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James Girton

Senior Oceanographer

Affiliate Assistant Professor, Oceanography

Email

girton@apl.washington.edu

Phone

206-543-8467

Research Interests

Overflows and Deep-Water Formation, Internal Waves, Mesoscale Eddies, Oceanic Surface and Bottom Boundary Layers, Measurements of Ocean Velocity Through Motionally-Induced Voltages

Biosketch

James Girton's research primarily investigates ocean processes involving small-scale turbulence and mixing and their influence on larger-scale flows. An important part of physical oceanography is the collection of novel datasets to shed new light on important physical processes, and to this end Dr. Girton's research has frequently drawn upon the widely under-utilized electromagnetic velocity profiling technique developed by Tom Sanford (his Ph.D. advisor and frequent collaborator). Instruments utilizing this technique include the expendable XCP, the full-depth free-falling AVP, and the autonomous long-duration EM-APEX. Each of these instruments has a unique role to play in the study of phenomena ranging from deep boundary currents and overflows to upper ocean mixing and internal waves. In addition to being less well-understood elements of ocean physics, many of these phenomena are potentially important for the behavior of the large-scale ocean circulation, particularly the meridional overturning that transports heat to subpolar and polar regions and sequesters atmospheric gases in the deep ocean. Prediction of future climate change by coupled ocean-atmosphere models requires reliable predictions of ocean circulation, so physically-based improvements to parameterizations of mixing, boundary stresses and internal waves in such models are an ongoing goal.

Department Affiliation

Ocean Physics

Education

B.A. Physics, Swarthmore College, 1993

Ph.D. Oceanography, University of Washington, 2001

Publications

2000-present and while at APL-UW

Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage

Alford, M.H., J.B. Girton, G. Voet, G.S. Carter, J.B. Mickett, and J.M. Klymak, "Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage," Geophys. Res. Lett., 40, 4668-4674, doi:10.1002/grl.50684, 2013.

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16 Sep 2013

We report the first direct turbulence observations in the Samoan Passage (SP), a 40-km wide notch in the South Pacific bathymetry through which flows most of the water supplying the North Pacific abyssal circulation. The observed turbulence is 1000 to 10,000 times typical abyssal levels — strong enough to completely mix away the densest water entering the passage — confirming inferences from previous coarser temperature and salinity sections. Accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence. Specifically, the flow accelerates substantially at the primary sill within the passage, reaching speeds as great as 0.55 m s-1. A strong hydraulic response is seen, with layers first rising to clear the sill and then plunging hundreds of meters downward. Turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill. In addition to the primary sill, other locations along the multiple interconnected channels through the Samoan Passage also have an effect on the mixing of the dense water. In fact, quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers, suggesting that abyssal mixing depends sensitively on bathymetric details on small scales.

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.

Mapping low-mode internal tides from multisatellite altimetry

Zhao, Z., M.H. Alford, and J.B. Girton, "Mapping low-mode internal tides from multisatellite altimetry," Oceanography, 25, 42-51, doi:10.5670/oceanog.2012.40, 2012.

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1 Jun 2012

Low-mode internal tides propagate over thousands of kilometers from their generation sites, distributing tidal energy across the ocean basins. Though internal tides can have large vertical displacements (often tens of meters or more) in the ocean interior, they deflect the sea surface only by several centimeters. Because of the regularity of the tidal forcing, this small signal can be detected by state-of-the-art, repeat-track, high-precision satellite altimetry over nearly the entire world ocean. Making use of combined sea surface height measurements from multiple satellites (which together have denser ground tracks than any single mission), it is now possible to resolve the complex interference patterns created by multiple internal tides using an improved plane-wave fit technique. As examples, we present regional M2 internal tide fields around the Mariana Arc and the Hawaiian Ridge and in the North Pacific Ocean. The limitations and some perspective on the multisatellite altimetric methods are discussed.

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Turbulent mixing and exchange with interior waters on sloping boundaries

Kunze, E., C. MacKay, E.E. McPhee-Shaw, K. Morrice, J.B. GIrton, and S.R. Terker, "Turbulent mixing and exchange with interior waters on sloping boundaries," J. Phys. Oceanogr., 42, 910-927, doi:10.1175/JPO-D-11-075.1, 2012.

