO-D-13-0104.1, 2014.

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1 Jul 2014

The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10-4) m2 s-1 and above 1000-m depth is O(10-5) m2 s-1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.

Observations of the cold wake of Typhoon Fanapi (2010)

Mrvaljevic, R.K., P.G. Black, L.R. Centurioni, Y.-T. Chang, E.A. D'Asaro, S.R. Jayne, C.M. Lee, R.-C. Lien, I.-I. Lin, J. Morzel, P.P. Niiler, L. Rainville, and T.B. Sanford, "Observations of the cold wake of Typhoon Fanapi (2010)," Geophys. Res. Lett., 40, 316-321, doi:10.1002/grl.50096, 2013.

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28 Jan 2013

Several tens of thousands of temperature profiles are used to investigate the thermal evolution of the cold wake of Typhoon Fanapi, 2010. Typhoon Fanapi formed a cold wake in the Western North Pacific Ocean on 18 September characterized by a mixed layer that was >2.5°C cooler than surrounding water, and extending to >80 m, twice as deep as the pre-existing mixed layer. The initial cold wake became capped after 4 days as a warm, thin surface layer formed. The thickness of the capped wake, defined as the 26°C to 27°C layer, decreased, approaching the background thickness of this layer with an e-folding time of 23 days, almost twice the e-folding lifetime of the Sea Surface Temperature (SST) cold wake (12 days). The wake was advected several hundreds of kilometers from the storm track by a pre-existing mesoscale eddy. The observations reveal new intricacies of cold wake evolution and demonstrate the challenges of describing the thermal structure of the upper ocean using sea surface information alone.

Internal tides on the East China Sea continental slope

Lien, R.-C., T.B. Sanford, S. Jan, M.-H. Chang, and B.B. Ma, "Internal tides on the East China Sea continental slope," J. Mar. Res., 71, 151-186, doi:10.1357/002224013807343461, 2013.

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1 Jan 2013

Strong semidiurnal internal tides are observed on the continental slope of the East China Sea (ECS) using an array of subsurface moorings and EM-APEX floats. A Princeton Ocean Model (POM) is used to simulate the effects of stratification profiles on the generation and propagation of M2 internal tides; model simulations are compared with observations. On the ECS continental slope northeast of Taiwan, the semidiurnal barotropic tidal current flows nearly perpendicular to the shelf break and continental slope, favoring the generation of internal tides. Both the critical slope analysis and numerical model results suggest multiple generation sites on the continental slope, shelf break and around North MienHua Canyon. Unique high-wavenumber semidiurnal internal tides with a dominant vertical scale of ~100 m are found on the continental slope. The high-wavenumber semidiurnal internal tides appear in a form of spatially coherent shear layers across the ECS slope. They propagate vertically both upward and downward. Patches of strong energy and shear at a typical vertical scale of O(50 m) are present at the intersections of the upward and downward propagating high-wavenumber internal tides. The strong shear of high-wavenumber semidiurnal tides could play an important role in triggering shear instability on the ECS slope. The semidiurnal internal tidal energy flux, primarily in low wavenumbers, on the ECS slope, exhibits strong temporal and spatial variations. The observed depth integrated energy flux is 3.0–10.7 kW m-1, mostly seaward from the continental shelf/slope. The POM model predicts similar seaward energy fluxes at a slightly weaker magnitude, 1.0–7.2 kW m-1. The difference may be due to the absence of mesoscale processes in the model, e.g., the Kuroshio Current and eddies, the assumed horizontally uniform stratification profiles, insufficient model resolution for the abrupt canyon bathymetry, and the lack of the other major semidiurnal tidal constituent, S2, in the model. On the ECS slope, the total energy in the internal wave continuum, between 0.3 cph (beyond semidiurnal tidal harmonics) and the buoyancy frequency, is 6-13 times that of the Garrett–Munk model, presumably as a result of the energy cascade from strong inertial waves and internal tides in the region.

Spatial structure of thermohaline and abyssal internal waves in the Sargasso Sea

Sanford, T.B., "Spatial structure of thermohaline and abyssal internal waves in the Sargasso Sea," Deep Sea Res., Part II, 85, 195-209, doi:10.1016/j.dsr2.2012.07.021, 2013.

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1 Jan 2013

Vertical and horizontal spatial structures are analyzed for the steady and internal wave velocity contributions in one hundred full-water-depth velocity profiles collected in the Sargasso Sea in water depths between 4500 and 5500 m. Temporal decompositions into subinertial, near-inertial, and high-frequency velocity contributions are obtained from multiple, but brief time series at several locations. Horizontal spatial variability is evaluated from two simultaneous velocity profiles at separations ranging from 15 m to 12.5 km. The total internal wave field exhibits equipartition between east and north velocity components, a decrease in energy density at the smallest vertical wavenumbers, and an overall dependence for kinetic energy (KE) on vertical wavenumber as m-2.5. Most of the internal wave energy is in near-inertial motions and, of this, most occurs at Wentzel–Kramers–Brillouin (WKB) normalized vertical wavelengths of 150–800 stretched-m (i.e., sm) with a spectral peak at 500-sm wavelength (for No=3 cph) and average surrounding the peak of 3 c/skm (330 sm). Near-inertial contributions exhibit a power law of m-3, while higher-frequency internal waves (ω>2f) a slope of m-2. There is strong vertical polarization (clockwise>anticlockwise) (CW>ACW) of the near-inertial contribution but little or none for higher-frequency motions. There is more WKB normalized near-inertial KE in the lower than in the upper half of the WKB-scaled water column while high-frequency internal waves have comparable upper and lower halves energies. The upper half shows a deficit compared to the Garrett and Munk model spectrum at vertical wavelengths shorter than 100 sm. Time-mean shear is largest in the upper half, so critical-layer processes may play a role. The internal wave KE of simultaneous but spatially separated profiles has a zero-correlation scale of 15–20 km, dominantly due to near-inertial waves. Thus, deep near-inertial motions exhibit wavelengths of 60–80 km in contrast to longer scales reported in the surface mixed layer and upper pycnocline. The aspect ratio k/m (330 sm/70 km) corresponds to a wave frequency of 1.05f. The downward group velocity is 0.6 mm s-1, with a vertical energy flux for the near-inertial motions of 0.6 mW m-2.

