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Mike Gregg

Professor Emeritus, Oceanography

Professor, Oceanography

Email

gregg@apl.washington.edu

Phone

206-543-1353

Biosketch

The idea that the cumulative action of centimeter-scale mixing affects the ocean's largest scales guides Mike Gregg's research. Evolving technology now enables the mixing to be put into the context of the meter-to-kilometer-scale processes directly producing it, such as internal waves, bottom and surface boundary layers, thermohaline staircases and intrusions, and hydraulic responses to flow constrictions. Because large-scale models, particularly coupled climate models, have grid scales vastly larger than those of the mixing and even of the intermediate-scale processes producing it, it is a goal to always try to work toward parameterizations that can be used in these models.

Department Affiliation

Ocean Physics

Education

B.S. Physics, Yale University, 1961

Ph.D. Physical Oceanography, Scripps Institution of Oceanography, 1971

Publications

2000-present and while at APL-UW

Transition from partly standing to progressive internal tides in Monterey Submarine Canyon

Hall, R.A., M.H. Alford, G.S. Carter, M.C. Gregg, R.-C. Lien, D.J. Wain, and Z. Zhao, "Transition from partly standing to progressive internal tides in Monterey Submarine Canyon," Deep Sea Res. II, 104, 164-173, doi:10.1016/j.dsr2.2013.05.039, 2014.

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1 Jun 2014

Monterey Submarine Canyon is a large, sinuous canyon off the coast of California, the upper reaches of which were the subject of an internal tide observational program using moored profilers and upward-looking moored ADCPs. The mooring observations measured a near-surface stratification change in the upper canyon, likely caused by a seasonal shift in the prevailing wind that favoured coastal upwelling. This change in near-surface stratification caused a transition in the behaviour of the internal tide in the upper canyon from a partly standing wave during pre-upwelling conditions to a progressive wave during upwelling conditions. Using a numerical model, we present evidence that either a partly standing or a progressive internal tide can be simulated in the canyon, simply by changing the initial stratification conditions in accordance with the observations. The mechanism driving the transition is a dependence of down-canyon (supercritical) internal tide reflection from the canyon floor and walls on the depth of maximum stratification. During pre-upwelling conditions, the main pycnocline extends down to 200 m (below the canyon rim) resulting in increased supercritical reflection of the up-canyon propagating internal tide back down the canyon. The large up-canyon and smaller down-canyon progressive waves are the two components of the partly standing wave. During upwelling conditions, the pycnocline shallows to the upper 50 m of the watercolumn (above the canyon rim) resulting in decreased supercritical reflection and allowing the up-canyon progressive wave to dominate.

Wind-driven submesoscale subduction at the north Pacific subtropical front

Hosegood, P.J., M.C. Gregg, and M.H. Alford, "Wind-driven submesoscale subduction at the north Pacific subtropical front," J. Geophys. Res., 118, 5333-5352, doi:10.1002/jgrc.20385, 2013.

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1 Oct 2013

Upper ocean observations from the north Pacific subtropical front during late winter demonstrate the generation of submesoscale intrusions by buoyancy loss. Prior to generation, a sharp thermohaline front was intensified by confluent flow of 1–2 x 10-5 s-1. Relative vertical vorticity across a surface-intensified, along-front jet on the warm side of a frontal trough was 0.5 f. During the storm, buoyancy loss arose due to cooling of ~650 W m-2 and down-front wind stress <0.5 N m-2 that generated a southward, cross-front Ekman transport of dense water over light. The resulting wind-driven buoyancy flux was concentrated at the front where it exceeded that due to convection by an order of magnitude. The intrusions appeared immediately following the storm both within the surface mixed layer and beneath the seasonal pycnocline. They were approximately 20 m thick and horizontally elongated in the cross-frontal direction. The near-surface intrusions had cool and fresh properties characteristic of the water underlying the seasonal pycnocline, whereas the subsurface intrusions were composed of warm and saline water from the surface. The apparent vertical exchange was constrained within a thin filament of 2 km zonal extent that was characterized by O(1) Rossby and Richardson numbers, pronounced cyclonic veering in the horizontal velocity throughout the surface mixed layer, and sloping isopycnals. The intrusion properties, background environmental context, and forcing history are consistent with prior numerical modeling results for the generation of ageostrophic vertical circulations by frontogenesis intensified by buoyancy loss, possibly resulting in symmetric instability.

Observations of near-inertial internal gravity waves radiating from a frontal jet

Alford, M.H., A.Y. Shcherbina, and M.C. Gregg, "Observations of near-inertial internal gravity waves radiating from a frontal jet," J. Phys. Oceanogr., 43, 1225-1239, doi:10.1175/JPO-D-12-0146.1, 2013.

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1 Jun 2013

Shipboard ADCP and towed CTD measurements are presented of a near-inertial internal gravity wave radiating away from a zonal jet associated with the Subtropical Front in the North Pacific. Three-dimensional spatial surveys indicate persistent alternating shear layers sloping downward and equatorward from the front. As a result, depth-integrated ageostrophic shear increases sharply equatorward of the front. The layers have a vertical wavelength of about 250 m and a slope consistent with a wave of frequency 1.01 f. They extend at least 100 km south of the front. Time series confirm that the shear is associated with a downward-propagating near-inertial wave with frequency within 20% of f. A slab mixed layer model forced with shipboard and NCEP reanalysis winds suggests that wind forcing was too weak to generate the wave. Likewise, trapping of the near-inertial motions at the low-vorticity edge of the front can be ruled out because of the extension of the features well south of it. Instead, the authors suggest that the wave arises from an adjustment process of the frontal flow, which has a Rossby number about 0.2–0.3.

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Internal tides and mixing in a submarine canyon with time-varying stratification

Zhao, Z., M.H. Alford, R.-C. Lien, M.C. Gregg, and G.S. Carter, "Internal tides and mixing in a submarine canyon with time-varying stratification," J. Phys. Oceanogr., 42, 2121-2142, doi:10.1175/JPO-D-12-045.1, 2012.

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1 Dec 2012

The time variability of the energetics and turbulent dissipation of internal tides in the upper Monterey Submarine Canyon (MSC) is examined with three moored profilers and five ADCP moorings spanning February–April 2009. Highly resolved time series of velocity, energy, and energy flux are all dominated by the semidiurnal internal tide and show pronounced spring-neap cycles. However, the onset of springtime upwelling winds significantly alters the stratification during the record, causing the thermocline depth to shoal from about 100 to 40 m. The time-variable stratification must be accounted for because it significantly affects the energy, energy flux, the vertical modal structures, and the energy distribution among the modes. The internal tide changes from a partly horizontally standing wave to a more freely propagating wave when the thermocline shoals, suggesting more reflection from up canyon early in the observational record. Turbulence, computed from Thorpe scales, is greatest in the bottom 50–150 m and shows a spring-neap cycle. Depth-integrated dissipation is 3 times greater toward the end of the record, reaching 60 mW m-2 during the last spring tide. Dissipation near a submarine ridge is strongly tidally modulated, reaching 10-5 W kg-1 (10–15-m overturns) during spring tide and appears to be due to breaking lee waves. However, the phasing of the breaking is also affected by the changing stratification, occurring when isopycnals are deflected downward early in the record and upward toward the end.