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1 Jun 2012

Microstructure measurements along the axes of Monterey and Soquel Submarine Canyons reveal 200%u2013300-m-thick well-stratified turbulent near-bottom layers with average turbulent kinetic energy dissipation rates 4 x 10-8 W kg-1 and eddy diffusivities 16 x 10-4 m2 s-1 (assuming mixing efficiency γ = 0.2) to at least thalweg depths of 1200 m. Turbulent dissipation rates are an order of magnitude lower in overlying waters, whereas buoyancy frequencies are only 25% higher. Well-mixed bottom boundary layer thicknesses hN are an order of magnitude thinner than the stratified turbulent layer. Because well-stratified turbulent layers are commonly observed above slopes, arguments that mixing efficiency should be reduced on sloping boundaries do not hold in cases of energetic internal-wave generation or interaction with topography. An advective–diffusive balance is used to infer velocities and transports, predicting horizontal upslope flows of 10–50 m day-1. Extrapolating this estimate globally suggests that canyon turbulence may contribute 2–3 times as much diapycnal transport to the World Ocean as interior mixing. The upcanyon turbulence-driven transports are not uniform, and the resulting upslope convergences will drive exchange between the turbulent layer and more quiescent interior. Predicted density surfaces of detrainment and entrainment are consistent with observed isopycnals of intermediate nepheloid and clear layers. These data demonstrate that turbulent mixing dynamics on sloping topography are fundamentally 2D or 3D in the ocean, so they cannot be accurately described by 1D models.

Observations of internal waves and parametric subharmonic instability in the Philippines archipelago

Chinn, B.S., J.B. Girton, and M.H. Alford, "Observations of internal waves and parametric subharmonic instability in the Philippines archipelago," J. Geophys. Res., 117, doi:10.1029/2011JC007392, 2012.

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12 May 2012

Internal waves contain a significant fraction of the kinetic energy in the ocean and are important intermediaries between the forcing (by wind and tide) and interior diapycnal mixing. We report here on measurements from Mindoro Strait in the Philippines (connecting the South China Sea to the Sulu Sea) of an internal wave field with a number of surprising properties that point to previously-unrecognized processes at work in the region. Continuum spectral levels are very close to typical "background" values found in the open ocean, but internal tide energy in both the diurnal and semidiurnal frequency bands is significantly elevated—and higher at the northern mooring (MP1) than the southern (MP2). Two particularly energetic depth ranges stand out at MP1: an upper layer centered near 300 m, and one at the bottom of the water column, near 1800 m. The upper layer contains both internal tides and a near-inertial band with upward and downward propagating waves and an apparent spring-neap cycle. The combination is suggestive of Parametric Subharmonic Instability as the forcing for the near-inertial band—a conclusion supported by bicoherence estimates. Mixing, estimated from density overturns, is weak over much of the water column but enhanced by about an order of magnitude in the deep layer and closely tied to the internal tide—both diurnal and semidiurnal. Near-inertial currents in this deep layer are dominantly rectilinear and not well-correlated with the mixing. Bulk mixing rates at the two sites are less than required to produce property changes seen in hydrography, suggesting additional enhancement elsewhere in the archipelago.

Internal tides around the Hawaiian Ridge estimated from multisatellite altimetry

Zhao, Z., M.H. Alford, J. Girton, T.M.S. Johnston, and G. Carter, "Internal tides around the Hawaiian Ridge estimated from multisatellite altimetry," J. Geophys. Res., 116, doi:10.1029/2011JC007045, 2011.

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24 Dec 2011

Satellite altimetric sea surface height anomaly (SSHA) data from Geosat Follow-on (GFO) and European Remote Sensing (ERS), as well as TOPEX/Poseidon (T/P), are merged to estimate M2 internal tides around the Hawaiian Ridge, with higher spatial resolution than possible with single-satellite altimetry. The new estimates are compared with numerical model runs. Along-track analyses show that M2 internal tides can be resolved from both 8 years of GFO and 15.5 years of ERS SSHA data. Comparisons at crossover points reveal that the M2 estimates from T/P, GFO, and ERS agree well. Multisatellite altimetry improves spatial resolution due to its denser ground tracks. Thus M2 internal tides can be plane wave fitted in 120 km x 120 km regions, compared to previous single-satellite estimates in 4° lon x 3° lat or 250 km x 250 km regions. In such small fitting regions the weaker and smaller-scale mode 2 M2 internal tides can also be estimated.

The higher spatial resolution leads to a clearer view of the M2 internal tide field around the Hawaiian Ridge. Discrete generation sites and internal tidal beams are clearly distinguishable, and consistent with the numerical model runs. More importantly, multisatellite altimetry produces larger M2 internal tidal energy fluxes, which agree better with model results, than previous single-satellite estimates. This study confirms that previous altimetric underestimates are partly due to the more widely spaced ground tracks and consequently larger fitting region. Multisatellite altimetry largely overcomes this limitation.

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.

Development of a hindcast/forecast model for the Philippine Archipelago

Arango, H.G., J.C. Levin, E.N. Curchitser, B. Zhang, A.M. Moore, W. Han, A.L. Gordon, C.M. Lee, and J.B. Girton, "Development of a hindcast/forecast model for the Philippine Archipelago," Oceanography, 24, 58-69, doi:10.5670/oceanog.2011.04, 2011.