The EM-POGO: a simple, absolute velocity profiler

Terker, S.R., T.B. Sanford, J.H. Dunlap, and J.B. Girton, "The EM-POGO: a simple, absolute velocity profiler," Deep Sea Res. II, 85, 220-227, doi: 10.1016/j.dsr2.2012.07.026, 2013.

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1 Jan 2013

Electromagnetic current instrumentation has been added to the Bathy Systems, Inc. POGO transport sondes to produce a free-falling absolute velocity profiler called EM-POGO. The POGO is a free-fall profiler that measures a depth-averaged velocity using GPS fixes at the beginning and end of a round trip to the ocean floor (or a pre-set depth). The EM-POGO adds a velocity profile determined from measurements of motionally-induced electric fields generated by the ocean current moving through the vertical component of the Earth's magnetic field. In addition to providing information about the vertical structure of the velocity, the depth-dependent measurements improve transport measurements by correcting for the non-constant fall-rate. Neglecting the variable fall rate results in errors O(1 cm s-1). The transition from POGO to EM-POGO included electrically isolating the POGO and electric-field-measuring circuits, installing a functional GPS receiver, finding a pressure case that provided an optimal balance among crush-depth, price and size, and incorporating the electrodes, electrode collar, and the circuitry required for the electric field measurement. The first EM-POGO sea-trial was in July 1999. In August 2006 a refurbished EM-POGO collected 15 absolute velocity profiles; relative and absolute velocity uncertainty was ~ 1 cm s-1 and 0.5–5 cm s-1, respectively, at a vertical resolution of 25 m. Absolute velocity from the EM-POGO compared to shipboard ADCP measurements differed by ~ 1–2 cm s-1, comparable to the uncertainty in absolute velocity from the ADCP. The EM-POGO is thus a low-cost, easy to deploy and recover, and accurate velocity profiler.

Vertically averaged velocities in the North Atlantic Current from field trials of a Lagrangian electric-field float

Szuts, Z.B., and T.B. Sanford, "Vertically averaged velocities in the North Atlantic Current from field trials of a Lagrangian electric-field float," Deep Sea Res. II, 85, 210-219, doi:10.1016/j.dsr2.2012.07.022, 2013.

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1 Jan 2013

A subsurface Lagrangian float that utilizes motional induction to calculate vertically averaged velocities was tested in the North Atlantic Current (NAC), taking advantage of existing cruises and infrastructure. The Electric Field Float (EFF) is a RAFOS float with horizontal electrodes that measures its own velocity by RAFOS tracking and calculates vertically averaged velocities when merged with the electrode system. The observations showed depth-averaged velocities that were fast in the core of the NAC (0.6 – 0.9 m s-1) and moderate in adjacent recirculations and eddies (0.3 – 0.4 m s-1). A float at 850 dbar moved at close to the depth-averaged velocity, while shallower floats followed surface intensified flow on top of the depth-averaged motion. Integral time scales of depth-averaged velocity (1.3 – 1.6 ± 0.4 d) are slightly shorter than time scales of float velocity (1.6 – 2.0 ± 0.3 d), while integral length scales of depth-averaged water velocity (35 ± 10 km for u, 18 ± 6 km for v) are slightly shorter than length scales of float motion (53 ± 12 km for u, 28 ± 6 km for v). Velocity spectra of depth-averaged velocity show significant variance at inertial periods. Quantitative and qualitative validation with multiple independent data sets confirms the accuracy of the instrument and sampling strategy in the NAC, advancing the limited observational knowledge of depth-averaged circulation in subpolar regions.

Circulation and intrusions northeast of Taiwan: Chasing and predicting uncertainty in the cold dome

Gawarkiewicz, G., S. Jan, P.F.J. Lermusiaux, J.L. McClean, L. Centurioni, K. Taylor, B. Cornuelle, T.F. Duda, J. Wang, Y.J. Yang, T. Sanford, R.-C. Lien, C. Lee, M.-A. Lee, W. Leslie, P.J. Haley Jr., P.P. Niiler, G. Gopalakrishnan, P. Velez-Belchi, D.-K. Lee, and Y.Y. Kim, "Circulation and intrusions northeast of Taiwan: Chasing and predicting uncertainty in the cold dome," Oceanography 24, 110-121, doi:10.5670/oceanog.2011.99, 2011.

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5 Dec 2011

An important element of present oceanographic research is the assessment and quantification of uncertainty. These studies are challenging in the coastal ocean due to the wide variety of physical processes occurring on a broad range of spatial and temporal scales. In order to assess new methods for quantifying and predicting uncertainty, a joint Taiwan-US field program was undertaken in August/September 2009 to compare model forecasts of uncertainties in ocean circulation and acoustic propagation, with high-resolution in situ observations. The geographical setting was the continental shelf and slope northeast of Taiwan, where a feature called the "cold dome" frequently forms. Even though it is hypothesized that Kuroshio subsurface intrusions are the water sources for the cold dome, the dome's dynamics are highly uncertain, involving multiple scales and many interacting ocean features. During the experiment, a combination of near-surface and profiling drifters, broad-scale and high-resolution hydrography, mooring arrays, remote sensing, and regional ocean model forecasts of fields and uncertainties were used to assess mean fields and uncertainties in the region. River runoff from Typhoon Morakot, which hit Taiwan August 7%u20138, 2009, strongly affected shelf stratification. In addition to the river runoff, a cold cyclonic eddy advected into the region north of the Kuroshio, resulting in a cold dome formation event. Uncertainty forecasts were successfully employed to guide the hydrographic sampling plans. Measurements and forecasts also shed light on the evolution of cold dome waters, including the frequency of eddy shedding to the north-northeast, and interactions with the Kuroshio and tides. For the first time in such a complex region, comparisons between uncertainty forecasts and the model skill at measurement locations validated uncertainty forecasts. To complement the real-time model simulations, historical simulations with another model show that large Kuroshio intrusions were associated with low sea surface height anomalies east of Taiwan, suggesting that there may be some degree of predictability for Kuroshio intrusions.