Internal wave measurements on the Cycladic Plateau of the Aegean Sea

Alford, M.H., M.C. Gregg, V. Zervakis, and H. Kontoyiannis, "Internal wave measurements on the Cycladic Plateau of the Aegean Sea," J. Geophys. Res., 117, doi:10.1029/2011JC007488, 2012.

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25 Jan 2012

The internal wave climate in the southern Aegean Sea is examined with an array of two bottom-mounted acoustic Doppler current profilers and three profiling moorings deployed on the northern continental slope of the Cretan Sea for 3 months. Frequency spectra indicate an extremely weak internal wave continuum, about 4–10 times weaker than the Garrett-Munk and Levine reference levels. Spectra are instead dominated by semidiurnal internal tides and near-inertial waves, which are examined in detail by bandpass filtering. In the semidiurnal band, a barotropic tidal flow of –2 cm s-1 is observed, with a pronounced spring/neap modulation in phase with the lunar fortnightly cycle. One to two days following several of these spring tide periods, a distinct internal tide featuring 10–20 m vertical displacements and 15–20 cm s-1 baroclinic velocities is detectable propagating upward and to the southeast. Time-mean energy increases a factor of 2–5 within about 100 m from the bottom, implying generation and/or scattering from the bottom, whose slope is nearly critical to semidiurnal internal waves over much of the array. Several strong, downward propagating near-inertial events are also seen, each of which occurs following a period of work done by the wind on the mixed layer as estimated from a nearby surface mooring. The high-frequency internal wave continuum is more temporally constant but increases substantially toward the end of the deployment. Significant but unexplained differences in kinetic energy occur between successive spring tide periods in the case of the internal tides and between successive wind events in the case of the near-inertial signals. Substantial variability is observed in the low-frequency flows, which likely contributes to the time variability of the internal wave signals.

Mixing over the steep side of the Cycladic Plateau in the Aegean Sea

Gregg, M.C., M.H. Alford, H. Kontoyiannis, V. Zervakis, and D. Winkel, "Mixing over the steep side of the Cycladic Plateau in the Aegean Sea," J. Mar. Syst., 89, 30-47, doi:10.1016/j.marsys.2011.07.009, 2012.

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1 Jan 2012

Intensive microstructure sampling over the southern slope of the Cycladic Plateau found very weak mixing in the pycnocline, centered on a thin minimum of diapycnal diffusivity with Kρ=1.5 x 10-6 m2 s-1. Below the pycnocline, Kρ increased exponentially in the bottom 200 m, reaching 1 x 10-4 m2 s-1 a few meters above the bottom. Near-bottom mixing was most intense where the bottom slope equaled the characteristic slope of the semi-diurnal internal tide. This suggests internal wave scattering and/or generation at the bottom, a conclusion supported by near-bottom dissipation rates increasing following rising winds and with intensifying internal waves. Several pinnacles on the slope were local mixing hotspots. Signatures included a vertical line of strong mixing in a pinnacle's wake, an hydraulic jump or lee wave over a downstream side of the summit, and a 'beam' sloping upward at the near-inertial characteristic slope. Because dissipation rate averages were dominated by strong turbulence, ε/vN2 > 100, the effect on Kρ of alternate mixing efficiencies proposed for this range of turbulent intensity is explored.

Flow and mixing in Ascension, a steep, narrow canyon

Gregg, M.C., R.A. Hall, G.S. Carter, M.H. Alford, R.-C. Lien, D.P. Winkel, and D.J. Wain, "Flow and mixing in Ascension, a steep, narrow canyon," J. Geophys. Res., 116, doi: 10.1029/2010JC006610, 2011.

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20 Jul 2011

A thin gash in the continental slope northwest of Monterey Bay, Ascension Canyon, is steep, with sides and axis both strongly supercritical to M2 internal tides. A hydrostatic model forced with eight tidal constituents shows no major sources feeding energy into the canyon, but significant energy is exchanged between barotropic and baroclinic flows along the tops of the sides, where slopes are critical. Average turbulent dissipation rates observed near spring tide during April are half as large as a two week average measured during August in Monterey Canyon. Owing to Ascension's weaker stratification, however, its average diapycnal diffusivity, 3.9 x 10^-3 m^2 s^-1, exceeded the 2.5 x 10^-3 m^2 s^-1 found in Monterey. Most of the dissipation occurred near the bottom, apparently associated with an internal bore, and just below the rim, where sustained cross-canyon flow may have been generating lee waves or rotors. The near-bottom mixing decreased sharply around Ascension's one bend, as did vertically integrated baroclinic energy fluxes. Dissipation had a minor effect on energetics, which were controlled by flux divergences and convergences and temporal changes in energy density. In Ascension, the observed dissipation rate near spring tide was 2.1 times that predicted from a simulation using eight tidal constituents averaged over a fortnightly period. The same observation was 1.5 times the average of an M2-only prediction. In Monterey, the previous observed average was 4.9 times the average of an M2-only prediction.

Tidally driven exchange in an archipelago strait: Biological and optical responses

Jones, B.H., C.M. Lee, G. Toro-Farmer, E.S. Boss, M.C. Gregg, and C.L. Villanoy, "Tidally driven exchange in an archipelago strait: Biological and optical responses," Oceanography, 24, 142-155, doi:10.5670/oceanog.2011.11, 2011.

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1 Mar 2011

Measurements in San Bernardino Strait, one of two major connections between the Pacific Ocean and the interior waters of the Philippine Archipelago, captured tidal currents that drove vertical mixing and net landward transport. A TRIAXUS towed profiling vehicle equipped with physical and optical sensors was used to repeatedly map subregions within the strait, employing survey patterns designed to resolve tidal variability of physical and optical properties. Strong flow over the sill between Luzon and Capul islands resulted in upward transport and mixing of deeper high-salinity, low-oxygen, high-particle-and-nutrient-concentration water into the upper water column, landward of the sill. During the high-velocity ebb flow, topography influences the vertical distribution of water, but without the diapycnal mixing observed during flood tide. The surveys captured a net landward flux of water through the narrowest part of the strait. The tidally varying velocities contribute to strong vertical transport and diapycnal mixing of the deeper water into the upper layer, contributing to the observed higher phytoplankton biomass within the interior of the strait.

Three-dimensional structure and temporal evolution of submesoscale thermohaline intrusions in the North Pacific subtropical frontal zone

Shcherbina, A.Y., M.C. Gregg, M.H. Alford, M.H., and R.R. Harcourt, "Three-dimensional structure and temporal evolution of submesoscale thermohaline intrusions in the North Pacific subtropical frontal zone," J. Phys. Oceanogr., 40, 1669-1689, doi:10.1175/2010JPO4373.1, 2010.

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1 Aug 2010

Four instances of persistent intrusive deformation of the North Pacific Subtropical Front were tagged individually by a Lagrangian float and tracked for several days. Each feature was mapped in three dimensions using repeat towed observations referenced to the float. Isohaline surface deformations in the frontal zone included sheetlike folds elongated in the alongfront direction and narrow tongues extending across the front. All deformations appeared as protrusions of relatively cold, and fresh, water across the front. No corresponding features of the opposite sign or isolated lenslike structures were observed. The sheets were O(10 m) thick, protruded about 10 km into the warm saline side of the front, and were coherent for 10–30 km along the front. Having about the same thickness and cross-frontal extent as the sheets, tongues extended less than 5 km along the front.