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1 Mar 2011

This article discusses the challenges of developing a regional ocean prediction model for the Philippine Archipelago, a complex area in terms of geometry, bathymetry-dominated dynamics and variability, and strong local and remote wind forcing, where there are limited temporal and spatial ocean measurements. We used the Regional Ocean Modeling System (ROMS) for real-time forecasting during the Philippine Straits Dynamics Experiment (2007-2009) observational program. The article focuses on the prediction experiments before and during the exploratory cruise period, June 6 - July 3, 2007. The gathered observations were not available in real time, so the 4-Dimensional Variational (4D-Var) data assimilation experiments were carried out in hindcast mode. The best estimate of ocean state (nowcast) is determined by combining satellite-derived products for sea surface temperature and height, and subsurface temperature and salinity measurements from several hydrographic assets over a sequential five-day data assimilation window. The largest source of forecast uncertainty is from the prescribed lateral boundary conditions in the nearby Pacific Ocean, especially excessive salt flux. This result suggests that remote forcing and inflows from the Pacific are crucial for predicting ocean circulation in the Philippine Archipelago region. The lateral boundary conditions are derived from 1/12 degree global HYbrid Coordinate Ocean Model (HYCOM) daily snapshots. The incremental, strong-constraint 4D-Var data assimilation successfully decreased temperature and salinity errors of the real-time, nonassimilative control forecast by 38% and 49%, respectively.

Internal wave climates of the Philippine Seas

Girton, J.B., B.S. Chinn, and M.H. Alford, "Internal wave climates of the Philippine Seas," Oceanography, 24, 100-111, doi:10.5670/oceanog.2011.07, 2011.

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1 Mar 2011

Internal wave measurements from moorings and profiling floats throughout the Philippine Archipelago, collected as part of the Office of Naval Research Philippine Straits Dynamics Experiment, reveal a wealth of subsurface processes, some of which have not been observed previously (in the Philippines or elsewhere). Complex bathymetry and spatially varying tide and wind forcing produce distinct internal wave environments within the network of seas and channels, ranging from quiescent interior basins to remotely forced straits. Internal tides in both the diurnal and semidiurnal bands dominate much of the velocity structure and are likely the dominant source of energy for mixing in the region. In addition, the transfer of energy from the internal tide directly to near-inertial motions through parametric subharmonic instability appears to be active and, rather than wind forcing, is the dominant source of near-inertial band energy.

Diapycnal mixing in the Antarctic Circumpolar Current

Ledwell, J., L. St. Laurent, J.B. Girton, and J. Toole, "Diapycnal mixing in the Antarctic Circumpolar Current," J. Phys. Oceanogr., 41, 241-246, doi:10.1175/2010JPO4557.1, 2011.

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1 Jan 2011

The vertical dispersion of a tracer released on a density surface near 1500-m depth in the Antarctic Circumpolar Current west of Drake Passage indicates that the diapycnal diffusivity, averaged over 1 yr and over tens of thousands of square kilometers, is (1.3 ± 0.2) x 10-5 m2 s-1. Diapycnal diffusivity estimated from turbulent kinetic energy dissipation measurements about the area occupied by the tracer in austral summer 2010 was somewhat less, but still within a factor of 2, at (0.75 ± 0.07) x 10-5 m2 s-1. Turbulent diapycnal mixing of this intensity is characteristic of the midlatitude ocean interior, where the energy for mixing is believed to derive from internal wave breaking. Indeed, despite the frequent and intense atmospheric forcing experienced by the Southern Ocean, the amplitude of finescale velocity shear sampled about the tracer was similar to background amplitudes in the midlatitude ocean, with levels elevated to only 20%–50% above the Garrett–Munk reference spectrum. These results add to a long line of evidence that diapycnal mixing in the interior middepth ocean is weak and is likely too small to dictate the middepth meridional overturning circulation of the ocean.

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

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.

Is the Faroe Bank Channel overflow hydraulically controlled?

Girton, J.B., L.J. Pratt, D.A. Sutherland, and J.F. Price, "Is the Faroe Bank Channel overflow hydraulically controlled?" J. Phys. Oceanogr. 36, 2340-2349, doi:10.1175/JPO2969.1, 2006.