Typhoon-ocean interaction in the western North Pacific: Part 1

D'Asaro, E., P. Black, L. Centurioni, P. Harr, S. Jayne, I.-I Lin, C. Lee, J. Morzel, R. Mrvaljevic, P.P. Niiler, L. Rainville, T. Sanford, and T.Y. Tang, "Typhoon-ocean interaction in the western North Pacific: Part 1," Oceanography, 24, 24-31, doi:10.5670/oceanog.2011.91, 2011

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5 Dec 2011

The application of new technologies has allowed oceanographers and meteorologists to study the ocean beneath typhoons in detail. Recent studies in the western Pacific Ocean reveal new insights into the influence of the ocean on typhoon intensity.

Upper-ocean response to Hurricane Frances (2004) observed by profiling EM-APEX floats

Sanford, T.B., J.F. Price, and J.B. Girton, "Upper-ocean response to Hurricane Frances (2004) observed by profiling EM-APEX floats," J. Phys. Oceanogr., 41, 1041-1056, doi:10.1175/2010JPO4313.1, 2011.

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1 Jun 2011

Three autonomous profiling Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats were air deployed one day in advance of the passage of Hurricane Frances (2004) as part of the Coupled Boundary Layer Air-Sea Transfer (CBLAST)-High field experiment. The floats were deliberately deployed at locations on the hurricane track, 55 km to the right of the track, and 110 km to the right of the track. These floats provided profile measurements between 30 and 200 m of in situ temperature, salinity, and horizontal velocity every half hour during the hurricane passage and for several weeks afterward. Some aspects of the observed response were similar at the three locations - the dominance of near-inertial horizontal currents and the phase of these currents - whereas other aspects were different. The largest-amplitude inertial currents were observed at the 55-km site, where SST cooled the most, by about 2.2C, as the surface mixed layer deepened by about 80 m. Based on the time-depth evolution of the Richardson number and comparisons with a numerical ocean model, it is concluded that SST cooled primarily because of shear-induced vertical mixing that served to bring deeper, cooler water into the surface layer. Surface gravity waves, estimated from the observed high-frequency velocity, reached an estimated 12-m significant wave height at the 55-km site. Along the track, there was lesser amplitude inertial motion and SST cooling, only about 1.2C, though there was greater upwelling, about 25-m amplitude, and inertial pumping, also about 25-m amplitude. Previously reported numerical simulations of the upper-ocean response are in reasonable agreement with these EM-APEX observations provided that a high wind speed-saturated drag coefficient is used to estimate the wind stress. A direct inference of the drag coefficient CD is drawn from the momentum budget. For wind speeds of 32-47 m s^-1, CD ~ 1.4 x 10^-3.

Sensing the ocean.

Sanford, T.B., K.A. Kelly, and D.M. Farmer, "Sensing the ocean." Physics Today, 64, 24-28, doi:10.1063/1.3554313, 2011.

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1 Feb 2011

Radar, sonar, and electromagnetic measurements provide complementary information about oceanic processes, properties, and motions.

Using global arrays to investigate internal-waves and mixing

MacKinnon, J., et al., including M. Alford, J. Girton, M. Gregg, E. Kunze, and T. Sanford, "Using global arrays to investigate internal-waves and mixing," In Proceedings, OceanObs'09: Sustained Ocean Observations and Information for Society (Vol. 2), Venice, Italy, 21-25 September 2009, J. Hall, et al., eds. (ESA Publication WPP-306, doi:10.5270/OceanObs09.cwp.58, 2010).

15 Feb 2010

Climate monitoring of the Indonesian throughflow

You, Y., T. Sanford, and C.-T. Liu, "Climate monitoring of the Indonesian throughflow," EOS Trans. AGU, 91, doi:10.1029/2010EO020002, 2010.

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12 Jan 2010

Despite growing recognition of the need to monitor the global ocean circulation, existing sensor networks are unable to observe certain key parts of the world ocean. One such choke point is in the Indonesian seas, where the Indonesian Throughflow (ITF) flows from the Pacific Ocean to the Indian Ocean as an upper branch of the overturning circulation. To properly understand the role the ITF plays in the climate system, longer-term measurements are required on decadal time scales and beyond. The Indonesian seas currently are not sampled on a regular basis because the Argo float network is not able to operate in this extremely complicated marginal sea environment.

Since 2008, an international program called the Indonesian Throughflow: Pacific Source Water Investigation (PACSWIN [You et al., 2010]) has sought to fill this gap in the global ocean observing network. PACSWIN is a coordinated multinational ocean climate program that will be active in the Indonesian seas and adjacent regions for the coming decade and beyond. The cable component of PACSWIN, which also has other components, promises to meet the need for a cost-effective long-term monitoring system, in addition to providing a more diverse set of supporting and supplementary measurements. This cable component was the subject of a 9–10 September 2009 workshop, the Climate Variability and Predictability (CLIVAR)-endorsed First PACSWIN Submarine Cable Workshop, held in Taipei, Taiwan. An implementation plan currently is being developed.

Heat and turbulence kinetic energy for surface layer cooling induced by the passage of Hurricane Frances (2004)

Huang, P., T.B. Sanford, and J. Imberger, "Heat and turbulence kinetic energy for surface layer cooling induced by the passage of Hurricane Frances (2004)," J. Geophys. Res., 114, doi:10.1029/2009JC005603, 2009.