All of the intrusions persisted as long as they were followed, several days to one week. Their structures evolved on both inertial (23 h) and subinertial (10 days) time scales in response to differential lateral advection. The water mass surrounding the intrusions participated in gradual anticyclonic rotation as a part of a mesoscale meander of the subtropical front. The intrusions may be interpreted as a manifestation of three-dimensional submesoscale turbulence of the frontal zone, driven by the mesoscale. Absence of large features of the opposite sign may be indicative of the asymmetry of the underlying dynamics.

Flow and hydraulics near the sill of Hood Canal, a strongly sheared, continuously stratified fjord

Gregg, M.C., and L.J. Pratt, "Flow and hydraulics near the sill of Hood Canal, a strongly sheared, continuously stratified fjord," J. Phys. Oceanogr., 40, 1087-1105, doi: 10.1175/2010JPO4312.1, 2010.

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1 May 2010

Hood Canal, a long fjord in Washington State, has strong tides but limited deep-water renewal landward of a complex constriction. Tide-resolving hydrographic and velocity observations at the constriction, with a depth-cycling towed body, varied markedly during three consecutive years, partly because of stratification variations. To determine whether hydraulic control is generally important and to interpret observations of lee waves, blocking, and other features, hydraulic criticality is estimated over full tidal cycles for channel wide internal wave modes 1, 2, and 3, at five cross-channel sections, using mode speeds from the extended Taylor–Goldstein equation.

These modes were strongly supercritical during most of ebb and flood on the gentle seaward sill face and for part of flood at the base of the steep landward side. Examining local criticality along the thalweg found repeated changes between mode 1 being critical and supercritical approaching the sill crest during flood, unsurprising given local minima and maxima in the cross-sectional area, with the sill crest near a maximum. Density crossing the sill sometimes resembled an overflow with an internal hydraulic control at the sill, followed by a hydraulic jump or lee wave. Long-wave speeds, however, suggest cross waves, particularly along the shallower gentler side, where flow downstream of a large-amplitude wave was uniformly supercritical. Supercritical approaching the sill, peak ebb was critical to mode 1 and supercritical to modes 2 and 3 at the base while forming a sluggish dome of dense water over the sill. Full interpretation exceeds observations and existing theory.

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

Characterizing thermohaline intrusions in the North Pacific subtropical frontal zone

Shcherbina, A.Y., M.C. Gregg, M.H. Alford, and R.R. Harcourt, "Characterizing thermohaline intrusions in the North Pacific subtropical frontal zone," J. Phys. Oceanogr., 39, 2735-2756, 2009.

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1 Nov 2009

A monthlong field survey in July 2007, focused on the North Pacific subtropical frontal zone (STFZ) near 30°N, 158°W, combined towed depth-cycling conductivity-temperature-depth (CTD) profiling with shipboard current observations. Measurements were used to investigate the distribution and structure of thermohaline intrusions. The study revealed that local extrema of vertical salinity profiles, often used as intrusion indicators, were only a subset of a wider class of distortions in thermohaline fields due to interleaving processes. A new method to investigate interleaving based on diapycnal spiciness curvature was used to describe an expanded class of laterally coherent intrusions. STFZ intrusions were characterized by their overall statistics and by a number of case studies. Thermohaline interleaving was particularly intense within 5 km of two partially compensated fronts, where intrusions with both positive and negative salinity anomalies were widespread. The vertical and cross-frontal scales of the intrusions were on the order of 10 m and 5 km, respectively. Though highly variable, the slopes of these features were typically intermediate between those of isopycnals and isohalines. Although the influence of double-diffusive processes sometime during the evolution of intrusions could not be excluded, the broad spectrum of the observed features suggests that any role of double diffusion was secondary.

Sea surface cooling at the equator by subsurface mixing in tropical instability waves

Moum, J.N., R.-C. Lien, A. Perlin, J.D. Nash, M.C. Gregg, and P.J. Wiles, "Sea surface cooling at the equator by subsurface mixing in tropical instability waves," Nature Geosci., 2, 761-765, 2009.

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11 Oct 2009

Changes in sea surface temperature of equatorial waters have critical effects on the large-scale atmospheric circulation. So far, large-scale, energetic tropical instability waves in equatorial waters have been thought to warm the sea surface through both meridional and zonal advection. Here, we present shipboard profiling measurements of turbulence kinetic-energy dissipation rate that reveal unanticipated vigorous mixing associated with tropical instability waves. The meridional tropical instability-wave shear increases the shear above the core of the Equatorial Undercurrent, which is already large, nudging the flow toward instability. As a consequence, turbulence dissipation rates and heat fluxes are many times greater than previous measurements at the same location but in the absence of tropical instability waves. The vertical divergence of turbulence heat flux is sufficient to cool the upper layer by 2 K per month, and heat the core of the Equatorial Undercurrent by 10 K per month. Long-term records at 140°W further reveal that cooling of the sea surface is significantly correlated to tropical-instability-wave kinetic energy. Thus, seasonal surface cooling in the central equatorial Pacific may be largely caused by mixing induced by tropical instability waves.

The CLIMODE field campaign: Observing the cycle of convection and restratification over the Gulf Stream

Marshall, J., et al. including M.C. Gregg and K.A. Kelly, "The CLIMODE field campaign: Observing the cycle of convection and restratification over the Gulf Stream," Bull. Am. Met. Soc., 90, 1337-1350, 2009.

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1 Sep 2009

A major oceanographic field experiment is described, which is designed to observe, quantify, and understand the creation and dispersal of weakly stratified fluid known as "mode water" in the region of the Gulf Stream. Formed in the wintertime by convection driven by the most intense air-sea fluxes observed anywhere over the globe, the role of mode waters in the general circulation of the subtropical gyre and its biogeo-chemical cycles is also addressed. The experiment is known as the CLIVAR Mode Water Dynamic Experiment (CLIMODE). Here we review the scientific objectives of the experiment and present some preliminary results.

Turbulence, acoustic backscatter and pelagic nekton in Monterey Bay

Gregg, M.C., and J.K. Horne, "Turbulence, acoustic backscatter and pelagic nekton in Monterey Bay," J. Phys. Oceanogr., 38, 1097-1114, 10.1175/2008JPO4033.1, 2009

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30 May 2009

During August 2006 aggregations of nekton, most likely small fish, intersected microstructure survey lines in Monterey Bay, California, providing an opportunity to examine biologically generated mixing. Some aggregations filled the water column, 80 m deep, and extended 100–200 m along the survey track. Others were half that size, and some were much smaller. Acoustic energy backscattered from the aggregations was measured with a calibrated echosounder and yielded volume backscattering strength Sv values of –80 to –60 dB re 1 m–1.