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1 Dec 2006

The overflow of dense water from the Nordic Seas through the Faroe Bank Channel (FBC) has attributes suggesting hydraulic control—primarily an asymmetry across the sill reminiscent of flow over a dam. However, this aspect has never been confirmed by any quantitative measure, nor is the position of the control section known. This paper presents a comparison of several different techniques for assessing the hydraulic criticality of oceanic overflows applied to data from a set of velocity and hydrographic sections across the FBC. These include 1) the cross-stream variation in the local Froude number, including a modified form that accounts for stratification and vertical shear, 2) rotating hydraulic solutions using a constant potential vorticity layer in a channel of parabolic cross section, and 3) direct computation of shallow water wave speeds from the observed overflow structure. Though differences exist, the three methods give similar answers, suggesting that the FBC is indeed controlled, with a critical section located 20–90 km downstream of the sill crest. Evidence of an upstream control with respect to a potential vorticity wave is also presented. The implications of these results for hydraulic predictions of overflow transport and variability are discussed.

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.

Deepwater overflow through Luzon Strait

Qu, T.D., J.B. Girton, and J.A Whitehead, "Deepwater overflow through Luzon Strait," J. Geophys. Res., 111, doi:10.1029/2005JC003139, 2006.

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10 Jan 2006

This study examines water property distributions in the deep South China Sea and adjoining Pacific Ocean using all available hydrographic data. Our analysis reveals that below about 1500 m there is a persistent baroclinic pressure gradient driving flow from the Pacific into the South China Sea through Luzon Strait. Applying hydraulic theory with assumptions of zero potential vorticity and flat bottom to the Luzon Strait yields a transport estimate of 2.5 Sv (1 Sv = 106 m3 s-1). Some implications of this result include: (1) a residence time of less than 30 years in the deep South China Sea, (2) a mean diapycnal diffusivity as large as 10-3 m2 s-1, and (3) an abyssal upwelling rate of about 3 x 10-6 m s-1. These quantities are consistent with residence times based on oxygen consumption rates. The fact that all of the inflowing water must warm up before leaving the basin implies that this marginal sea contributes to the water mass transformations that drive the meridional overturning circulation in the North Pacific. Density distributions within the South China Sea basin suggest a cyclonic deep boundary current system, as might be expected for an overflow-driven abyssal circulation.

Comparison of entrainment in overflows simulated by z-coordinate, isopycnal and non-hydrostatic models

Legg, S., R.W. Hallberg, and J.B. Girton, "Comparison of entrainment in overflows simulated by z-coordinate, isopycnal and non-hydrostatic models," Ocean Modelling, 11, 69-97, doi:10.1016/j.ocemod.2004.11.006, 2006.

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1 Jan 2006

A series of idealised numerical simulations of dense water flowing down a broad uniform slope are presented, employing both a z-coordinate model (the MIT general circulation model) and an isopycnal coordinate model (the Hallberg Isopycnal Model). Calculations are carried out at several different horizontal and vertical resolutions, and for a range of physical parameters. A subset of calculations are carried out at very high resolution using the non-hydrostatic variant of the MITgcm. In all calculations dense water descends the slope while entraining and mixing with ambient fluid. The dependence of entrainment, mixing and down-slope descent on resolution and vertical coordinate are assessed. At very coarse resolutions the z-coordinate model generates excessive spurious mixing, and dense water has difficulty descending the slope. However, at intermediate resolutions the mixing in the z-coordinate model is less than found in the high-resolution non-hydrostatic simulations, and dense water descends further down the slope. Isopycnal calculations show less resolution dependence, although entrainment and mixing are both reduced slightly at coarser resolution. At intermediate resolutions the z-coordinate and isopycnal models produce similar levels of mixing and entrainment. These results provide a benchmark against which future developments in overflow entrainment parameterizations in both z-coordinate and isopycnal models may be compared.

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.

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.

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.

Dynamics of Transport and Variability in the Denmark Strait Overflow

Girton, J.B., "Dynamics of Transport and Variability in the Denmark Strait Overflow," APL-UW TR 0103, August 2001.

1 Aug 2001

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.

In The News

Samoan Passage cruise on the R/V Thompson

UW News and Information, Hannah Hickey

Oceanographers from the University of Washington's Applied Physics Laboratory are in Samoa for six weeks, the third and final trip studying skyscraper-sized waves that break in a narrow channel in the South Pacific Ocean.

27 Jan 2014

Dot Earth: Deep-ocean waves

The New York Times, Andrew Revkin

Scientists at the University of Washington have found skyscraper-height waves in deep ocean layers in the South Pacific — in some cases breaking in slow motion like surf on a beach. The finding sheds light on processes that allow heat in shallower ocean waters to mix with abyssal currents.

11 Sep 2013

Breaking deep-sea waves reveal mechanism for global ocean mixing

UW News & Information, Hannah Hickey

A University of Washington study for the first time recorded wave breaking in a key bottleneck for circulation in the world%u2019s largest ocean — the Samoan Passage, a narrow channel in the South Pacific Ocean that funnels water flowing from Antarctica.

9 Sep 2013

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