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29 Dec 2009

Heat and turbulent kinetic energy budgets of the ocean surface layer during the passage of Hurricane Frances were examined using a three-dimensional hydrodynamic model. In situ data obtained with the Electromagnetic-Autonomous Profiling Explorer (EM-APEX) floats were used to set up the initial conditions of the model simulation and to compare to the simulation results. The spatial heat budgets reveal that during the hurricane passage, not only the entrainment in the bottom of surface mixed layer but also the horizontal water advection were important factors determining the spatial pattern of sea surface temperature. At the free surface, the hurricane-brought precipitation contributed a negligible amount to the air-sea heat exchange, but the precipitation produced a negative buoyancy flux in the surface layer that overwhelmed the instability induced by the heat loss to the atmosphere. Integrated over the domain within 400 km of the hurricane eye on day 245.71 of 2004, the rate of heat anomaly in the surface water was estimated to be about 0.45 PW (1 PW = 1015 W), with about 20% (0.09 PW in total) of this was due to the heat exchange at the air-sea interface, and almost all the remainder (0.36 PW) was downward transported by oceanic vertical mixing. Shear production was the major source of turbulent kinetic energy amounting 88.5% of the source of turbulent kinetic energy, while the rest (11.5%) was attributed to the wind stirring at sea surface. The increase of ocean potential energy due to vertical mixing represented 7.3% of the energy deposited by wind stress.

The First PACSWIN Submarine Cable Workshop

You, Y., T. Sanford, C.-T. Liu, P. Sigray, M. Koga, W. Pandoe, J. H. Lee, N. Palshin, Z. Szuts, and K. Taira, "The First PACSWIN Submarine Cable Workshop," In CLIVAR
Exchanges, 14, 11-13, 2009.

1 Oct 2009

Vorticity and turbulence in the wake of a bridge pier

Lien, R.-C., T.B. Sanford, "Vorticity and turbulence in the wake of a bridge pier," IEEE J. Ocean. Eng., 34, 307-314, doi:10.1109/JOE.2009.2019383, 2009.

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1 Jul 2009

Dissipation rates of turbulence kinetic energy (TKE) epsiv and enstrophy zeta2 are reported in a high Reynolds number turbulent wake. Previous turbulent wake observations have been made in laboratory experiments with relatively low Reynolds number flows O (103). Results presented here are from a set of rare field observations of vorticity and turbulence in a turbulent wake with a high Reynolds number O (107). The turbulent wake was formed by an unsteady strong tidal current interacting with a bridge pier. Measurements were taken mostly in the intermediate wake, i.e., 10 les x/d les 60, where x is the downstream distance and d is the width of the bridge pier. Both epsiv and zeta2 show a similar downstream decay rate that is faster than that predicted by the self-preservation similarity in the far wake. The theoretical relation epsiv = nuzeta2 for high Reynolds number flow is confirmed by field observations. The magnitudes of the vertical and horizontal components of enstrophy do not differ significantly. The turbulence internal intermittency is ~ 0.2, estimated from autocorrelation coefficients of enstrophy; this value is close to that reported previously in turbulent wakes and jets.

Observations of turbulence mixing and vorticity in a littoral surface boundary layer

Lien, R.-C., T.B. Sanford, and W.-T. Tsai, "Observations of turbulence mixing and vorticity in a littoral surface boundary layer," J. Phys. Oceanogr., 38, 648-669, 2008.

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1 Mar 2008

Measurements of small-scale vorticity, turbulence velocity, and dissipation rates of turbulence kinetic energy were taken in a littoral fetch-limited surface wave boundary layer. Drifters deployed on the surface formed convergence streaks with 1-m horizontal spacing within a few minutes. In the interior, however, no organized pattern of velocity, vorticity, or turbulence mixing intensity was found at a similar horizontal spatial scale. The turbulent Langmuir number La was 0.6–1.3, much larger than the 0.3 of the typical open ocean, suggesting comparable importance of wind-driven turbulence and Langmuir circulation. Observed turbulent kinetic energy values are explained by the wind-driven shear turbulence. The production rate of turbulence kinetic energy associated with the vortex force is about 10-7 W kg-1, slightly smaller than that generated by the wind-driven turbulence. The rms values of the streakwise component of vorticity and the vertical component of vorticity have a similar magnitude of ~0.02 s-1. Vertical profiles of turbulent kinetic energy, streakwise and vertical components of vorticity showed a monotonic decrease from the surface. Traditionally, surface convergence streaks are regarded as signatures of Langmuir circulation. Two large-eddy simulations with and without Stokes drift were performed. Both simulations produced surface convergence streaks and vertical profiles of turbulent kinetic energy, vorticity, and velocity consistent with observations. The observations and model results suggest that the presence of surface convergence streaks does not necessarily imply the existence of Langmuir circulation. In a littoral surface boundary layer where surface waves are young, fetch-limited, and weak, and La = O(1), the turbulence mixing in the surface mixed layer is primarily due to the wind-driven shear turbulence, and convergence streaks exist with or without surface waves.

Cold wake of Hurricane Frances

D'Asaro, E.A., T.B. Sanford, P.P. Niiler, and E.J. Terrill, "Cold wake of Hurricane Frances," Geophys. Res. Lett., 34, doi:10.1029/2007GL029922, 2007.