Restratification of the surface mixed layer with submesoscale lateral density gradients: Diagnosing the importance of horizontal dimensions

Hosegood, P.J., M.C. Gregg, and M.H. Alford, "Restratification of the surface mixed layer with submesoscale lateral density gradients: Diagnosing the importance of horizontal dimensions," J. Phys. Oceanogr., 38, 2438-2460, 2008.

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1 Nov 2008

A depth-cycling towed conductivity–temperature–depth (CTD) and vessel-mounted acoustic Doppler current profiler (ADCP) were used to obtain four-dimensional measurements of the restratification of the surface mixed layer (SML) at a submesoscale lateral density gradient near the subtropical front. With the objective of studying the role of horizontal processes in restratification, the thermohaline and velocity fields were monitored for 33 h by 16 small-scale (≤15 km2) surveys centered on a drogued float. Daytime warming by insolation caused a unidirectional displacement of the initially vertical isopycnals toward increasing density. Across the entire SML (50-m vertical scale), solar insolation accounted for 60% of observed restratification, but over 10-m scales, the percentage decreased with depth from 80% at 25–35 m to ≤25% at 55–65 m. Below 35 m, stratification was enhanced by the vertically sheared horizontal advection of the lateral density gradient due to a near-inertial wave of 100-m vertical wavelength that rotated anticyclonically at the inertial frequency. The phase and similar period (25.4 h) of the local inertial period to the diurnal cycle ensured constructive interference with isopycnal displacements due to insolation. Restratification by sheared advection matched that predicted due to vertically sheared inertial oscillations generated during the geostrophic adjustment of a density front, but direct wind forcing may also have generated the wave that was subsequently modified by interaction with mesoscale vorticity associated with a nearby large-scale front. By further including the effects of lateral uncompensated thermohaline inhomogeneity, the authors can account for 100% ± 20% of the observed N2 during daytime restratification. No detectable restratification due to the slumping of horizontal density gradients under gravity alone was found.

Energetics of M2 barotropic to baroclinic conversion at the Hawaiian Islands

Carter, G.S., M.A. Merrifield, J. Becker, K. Katsumata, M.C. Gregg, D.S. Luther, M.D. Levine, T.J. Boyd, and Y.L. Firing, "Energetics of M2 barotropic to baroclinic conversion at the Hawaiian Islands," J. Phys. Oceanogr., 38, 2205-2223, 10.1175/2008JPO3860.1, 2008.

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30 Oct 2008

A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (M2) over a region of the Hawaiian Ridge from Niihau to Maui. This region includes the Kaena Ridge, one of the three main internal tide generation sites along the Hawaiian Ridge and the main study site of the Hawaii Ocean Mixing Experiment. The 0.01°-horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations, moored ADCP currents, and notably reasonable agreement with microstructure data. Barotropic and baroclinic energy equations are derived from the model's sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The M2 barotropic tide loses 2.7 GW of energy over the study region. Of this, 163 MW (6%) is dissipated by bottom friction and 2.3 GW (85%) is converted into internal tides. Internal tide generation primarily occurs along the flanks of the Kaena Ridge and south of Niihau and Kauai. The majority of the baroclinic energy (1.7 GW) is radiated out of the model domain, while 0.45 GW is dissipated close to the generation regions. The modeled baroclinic dissipation within the 1000-m isobath for the Kaena Ridge agrees to within a factor of 2 with the area-weighted dissipation from 313 microstructure profiles. Topographic resolution is important, with the present 0.01° resolution model resulting in 20% more barotropic-to-baroclinic conversion compared to when the same analysis is performed on a 4-km resolution simulation. A simple extrapolation of these results to the entire Hawaiian Ridge is in qualitative agreement with recent estimates based on satellite altimetry data.

Sub-mesoscale lateral density structure in the oceanic surface mixed layer

Hosegood, P., M.C. Gregg, and M.H. Alford, "Sub-mesoscale lateral density structure in the oceanic surface mixed layer," Geophys. Res. Lett., 33, 10.1029/2006GL026797, 2006.

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23 Nov 2006

We investigate the horizontal scales of density variability in the surface mixed layer (SML) in the North Pacific Subtropical Front (STF) during a period of highly variable atmospheric forcing. Wavelet analysis shows that horizontal density variability is not restricted to scales, L, ≥10 km as previously suggested but extends to L = 2 km. The limiting L varies strongly with location and corresponds to a local internal Rossby radius that accounts for transient stratification above the seasonal thermocline. Density compensation in the SML, achieved when temperature and salinity effects cancel in their effect on density, occurs at 30°N at the climatological front associated with the northern boundary of the STF where large thermohaline gradients are observed. At 28°N, however, temperature gradients within the SML are not compensated by salinity, and horizontal density gradients result in 2 ≤ L ≤ 10 km. Our observations suggest dynamic processes restratify the SML at scales rarely resolved by numerical models of the SML.

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.

Flow and mixing around a small seamount on Kaena Ridge, Hawaii

Carter, G.S., M.C. Gregg, and M.A. Merrifield, "Flow and mixing around a small seamount on Kaena Ridge, Hawaii," J. Phys. Oceanogr., 36, 1036-1052, 2006.

1 Jun 2006

Persistent near-diurnal internal waves observed above a site of M-2 barotropic-to-baroclinic conversion

Carter, G.S., and M.C. Gregg, "Persistent near-diurnal internal waves observed above a site of M-2 barotropic-to-baroclinic conversion," J. Phys. Oceanogr., 36, 1136-1147, doi:10.1175/JPO2884.1, 2006.

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1 Jun 2006

Near-diurnal internal waves were observed in velocity and shear measurements from a shipboard survey along a 35-km section of the Kaena Ridge, northwest of Oahu. Individual waves with upward phase propagation could be traced for almost 4 days even though the ship transited approximately 20 km. Depth–time maps of shear were dominated by near-diurnal waves, despite the fact that Kaena Ridge is a site of considerable M2 barotropic-to-baroclinic conversion. Guided by recent numerical and observational studies, it was found that a frequency of 1/2M2 (i.e., 24.84-h period) was consistent with these waves. Nonlinear processes are able to transfer energy within the internal wave spectrum. Bicoherence analysis, which can distinguish between nonlinearly coupled waves and waves that have been independently excited, suggested that the 1/2M2 waves were nonlinearly coupled with the dominant M2 internal tide only between 525- and 595-m depth. This narrow depth range corresponded to an observed M2 characteristic emanating from the northern edge of the ridge. The observations occurred in close proximity to the internal tide generation region, implying a rapid transfer of energy between frequencies. Strong nonlinear interactions seem a likely mechanism. Nonlinear transfers such as these could complicate attempts to close local single-constituent tidal energy budgets.

Structure, propagation, and mixing of energetic baroclinic tides in Mamala Bay, Oahu, Hawaii

Alford, M.H., M.C. Gregg, and M.A. Merrifield, "Structure, propagation, and mixing of energetic baroclinic tides in Mamala Bay, Oahu, Hawaii," J. Phys. Oceanogr., 36, 997-1018, doi:10.1175/JPO2877.1, 2006.