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11 Aug 2007

An array of instruments air-deployed ahead of Hurricane Frances measured the three-dimensional, time dependent response of the ocean to this strong (60 m s-1) storm. Sea surface temperature cooled by up to 2.2°C with the greatest cooling occurring in a 50-km-wide band centered 60–85 km to the right of the track. The cooling was almost entirely due to vertical mixing, not air-sea heat fluxes. Currents of up to 1.6 m s-1 and thermocline displacements of up to 50 m dispersed as near-inertial internal waves. The heat in excess of 26°C, decreased behind the storm due primarily to horizontal advection of heat away from the storm track, with a small contribution from mixing across the 26°C isotherm. SST cooling under the storm core (0.4°C) produced a 16% decrease in air-sea heat flux implying an approximately 5 m s-1 reduction in peak winds

Highly resolved observations and simulations of the ocean response to a hurricane

Sanford, T.B., J.F. Price, J.B. Girton, and D.C. Webb, "Highly resolved observations and simulations of the ocean response to a hurricane," Geophys. Res. Lett., 34, doi:10.1029/2007GL029679, 2007.

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7 Jul 2007

An autonomous, profiling float called EM-APEX was developed to provide a quantitative and comprehensive description of the ocean side of hurricane-ocean interaction. EM-APEX measures temperature, salinity and pressure to CTD quality and relative horizontal velocity with an electric field sensor. Three prototype floats were air-deployed into the upper ocean ahead of Hurricane Frances (2004). All worked properly and returned a highly resolved description of the upper ocean response to a category 4 hurricane. At a float launched 55 km to the right of the track, the hurricane generated large amplitude, inertially rotating velocity in the upper 120 m of the water column. Coincident with the hurricane passage there was intense vertical mixing that cooled the near surface layer by about 2.2°C. We find consistent model simulations of this event provided the wind stress is computed from the observed winds using a high wind-speed saturated drag coefficient.

Air-sea exchange in hurricanes: Synthesis of observations from the Couple Boundary Layer Air-Sea Transfer experiment

Black, P.G., E.A. D'Asaro, W.M. Drennan, J.R. French, P.P. Niiler, T.B. Sanford, E.J. Terrill, E.J. Walsh, and J.A. Zhang, "Air-sea exchange in hurricanes: Synthesis of observations from the Couple Boundary Layer Air-Sea Transfer experiment," Bull. Am. Meterol. Soc., 88, 357-374, doi:10.1175/BAMS-88-3-357, 2007.

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1 Mar 2007

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (CD) values derived from CBLAST momentum flux measurements show CD becoming invariant with wind speed near a 23 m s-1 threshold rather than a hurricane-force threshold near 33 m s-1. Values above 23 m s-1 are lower than previous open-ocean measurements.

The Dalton number estimates (CE) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s-1, which is in approximate agreement with previous measurements at lower winds. These observations imply a CE/CD ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be "roll vortex" or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery.

As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and sub-surface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s-1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.

An estimate of tidal energy lost to turbulence at the Hawaiian Ridge

Klymak, J.M., J.N. Moum, J.D. Nash, E. Kunze, J.B. Girton, G.S. Carter, C.M. Lee, T.B. Sanford, and M.C. Gregg, "An estimate of tidal energy lost to turbulence at the Hawaiian Ridge," J. Phys. Oceanogr., 36, 1148-1164, doi: 10.1175/JPO2885.1, 2006.

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1 Jun 2006

An integrated analysis of turbulence observations from four unique instrument platforms obtained over the Hawaiian Ridge leads to an assessment of the vertical, cross-ridge, and along-ridge structure of turbulence dissipation rate and diffusivity. The diffusivity near the seafloor was, on average, 15 times that in the midwater column. At 1000-m depth, the diffusivity atop the ridge was 30 times that 10 km off the ridge, decreasing to background oceanic values by 60 km. A weak (factor of 2) spring–neap variation in dissipation was observed. The observations also suggest a kinematic relationship between the energy in the semidiurnal internal tide (E) and the depth-integrated dissipation (D), such that D ~ E1±0.5 at sites along the ridge. This kinematic relationship is supported by combining a simple knife-edge model to estimate internal tide generation, with wave–wave interaction time scales to estimate dissipation. The along-ridge kinematic relationship and the observed vertical and cross-ridge structures are used to extrapolate the relatively sparse observations along the length of the ridge, giving an estimate of 3 ± 1.5 GW of tidal energy lost to turbulence dissipation within 60 km of the ridge. This is roughly 15% of the energy estimated to be lost from the barotropic tide.

Internal tides and turbulence along the 3000-m isobath of the Hawaiian Ridge

Lee, C.M., E. Kunze, T.B. Sanford, J.D. Nash, M.A. Merrifield, and P.E. Holloway, "Internal tides and turbulence along the 3000-m isobath of the Hawaiian Ridge," J. Phys. Oceanogr., 36, 1165-1183, doi: 10.1175/JPO2886.1, 2006.

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1 Jun 2006

Full-depth velocity and density profiles taken along the 3000-m isobath characterize the semidiurnal internal tide and bottom-intensified turbulence along the Hawaiian Ridge. Observations reveal baroclinic energy fluxes of 21 ± 5 kW m-1 radiating from French Frigate Shoals, 17 ± 2.5 kW m-1 from Kauai Channel west of Oahu, and 13 ± 3.5 kW m-1 from west of Nihoa Island. Weaker fluxes of 14 ± 2 kW m-1 radiate from the region near Necker Island and east of Nihoa Island. Observed off-ridge energy fluxes generally agree to within a factor of 2 with those produced by a tidally forced numerical model. Average turbulent diapycnal diffusivity K is (0.5–1) x 10-4 m2 s-1 above 2000 m, increasing exponentially to 20 x 10-4 m2 s-1 near the bottom. Microstructure values agree well with those inferred from a finescale internal wave-based parameterization. A linear relationship between the vertically integrated energy flux and vertically integrated turbulent dissipation rate implies that dissipative length scales for the radiating internal tide exceed 1000 km.

Structure of the baroclinic tide generated at Kaena Ridge, Hawaii

Nash, J.D., E. Kunze, C.M. Lee, and T.B. Sanford, "Structure of the baroclinic tide generated at Kaena Ridge, Hawaii," J. Phys. Oceanogr., 36, 1123-1135, doi:10.1175/JPO2883.1, 2006.