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1 Jun 2006

Large semidiurnal vertical displacements (≈100 m) and strong baroclinic currents (≈0.5 m s-1; several times as large as barotropic currents) dominate motions in Mamala Bay, outside the mouth of Pearl Harbor, Hawaii. During September 2002, the authors sought to characterize them with a 2-month McLane moored profiler deployment and a 4-day intensive survey with a towed CTD/ADCP and the Research Vessel (R/V) Revelle hydrographic sonar. Spatial maps and time series of turbulent dissipation rate ε, diapycnal diffusivity Kρ, isopycnal displacement η, velocity u, energy E, and energy flux F are presented. Dissipation rate peaks in the lower 150 m during rising isopycnals and high strain and shows a factor-of-50 spring-neap modulation. The largest Kρ values, in the western bay near a submarine ridge, exceed 10-3 m2 s-1. The M2 phases of η and u increase toward the west, implying a westward phase velocity cp ≈ 1 m s-1 and horizontal wavelength ≈60 km, consistent with theoretical mode-1 values. These phases vary strongly (≈±45°) in time relative to astronomical forcing, implying remotely generated signals. Energy and energy flux peak 1–3 days after spring tide, supporting this interpretation. The group velocity, computed as the ratio F/E, is near ≈1 m s-1, also in agreement with theoretical mode-1 values. Spatial maps of energy flux agree well with results from the Princeton Ocean Model, indicating converging fluxes in the western bay from waves generated to the east and west. The observations indicate a time-varying interference pattern between these waves that is modulated by background stratification between their sources and Mamala Bay.

Mixing, 3D mapping, and Lagrangian evolution of a thermohaline intrusion

Alford, M.H., M.C. Gregg, and E.A. D'Asaro, "Mixing, 3D mapping, and Lagrangian evolution of a thermohaline intrusion," J. Phys. Oceanogr., 35, 1689-1711, 2005

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30 Sep 2005

Observations of the three-dimensional structure and evolution of a thermohaline intrusion in a wide, deep fjord are presented. In an intensive two-ship study centered on an acoustically tracked neutrally buoyant float, a cold, fresh, low-oxygen tongue of water moving southward at about 0.03 m s-1 out of Possession Sound, Washington, was observed. The feature lay across isopycnal surfaces in a 50–80-m depth range. The large-scale structures of temperature, salinity, velocity, dissolved oxygen, and chlorophyll were mapped with a towed, depth-cycling instrument from one ship while the other ship measured turbulence close to the float with loosely tethered microstructure profilers. Observations from both ships were expressed in a float-relative (Lagrangian) reference frame, minimizing advection effects. A float deployed at the tongue's leading edge warmed 0.2°C in 24 h, which the authors argue resulted from mixing. Diapycnal heat fluxes inferred from microstructure were 1–2 orders of magnitude too small to account for the observed warming. Instead, lateral stirring along isopycnals appears responsible, implying isopycnal diffusivities O(1 m2 s-1). These are consistent with estimates, using measured temperature microstructure, from an extension of the Osborn–Cox model that allows for lateral gradients. Horizontal structures with scales O(100 m) are seen in time series and spatial maps, supporting this interpretation.

Upper ocean heat and freshwater budgets in the eastern Pacific warm pool

Wijesekera, H.W., D.L. Rudnick, C.A. Paulson, S.D. Pierce, W.S. Pegau, J. Mickett, and M.C. Gregg, "Upper ocean heat and freshwater budgets in the eastern Pacific warm pool," J. Geophys. Res., 110, 10.1029/2004JC002511, 2005

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11 Aug 2005

This study focuses on upper ocean budgets of heat and freshwater, which yield estimates of net surface heat flux and rainfall minus evaporation. The budgets are based on a 19 day ship survey conducted as part of the Eastern Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System 2001 in September 2001. Underway measurements included temperature and salinity sections from an undulating platform, SeaSoar, and horizontal currents from an acoustic Doppler current profiler along a 146 x 146 km survey pattern centered near 10°N, 95°W in the eastern Pacific warm pool. Additional measurements from a second ship at the center of the survey pattern included radar backscatter from rainfall, air-sea fluxes, and vertical profiles of temperature, salinity, microstructure, and horizontal velocity. Satellite measurements of surface height, temperature, and rainfall were also analyzed. The heat budget of 20 and 25 m surface layers indicated that storage, advection, turbulent transport, and penetrative solar radiation were all significant components of the heat budget with a net surface cooling of 41 W m-2 estimated as a residual, which agreed with atmospheric measurements (30 W m-2). The precipitation rate from the freshwater budget was 29 mm d-1, which was in excellent agreement with in situ measurements on both ships and in good agreement with satellite estimates for the same period. Lateral transports of heat and salt were influenced by an anticyclonic eddy in the survey area, and it is suggested that anticyclonic eddies, which form near the Central American coast, may carry anomalously warm sea surface temperature toward the west and become preferential sites for heavy rainfall.

Internal waves, solitary-like waves, and mixing on the Monterey Bay shelf

Carter, G.S., M.C. Gregg, and R.-C. Lien, "Internal waves, solitary-like waves, and mixing on the Monterey Bay shelf," Continent. Shelf Res., 25, 1499-1520, DOI: 10.1016/j.csr.2005.04.011, 2005

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24 Jun 2005

Microstructure measurements taken on the Monterey Bay continental shelf, within 4 km of the shelf break, reveal a complex mixing environment. Depth- and time-averaged dissipation rates and diapycnal diffusivities were elevated above observations made over other continental shelves with no significant topography, but were below those influenced by topographic features. The close proximity of the shelf break/canyon rim, locally generated internal tides, and nonlinear internal waves all contributed to the elevated turbulence. The complex bathymetry associated with Monterey Submarine Canyon allowed an internal tide to be generated at depths greater than 1500 m, as well as at the shelf break. The observed velocity field was normally dominated by upward energy propagation from the local shelf break generated internal tide, but near low tide downward energy propagation from a surface reflection of the internal tide generated below 1500 m was observed. Turbulent dissipation rates were not well parameterized by either the open-ocean Gregg–Henyey model or the recently developed MacKinnon–Gregg shelf model. Like its application on the New England shelf, the MacKinnon–Gregg model had the correct functional dependence on shear and stratification (dissipation increasing with increasing shear and increasing stratification), however, the magnitude and range of values were too small.

The most surprising finding was the presence of what we believe to be large, high-aspect-ratio, downslope-propagating nonlinear internal solitary-like waves of elevation. Upon reaching the canyon rim, these waves propagated into deep water and transformed into waves of depression. On the shelf south of the canyon, the waves of elevation accounted for 20% of the observed turbulent kinetic energy dissipation. Off the shelf, where the solitary-like waves changed to downward displacement, their average dissipation increased 10-fold, and accounted for nearly half the dissipation in the upper 150 m.

Surface ventilation of the Black Sea's cold intermediate layer in the middle western gyre

Gregg, M.C., and E. Yakushev, "Surface ventilation of the Black Sea's cold intermediate layer in the middle western gyre," Geophys. Res. Lett., 32, 10.1029/2004GL021580, 2005.