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1 Jun 2006

Repeat transects of full-depth density and velocity are used to quantify generation and radiation of the semidiurnal internal tide from Kaena Ridge, Hawaii. A 20-km-long transect was sampled every 3 h using expendable current profilers and the absolute velocity profiler. Phase and amplitude of the baroclinic velocity, pressure, and vertical displacement were computed, as was the energy flux. Large barotropically induced isopycnal heaving and strong baroclinic energy-flux divergence are observed on the steep flanks of the ridge where upward and downward beams radiate off ridge. Directly above Kaena Ridge, strong kinetic energy density and weak net energy flux are argued to be a horizontally standing wave. The phasing of velocity and vertical displacements is consistent with this interpretation. Results compare favorably with the Merrifield and Holloway model.

The hurricane mixing front

D'Asaro, E.A., R. Harcourt, E. Terrill, P.P. Niiler, and T.B. Sanford, "The hurricane mixing front," Bull. Am. Meteorol. Soc., 87, 1492, 2006.

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26 Apr 2006

The temperature of the sea surface beneath the hurricane inner core is a key factor controlling the flux of enthalpy from the ocean to the hurricane and thus an important influence on hurricane intensification. Mixing caused by the hurricane winds produces a rapid cooling of the sea surface as cooler water is mixed upward from below. This produces a front in sea surface temperature beneath the storm. The position of this front relative to the eye should thus be related to SST-induced storm intensification. Data from the CBLAST measurements in hurricanes is used to map several examples of this front. The sensitivity of its position to storm properties is explored using simple models of ocean mixing.

Autonomous velocity and density profiler: EM-APEX

Sanford, T.B., J.H. Dunlap, J.A. Carlson, D.C. Webb, and J.B. Girton, "Autonomous velocity and density profiler: EM-APEX," Proceedings, IEEE/OES Eighth Working Conference on Current Measurement Technology, 152-156, doi:10.1109/CCM.2005.1506361, (IEEE, 2005)

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30 Jul 2005

We developed an autonomous ocean profiling velocity and density float that provides exceptional vertical coverage and temporal resolution to depths of 2000 m for deployments of many years. Electrodes were added to the exterior of standard WRC APEX floats, and electronics were added inside. The electrode voltages result from the motion of seawater and the instrument through the Earth's magnetic field. Other systems included magnetic compass, tilt, CTD, GPS, and Iridium (providing sampling/mission changes). Three EM-APEX floats were deployed from a C-130 aircraft ahead of Hurricane Frances. The floats profiled for 10 hr from the surface to 200 m, then continued profiling between 30 m and 200 m with excursions to 500 m every half inertial period. The velocity computations were performed onboard and saved for later transmission. After five days, the floats surfaced and transmitted the accumulated processed observations, then the floats profiled from 500 m every half inertial period until recovered early in October located by GPS and Iridium.

Circulation and mixing in the Faroese Channels

Mauritzen, C., J. Price, T. Sanford, and D. Torres, "Circulation and mixing in the Faroese Channels," Deep-Sea Res. I, 52, 883-913, doi:10.1016/j.dsr.2004.11.018, 2005

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1 Jun 2005

The hydrographic properties of the dense waters overflowing the Greenland–Scotland Ridge through the Faroese Channels are greatly modified during the transit. In this study, we consider conditions both upstream and downstream of the sill, using a combination of hydrographic measurements (CTD, nutrients) and direct velocity measurements (expendable current profilers, lowered acoustic Doppler current profiler) from several repeated lines across the channel system. The purpose is to identify and quantify regions of enhanced mixing and frictional drag. The method of quantification used in both cases (Thorpe scaling) is independent of budget calculations and thus lends itself useful for non-steady-state flows.

The method appears to be successful in this case as (1) the numbers obtained are non-random, (2) they support the qualitative interpretation of mixing made from water mass analysis, and (3) they are supported by companion log-layer estimates of bottom boundary layer friction. Large buoyancy fluxes were deduced within the dense water masses all along the channel system, also upstream of the sill where the velocities are low. The largest implied mixing (~3 x 10-7 W/kg) was observed roughly 100 km downstream of the sill, where the plume starts to descend. Frictional stress within the water column was also elevated throughout the channel system; the log-layer formulae yielding a drag coefficient for the entire data set of about 4 x 10-3. The average stress at the sill was roughly 2 Pa.

Turbulence spectra and local similarity scaling in a strongly stratified oceanic bottom boundary layer

Lien, R.-C., and T.B. Sanford, "Turbulence spectra and local similarity scaling in a strongly stratified oceanic bottom boundary layer," Cont. Shelf Res., 24, 375-392, doi:10.1016/j.csr.2003.10.007, 2004.

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1 Feb 2004

In the turbulence inertial subrange, wavenumber spectra of vertical velocity and streamwise velocity in a strongly stratified oceanic bottom boundary layer agree with the local similarity scaling laws found previously in the stable atmospheric boundary layer. At scales greater than the turbulence inertial subrange, oceanic velocity spectra exceed the universal spectra. This additional energy at large scales could be due either to internal waves, inappropriate estimates of turbulence parameters, or non-stationarity of the data. The strong stratification in the observed tidal bottom boundary layer is maintained by the advective density gradient. Results reported here include the effects of horizontal advection.

From tides to mixing along the Hawaiian Ridge

Rudnick, D.L., T.J. Boyd, R.E. Brainard, G.S. Carter, G.D. Egbert, M.C. Gregg, P.E. Holloway, J.M. Klymak, E. Kunze, C.M. Lee, M.D. Levine, D.S. Luther, J.P. Martin, M.A. Merrifield, J.N. Moum, J.D. Nash, R. Pinkel, L. Rainville, and T.B. Sanford, "From tides to mixing along the Hawaiian Ridge," Science, 301, 355-357, DOI: 10.1126/science.1085837, 2003.