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5 Feb 2005

Understanding the origin of the shallow temperature minimum, known as the Cold Intermediate Layer (CIL), in the Black Sea has long been hampered by the scarcity of winter observations. During March 2003, we observed a cold-air outbreak over the center of the Black Sea's Western Gyre. Freezing winds drove convection that cooled the surface mixed layer to 6.1°C and deepened it to 40 m, directly ventilating the upper 80% of the CIL, whose lower boundary was at 49 m. Concentrations of dissolved oxygen were 350 µM in the mixed layer and decreased rapidly to 70 µM at the base of the CIL, 9 m below the mixed layer. A few meters deeper, at the top of the Sub-Oxic Layer (SOL), both oxygen and hydrogen sulfide became undetectable (<5 µM and <1 µM, respectively). Microstructure profiles revealed intermittent turbulence in the oxycline below the mixed layer. Average rates of turbulent dissipation were 10-9 – 10-8 W kg-1. The accompanying mixing produced diapycnal diffusivities that were only (1–4) x 10-6 m2 s-1. Consequently, turbulent fluxes were too weak to renew significantly either the lower 20% of the CIL or the SOL, whose top was 4 m below the bottom of the CIL and hence well-removed from direct surface replenishment.

Fast response thin-film thermistor for measurements in ocean waters

Kukuruznyak, D.A., J.B. Miller, M.C. Gregg, and F.S. Ohuchi, "Fast response thin-film thermistor for measurements in ocean waters," Rev. Sci. Instr., 76, 24, 905-1-3, doi:10.1063/1.1851494, 2005.

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11 Jan 2005

This work describes a very fast thin-film thermistor for temperature measurements in aggressive and conducting liquids. In order to minimize the thermal mass of the sensor, we employed a thin-gauge hollow glass capillary as both a hermetic encapsulation as well as substrate for the thin resistive film. The thermistor thin film is deposited onto an inner surface of the capillary using a solution deposition technique. Microscopic electrical wires are deeply inserted into the capillary tube and fused to the thermistor film. The resistance element thus fabricated is attached to the supporting members and sealed at the open ends. Consequently, detrimental influence of the massive prongs on the dynamic characteristics of the sensor was also eliminated. The prototype temperature probes were compared to the fastest commercially available bead thermistors (with response time of 7 milliseconds). The new sensors exhibited superior temporal response.

Tidally generated turbulence over the Knight Inlet sill

Klymak, J.M., and M.C. Gregg, "Tidally generated turbulence over the Knight Inlet sill," J. Phys. Oceanogr., 34, 1135-1151, 2004.

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1 May 2004

Very high turbulent dissipation rates were observed in the nonlinear internal lee waves that form each tide over a sill in Knight Inlet, British Columbia. This turbulence was due to both shear instabilities and the jumplike adjustment of the wave to background flow conditions. Away from the sill, turbulent dissipation was significantly lower. Energy removed from the barotropic tide was estimated using a pair of tide gauges; a peak of 20 MW occurred during spring tide. Approximately two-thirds of the barotropic energy loss radiated away as internal waves, while the remaining one-third was lost to processes near the sill. Observed dissipation in the water column does not account for the near-sill losses, but energy lost to vortex shedding and near-bottom turbulence, though not measured, could be large enough to close the energy budget.

Direct measurements of diapycnal mixing in a fjord reach - Puget Sound's Main Basin

Mickett, J.B., M.C. Gregg, and H.E. Seim, "Direct measurements of diapycnal mixing in a fjord reach - Puget Sound's Main Basin," Estuar. Coast. Shelf Sci., 59, 539-558, doi:10.1016/j.ecss.2003.10.009, 2004.

1 Apr 2004

Small-scale processes in straits

Gregg, M.C., "Small-scale processes in straits," Deep-Sea Res. II, 51, 489-503, doi:10.1016/j.dsr2.2003.08.003, 2004.

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1 Feb 2004

Small-scale studies in straits reveal much about their internal dynamics. Highlights of recent work are described to provide an overview of the rapid progress being made in understanding processes within straits and how they affect outside regions. Owing to the early state of work on small-scale processes within straits, the emphasis is on identifying and understanding the dominant processes and mechanisms leading to mixing and the mixing itself. Theoretical, numerical, and observational approaches are included, and some suggestions are made about future directions.

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.

The role of upstream wave and a downstream density pool in the growth of lee waves: stratified flow over the Knight Inlet sill

Klymak, J.M., and M.C. Gregg, "The role of upstream wave and a downstream density pool in the growth of lee waves: stratified flow over the Knight Inlet sill," J. Phys. Oceanogr., 33, 1446-1461, DOI: 10.1175/1520-0485(2003)033<1446:TROUWA>2.0.CO;2, 2003.

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1 Jul 2003

Observations and modeling simulations are presented that illustrate the importance of a density contrast and the upstream response to the time dependence of stratified flow over the Knight Inlet sill. Repeated sections of velocity and density show that the flow during ebb and flood tides is quite different: a large lee wave develops early in flood tide, whereas lee-wave growth is suppressed until the second half of ebb tide. There is a large upstream response that displaces as much water as accumulates in the lee wave, one that is large enough to also block the flow at a depth roughly consistent with simple dynamics. There is a large density contrast between the seaward and landward sides of the sill, and a "salty pool" of water is found in the seaward basin that is not found landward. The interface with this salty pool demarks the point of flow separation during ebb, initially suppressing the lee wave and then acting as its lower boundary. A simple two-dimensional numerical model of the inlet was used to explore the important factors governing the flow. A base simulation that included the landward–seaward asymmetry of the sill shape, but not the density difference, yielded a response that was almost symmetric with a large lee wave forming early during both flood and ebb tide. The simulation behaves more like the observations when a salty pool of water is added seaward of the sill. This salty pool induces flow separation in the model and suppresses growth of the lee wave until late in ebb. This effect is termed "density-forced" flow separation, a modification of "postwave" flow separation that allows for a density gradient across an obstacle.

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.

Mixing on the late-summer New England shelf—solibores, shear, and stratification

MacKinnon, J.A., and M.C. Gregg, "Mixing on the late-summer New England shelf—solibores, shear, and stratification," J. Phys. Oceanogr., 33, 1475-1492, doi:10.1175/1520-0485(2003)033<1476:MOTLNE>2.0.CO;2, 2003.

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1 Mar 2003

Observations are presented of microstructure and velocity measurements made on the outer New England shelf in the late summer of 1996 as part of the Coastal Mixing and Optics Experiment. The depth- and time-averaged turbulent dissipation rate was 5–50 (x 10-9 W kg-1). The associated average diapycnal diffusivity in stratified water was 5–20 (x 10-6 m2 s-1), comparable to observed open-ocean thermocline values and too low to explain the strong variability observed in local water properties. Dissipation rates and diffusivity were both highly episodic. Turbulent boundary layers grew down from the surface and up from the bottom. The dissipation rate within the bottom boundary layer had an average of 1.2 x 10-7 W kg-1 and varied in magnitude with the strength of near-bottom flow from the barotropic tide, an along-shelf flow, and low-frequency internal waves. The average dissipation rate in the peak thermocline was 5 x 10-8 W kg-1; one-half of the thermocline dissipation was due to the strong shear and strain within six solibores that cumulatively lasted less than a day but contained 100-fold elevated dissipation and diffusivity. Nonsolibore, midcolumn dissipation was strongly correlated with shear from low-frequency internal waves. Dissipation was not well parameterized by Gregg–Henyey-type scaling. An alternate scaling, modified to account for observed coastal internal wave properties, was in good agreement with measured dissipation rates. At the end of the observational period Hurricane Edouard passed by, producing strong dissipation rates (4 x 10-6 W kg-1) and consequent mixing during and for several days following the peak winds.

Shear and baroclinic energy flux on the summer New England shelf

MacKinnon, J.A., and M.C. Gregg, "Shear and baroclinic energy flux on the summer New England shelf," J. Phys. Oceanogr., 33, 1462-1475, doi:10.1175/1520-0485(2003)033<1462:SABEFO>2.0.CO;2, 2003.

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1 Mar 2003

Observations are presented of internal wave properties and energy fluxes through a site near the 70-m isobath on the New England shelf in late summer. Data collected from a shipboard ADCP and microstructure profiler over a three-week period and projected onto dynamic vertical modes reveals large variations in the magnitude and vertical structure of internal waves. Baroclinic energy and shear were primarily associated with low-mode near-inertial and semidiurnal waves and, at times, high-frequency solibores. The energies in each mode varied by factors from 2 to 10 over several days and were not significantly correlated with one another. The associated shear variance was concentrated in the thermocline. However, the strength and vertical range of shear varied significantly throughout the research period and depended sensitively on both the magnitude and evolving vertical mode content of the wave field. Shear during the quasi-two-layer solibores was strong enough to temporarily lower the 4-m Richardson number below the threshold for shear instability. Energy flux through the site came primarily from the mode-1 internal tide, in both linear and nonlinear (solibore) forms. The average energy flux from the first five baroclinic modes was 130 W m-1. A comparison of energy fluxes from each mode and locally measured average dissipation rates suggests that near-inertial and high-mode waves were generated near the experimental site.

Intense, variable mixing near the head of Monterey Submarine Canyon

Carter, G.S., and M.C. Gregg, "Intense, variable mixing near the head of Monterey Submarine Canyon," J. Phys. Oceanogr., 32, 3145-3165, doi:10.1175/1520-0485(2002)032, 2002.

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1 Nov 2002

A microstructure survey near the head of Monterey Submarine Canyon, the first in a canyon, confirmed earlier inferences that coastal submarine canyons are sites of intense mixing. The data collected during two weeks in August 1997 showed turbulent kinetic energy dissipation and diapycnal diffusivity up to 103 times higher than in the open ocean. Dissipation and diapycnal diffusivity within 10 km of the canyon head were among the highest observed anywhere. Mixing occurred mainly in an on-axis stratified turbulent layer, with thickness and intensity increasing from neap to spring tide. Strain spectra showed a gentler than k-1z rolloff, suggesting that critical reflection and scattering may push energy into high wavenumbers. Dissipation dependence on shear appears to be much weaker in the canyon than in the open ocean, with indications that the dependence maybe as low as ε ∝ (S)1/2. Coastal canyons may account for a small but significant fraction of the internal tide energy budget. A crude estimate of global dissipation in canyons is 58 GW, 󖐯% of the estimated global M2 internal tide dissipation.

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.

Internal waves in Monterey Submarine Canyon

Kunze, E., L.K. Rosenfeld, G.S. Carter, and M.C. Gregg, "Internal waves in Monterey Submarine Canyon," J. Phys. Oceanogr., 32, 1890-1913, 2002.

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1 Jun 2002

Velocity, temperature, and salinity profile surveying in Monterey Submarine Canyon during spring tide reveals an internal wave field almost an order of magnitude more energetic than that in the open ocean. Semidiurnal fluctuations and their harmonics dominate, near-inertial motions are absent. The ratio of horizontal kinetic to available potential energy is less than one in much of the canyon, inconsistent with hydrostatic internal waves. The excess potential energy may be due to isopycnal displacements induced by barotropic tide flow over the sloping bottom. Removal of the expected barotropic contribution raises the energy ratio to 2.04–2.10, in line with the semidiurnal internal wave value of 2.13. Finescale shear and strain are also elevated. Finescale parameterizations for turbulent eddy diffusivities, which have proven successful in the open ocean, underestimate upper-canyon microstructure estimates of 100 x 10-4 m2 s-1 by a factor of 30. Energy fluxes and near-bottom velocities are strongly steered by the sinuous canyon topography. A vertically integrated influx of 5 kW m-1 at the mouth diminishes to ±1 kW m-1 toward the shallow end of the canyon. Both sinks and sources of internal wave energy are indicated by energy-flux convergences and divergences along the canyon axis. Along-axis energy-flux convergences are consistent with microstructure dissipation rates. The high diapycnal eddy diffusivities may drive strong nutrient fluxes to enhance bioproductivity.

Flow, water mass changes, and hydraulics in the Bosphorus

Gregg, M.C., and E. Ozsoy, "Flow, water mass changes, and hydraulics in the Bosphorus," J. Geophys. Res., 107, 10.1029/2000JC000485, 2002.

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7 Mar 2002

Using average sections along the Bosphorus taken in September 1994 with a loosely tethered profiler and an acoustic Doppler current profiler, Gregg et al. [1999] found the exchange flow between the Sea of Marmara and the Black Sea to be quasi-steady but far from satisfying the hydraulic control conditions for two-layered flows. Here we examine synoptic sections and use images from an acoustic backscatter system to provide the first detailed look at the flow and water mass changes in the Bosphorus and to assess how well the flow satisfies the hydraulic assumptions. Thirty kilometers long, 28–100 m deep, and 0.75–3 km wide, the Bosphorus has bathymetry far more complex than that used in analytic or numerical models of exchange flows. The particulars affect dynamics in important ways. For instance, owing to changes in channel shape, the narrowest section, known as the contraction, is not also the minimum in cross-sectional area. Rather, it is a transition between the wider northern half and the narrower southern half of the strait, and some places south of the contraction have slightly smaller areas. Sharp bends occur throughout the strait and often produce flow separations as well as directing upper and lower flows to opposite sides of the channel. Never <28% of the water column, the interface thickens to 75% in the southern half of the strait as a result of intense mixing downstream of the contraction. As a consequence of the strong mixing and numerous flow separations, we conclude that the exchange flow may be at least partly controlled by friction instead of being a simple hydraulic flow.

Mixing in the Bosphorus Strait and the Black Sea continental shelf: Observations and a model of the dense water outflow

Ozsoy, E., D. Di Iorio, M.C. Gregg, and J.O. Backhaus, "Mixing in the Bosphorus Strait and the Black Sea continental shelf: Observations and a model of the dense water outflow," J. Mar. Syst., 31, 99-135, 2001.