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18 Jul 2003

The cascade from tides to turbulence has been hypothesized to serve as a major energy pathway for ocean mixing. We investigated this cascade along the Hawaiian Ridge using observations and numerical models. A divergence of internal tidal energy flux observed at the ridge agrees with the predictions of internal tide models. Large internal tidal waves with peak-to-peak amplitudes of up to 300 meters occur on the ridge. Internal-wave energy is enhanced, and turbulent dissipation in the region near the ridge is 10 times larger than open-ocean values. Given these major elements in the tides-to-turbulence cascade, an energy budget approaches closure.

Descent and modification of the overflow plume in the Denmark Strait

Girton, J.B., and T.B. Sanford, "Descent and modification of the overflow plume in the Denmark Strait," J. Phys. Oceanogr., 33, 1351-1364, DOI: 10.1175/1520-0485(2003)033<1351:DAMOTO>2.0.CO;2, 2003.

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1 Jul 2003

Bulk properties of the Denmark Strait overflow (DSO) plume observed in velocity and hydrography surveys undertaken in 1997 and 1998 are described. Despite the presence of considerable short-term variability, it is found that the pathway and evolution of the plume density anomaly are remarkably steady. Bottom stress measurements show that the pathway of the plume core matches well with a rate of descent controlled by friction. The estimated entrainment rate diagnosed from the rate of plume dilution with distance shows a marked increase in entrainment at approximately 125 km from the sill, leading to a net dilution consistent with previous reports of a doubling of overflow transport measured by current meter arrays. The entrainment rate increase is likely related to the increased topographic slopes in the region, compounded by a decrease in interface stratification as the plume is diluted and enters a denser background.

Internal tide radiation from Mendocino Escarpment

Althaus, A.M., E. Kunze, and T.B. Sanford, "Internal tide radiation from Mendocino Escarpment," J. Phys. Oceanogr., 33, 1510-1527, 2003

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1 Jul 2003

Strong semidiurnal internal tides are observed near Mendocino Escarpment in full-depth profile time series of velocity, temperature, and salinity. Velocity and density profiles are combined to estimate the internal tide energy flux. Divergence of this flux demonstrates that its source is the barotropic tide interacting with the escarpment. A baroclinic energy flux of 7 kW m-1 radiates from the escarpment, corresponding to 3% of the 220 kW m-1 fluxing poleward in the surface tide. Energy and energy flux are concentrated in packets that emanate from the flanks of the ridge surmounting the escarpment and one site 90 km north of the escarpment. Coherent beamlike structure along semidiurnal ray paths remains identifiable until the first surface reflection. Beyond the first surface reflection north of the escarpment, the energy flux drops by 2 kW m-1 and beams are no longer discernible. Turbulence, as inferred from finescale parameterizations, is elevated by over two orders of magnitude relative to the open-ocean interior in localized 500-m-thick layers at the bottom over the ridge crest, near the surface at the station closest to the first surface reflection to the north, slightly north of the first bottom reflection to the north, and on the south flank of the escarpment. Despite its intensity, turbulent dissipation integrated over the ridge crest is only 1% of the energy flux in the internal tides. Thus, the bulk of surface tidal losses at the escarpment is radiating away as internal waves. High turbulent dissipation rates near the surface reflection suggest that loss of energy flux there may be turbulent. This turbulence may arise from (i) Wentzel–Kramers–Brillouin amplification of semidiurnal shear as the internal tide propagates into high near-surface stratification or (ii) superposition of incident and reflected waves enhancing nonlinear transfers to small scales and turbulence production. Localized mixing due to internal tide beams impinging on the base of the mixed layer may be an important unconsidered cause of nutrient and water-mass fluxes between the surface layer and the upper pycnocline.

Structure and variability of the Denmark Strait Overflow: Model and observations

Kase, R.H., J.B. Girton, and T.B. Sanford, "Structure and variability of the Denmark Strait Overflow: Model and observations," J. Geophys. Res., 108, 10.1029/2002JC001548, 2003.

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7 Jun 2003

We report on a combined modeling and observational effort to understand the Denmark Strait Overflow (DSO). Four cruises over the course of 3 years mapped hydrographic properties and velocity fields with high spatial resolution. The observations reveal the mean path of the dense water, as well as the presence of strong barotropic flows, energetic variability, and strong bottom friction and entrainment. A regional sigma coordinate numerical model of interbasin exchange using realistic bottom topography and an overflow forced only by an upstream reservoir of dense fluid is compared with the observations and used to further investigate these processes. The model successfully reproduces the volume transport of dense water at the sill, as well as the 1000-m descent of the dense water in the first 200 km from the sill and the intense eddies generated at 1–3 day intervals. Hydraulic control of the mean flow is indicated by a region supercritical to long gravity waves in the dense layer located approximately 100 km downstream of the sill in both model and observations. In addition, despite the differences in surface forcing, both model and observations exhibit similar transitions from mostly barotropic flow at the sill to a bottom-trapped baroclinic flow downstream, indicating the dominant role of the overflow in determining the full water column dynamics.

Reduced mixing from the breaking of internal waves in equatorial waters

Gregg, M.C., T.B. Sanford, and D.P. Winkel, "Reduced mixing from the breaking of internal waves in equatorial waters," Nature, 422, 513-515, doi:10.1038/nature01507, 2003.

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3 Apr 2003

In the oceans, heat, salt and nutrients are redistributed much more easily within water masses of uniform density than across surfaces separating waters of different densities. But the magnitude and distribution of mixing across density surfaces are also important for the Earth's climate as well as the concentrations of organisms. Most of this mixing occurs where internal waves break, overturning the density stratification of the ocean and creating patches of turbulence. Predictions of the rate at which internal waves dissipate were confirmed earlier at mid-latitudes. Here we present observations of temperature and velocity fluctuations in the Pacific and Atlantic oceans between 42° N and 2° S to extend that result to equatorial regions. We find a strong latitude dependence of dissipation in accordance with the predictions. In our observations, dissipation rates and accompanying mixing across density surfaces near the Equator are less than 10% of those at mid-latitudes for a similar background of internal waves. Reduced mixing close to the Equator will have to be taken into account in numerical simulations of ocean dynamics—for example, in climate change experiments.

Patterns of shear and turbulence across the Florida Current

Winkel, D.P., M.C. Gregg, and T.B. Sanford, "Patterns of shear and turbulence across the Florida Current," J. Phys. Oceanogr., 32, 3269-3285, doi:10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2, 2002.

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1 Nov 2002

Measurements by the Multi-Scale Profiler (MSP) at seven stations spanning the Straits of Florida characterize levels and patterns of internal wave activity and mixing in this vertically sheared environment. Contrasting properties suggest five mixing regimes. The largest and most vast is the interior regime, where the background flow has an inverse Richardson number (Ri1) ranging up to 0.55, shear is dominated by fluctuations that are 1–4 times stronger than in the open ocean, and turbulent diffusivities are similarly moderate at (1–4) x 10-5 m2 s-1. The high-velocity core of the current, near the surface at midchannel, is associated with weak mixing. To its west is a zone of high mean shear, where strong stratification results in background Ri-1 of only 0.4, fluctuations are weak, and diffusivity is moderate. Intermittent shear features beneath the core have mean Ri-1 > 1 and strong turbulence. Two regimes are related to channel topography. Adjacent to the steep eastern slope, finescale shear is predominately cross-channel, and turbulence varies from nearly the weakest to nearly the strongest. Within 100 m of the channel floor, turbulent stratified boundary layers are mixing at (2–6) x 10-4 m2 s-1 to account for one-half of the section-averaged diffusivity. Using existing finescale parameterizations, observed dissipation rates can be predicted within a factor of 2 for most of this dataset, despite significantly strong mean shear and generally anisotropic and asymmetric fluctuations. The exceptions are in the high mean shear zones, where total rather than fluctuating shear yields reasonable estimates, and in some of the more turbulent regions, where shear underestimates mixing. Given its overall moderate levels of turbulence and finescale shear, the Florida Current is not a hot spot for oceanic mixing.

Synoptic sections of the Denmark Strait overflow

Girton, J.B., T.B. Sanford, and R.H. Kase, "Synoptic sections of the Denmark Strait overflow," Geophys. Res. Lett., 28, 1619-1622, doi:10.1029/2000GL011970, 2001.

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15 Apr 2001

We report on a rapid high-resolution survey of the Denmark Strait overflow (DSO) as it crosses the sill, the first such program to incorporate full-water-column velocity profiles in addition to conventional hydrographic measurements. Seven transects with expendable profilers over the course of one week are used to estimate volume transport as a function of density. Our observations reveal the presence of a strongly barotropic flow associated with the nearly-vertical front dividing the Arctic and Atlantic waters. The seven-section mean transport of water denser than σΘ=27.8 is 2.7±0.6Sv, while the mean transport of water colder than 2.0°C is 3.8±0.8 Sv. Although this is larger than the 2.9 Sv of Θ < 2°C water measured by a 1973 current meter array, we find that a sampling of our sections equivalent to the extent of that array also measures 2.9Sv of cold water. Both the structure and magnitude of the measured flow are reproduced well by a high-resolution numerical model of buoyancy-driven exchange with realistic topography.

R/V Seward Johnson Cruise 9908: Cruise Report and Preliminary Results

Sanford, T.B., M.D. Allison, and J.H. Dunlap, "R/V Seward Johnson Cruise 9908: Cruise Report and Preliminary Results," APL-UW TM 7-00, August 2000.

1 Aug 2000

Spectral characteristics of velocity and vorticity fluxes in an unstratified turbulent boundary layer

Lien, R.-C., and T.B. Sanford, "Spectral characteristics of velocity and vorticity fluxes in an unstratified turbulent boundary layer," J. Geophys. Res., 105, 8659-8672, 2000.

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15 Apr 2000

Wavenumber spectral characteristics of the velocity and vorticity fluxes in an unstratified turbulent boundary layer are presented. The observed vertical and streamwise velocity spectra agree with empirical forms found in the atmospheric boundary layer. Spectral ratios of 4/3 between the vertical and streamwise velocity spectra and the agreement between the observed vorticity flux quad spectrum and that of isotropic turbulence suggest local isotropy at scales smaller than Z. The normalized cospectrum of the momentum flux agrees remarkably well with the empirical form found in the atmospheric boundary layer. In the inertial subrange the momentum flux cospectrum shows a clear spectral slope of –7/3. The observed composite vorticity flux cospectrum has most of its variance at the streamwise wavenumber kx =(1–10) Z-1 and has a spectral slope of –7/3 in the inertial subrange. The –7/3 spectral slope is consistent with a dimensional argument, assuming that the vorticity flux cospectrum is proportional to the gradient of the mean vorticity, and depends on the turbulence kinetic energy dissipation rate ε and the wavenumber. A model turbulent vorticity flux cospectrum is constructed based on the shape of observed spectra, a –7/3 spectral slope in the inertial subrange, and the similarity scaling of the vorticity flux in an unstratified turbulent boundary layer. The turbulence vorticity flux is directly related to the divergence of turbulence momentum flux, the force exerted by turbulence on the mean flow. Therefore our proposed empirical cospectral form of the vorticity fluxes might be useful for turbulence parameterization in numerical models.

Inventions

Remote Sensing of Salinity Profiles in a Marine Estuary

Record of Invention Number: 47312

Tom Sanford, Jim Carlson, John Dunlap

Disclosure

22 Apr 2015

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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