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1 Nov 2001

Counterflowing waters of the Black Sea and Mediterranean Sea are mixed by turbulent entrainment processes along their course through the Turkish Straits. In the Bosphorus Strait, the entrainment into the upper layer from below is abruptly increased when the flow is accelerated in the narrower southern reach, where the flow passes through a contraction. In contrast, the lower layer salinity decreases towards the north first by gradual entrainment within the Strait and later at an increased rate in the wide continental shelf region upon exit into the Black Sea. After passing over the sill located north of the Strait, the flow on the continental shelf proceeds in the form of a gravity current following the local slopes. The topography of the shelf region, assembled from various sources of high-resolution surveys and maps, is reminiscent of a river delta. The water properties and thickness of the Mediterranean plume is modified by turbulent entrainment, shelf currents, stratification, bottom friction and slope. The flow first spreads out on the mid-shelf slope, follows the delta features to reach the shelf edge and, finally, cascades down the steep continental slope. Horizontal spreading by convective instabilities and eastward propagation of anomalous properties along the continental slope are characteristic features of the deeper region adjacent to the shelf. The behaviour of the density current is revealed by results obtained from a reduced gravity model, suggesting that the slope and fine-scale features of the bottom topography are crucial elements in determining plume behaviour. The model results are found to be robust to environmental changes and in good correspondence with observed flow features, especially when the topography with realistic fine scales and slope are adequately represented.

Three-dimensional nature of flow near a sill

Klymak, J.M., and M.C Gregg, "Three-dimensional nature of flow near a sill," J. Geophys. Res., 106, 22,295-22,311, doi:10.1029/2001JC000933, 2001.

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15 Oct 2001

Velocity and density sections across Knight Inlet, British Columbia, demonstrate that lateral recirculations are a first-order feature of the flow in the lee of the sill. The flow over this sill has been used as a typical example of two-dimensional hydraulics, with a lower layer that thins and accelerates as it moves downstream below an almost stagnant layer that widens downstream, creating a distinctive wedge shape. However, we find that rather than being stagnant, the velocity in this wedge-shaped layer is actually quite large, consisting of a swiftly recirculating dipole vortex during flood tide and a monopole vortex during ebb tide, though we may have missed the matching half of a dipole with our sampling during ebb. The recirculations during flood tide carry an amount of water equal to 25% of the tidal flux, while the monopole during ebb tide carries 20% of the tidal flux. These recirculations bias along-channel estimates of volume flux, especially in the middle wedge-shaped layer, and demonstrate that accurate volume fluxes in the lee wave are only possible if three-dimensional surveys are made. Our three-dimensional survey shows that there is a net isopycnal convergence of water in the middle layer at a rate adequate to close the volume budget of the flow without recourse to diapycnal fluxes. We also calculate the strength of the vorticity in the recirculations observed during flood tide and attribute their formation to boundary layer separation.

Near-inertial mixing: Modulation of shear, strain and microstructure at low latitude

Alford, M.H., and M.C. Gregg, "Near-inertial mixing: Modulation of shear, strain and microstructure at low latitude," J. Geophys. Res., 106, 16,947-16,968, doi:10.1029/2000JC000370, 2001.

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15 Aug 2001

We report direct, quantitative measurements of mixing associated with three cycles of a single, energetic, downward-propagating near-inertial wave in the Banda Sea at 6.5°S, 128°E during October 1998. The wave dominates the shear, containing 70% of the total variance. Simultaneous depth/time series of shear, strain, Froude number (Fr), and microstructure allow direct computation of their coherence and phase from 50–120 m, for 14 days. In this depth range, 72% of diapycnal diffusivity (68% of dissipation) occurs in three distinct pulses, spaced at the inertial period of 4.4 days. These are collocated with maxima of transverse shear, strain and Fr. Inertial-band log diapycnal diffusivity, log10 Kp , is coherent at the 95% confidence level with both components of shear and Froude number. In this data set, strain is more important than shear in modulating Fr. Owing to the low latitude, the inertial frequency (fo=1/4.4 cycles per day) is much smaller than the diurnal and tidal frequencies. Consequently, near-inertial motions may be studied separately from tides and other motions via time-domain filtering. Semiempirical WKB plane-wave solutions with observed frequency ωo = 1.02fo and vertical scale 100 m explain 66% and 42% of inertial-band shear and strain variance, respectively. On the basis of the observed phase relationship between shear and strain, the wave is propagating equatorward, toward 295° true. Ratios of shear to strain and of parallel to transverse shear suggest that the wave's intrinsic frequency ωI≈1.18feff. This indicates that background vorticity ζ has lowered the effective Coriolis frequency, feff = fo ζ/2, relative to its planetary value, fo [Kunze, 1985]. Ray tracing suggests that the wave was generated near 6.9°S, 130.6°E, ~20 days prior to the cruise, coincident with the end of high winds associated with the SE monsoon. A slab mixed layer model [Pollard and Millard, 1970], forced with National Center for Environmental Prediction (NCEP) model surface winds, confirms that fluxes from the wind to the ocean at this time were sufficient to generate the wave. A very simple model shows that mixing by monsoon-generated inertial waves may add an important and strongly time-dependent aspect to some regions' energy budgets.

Controlled coordination and oxidation states of copper and manganese cations in complex nickel-copper-cobalt-manganese oxide thin films

Kukuruznyak, D.A., H. Sang-Wook, M.-H Lee, S.A. Bulkey, K.A. Omland, F.S. Ohuchi, and M.C. Gregg, "Controlled coordination and oxidation states of copper and manganese cations in complex nickel-copper-cobalt-manganese oxide thin films," J. Vac. Sci. Technol., 19, 1923-1929, doi:10.1116/1.1355361, 2001.

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1 Jul 2001

Thin thermistor films of Ni0.48Co0.24CuxMn2.28-xO4 (x=0.6 and 1.2) were prepared by the deposition of metalorganic solutions followed by furnace annealing at temperatures between 600 and 800°C. Annealing temperatures are decisive factors to control the electrical properties and electronic structure. X-ray photoelectron spectroscopy revealed that the specimens contained a mixture of Cu1+ and Cu2+ cations, and the annealing caused the change of the oxidation state from Cu1+ to Cu2+ , which was accompanied by the reduction of manganese cations from Mn4+ to Mn3+ . The Cu2p core level from the Cu1+ state along with Cu3d levels showed unusually large negative binding energy shifts (2p3/2 at 930.8 eV and 2p1/2 at 950.6 eV). Extended x-ray absorption fine structure showed that all manganese ions were located in octahedral sites of the spinel lattice, and both Cu1+ and Cu2+ cations occupy the tetrahedral sites regardless of the annealing temperature. X-ray absorption near edge structure spectra of the Mn K edge confirmed the reduction of manganese at high temperature. Cu K-edge spectra confirmed the presence of the cations with anomalous position in the 3d states, thus the negative shift of the Cu1+ core was attributed to the tetrahedral coordination of these cations in the spinel structure.

Preparation and properties of thermistor-thin-films by metal organic decomposition

Kukuruznyak, D.A., S.A. Bulkey, K.A. Omland, M.C. Gregg, E.A. Stern, and F.S. Ohuchi, "Preparation and properties of thermistor-thin-films by metal organic decomposition," Thin Solid Films, 385, 89-95, doi:10.1016/S0040-6090(00)01890-3, 2001.

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2 Apr 2001

Thin thermistor films of chemical composition Ni0.48Co0.24Cu0.6Mn1.68O4 were prepared by metal organic decomposition (MOD). Stock solution that was used for thin-film deposition was prepared by dissolving manganese (III) and nickel (II) acetylacetonates, cobalt acetate and copper nitrate in a mixture of methanol, ethylene glycol and acetic acid. Thin-films were fabricated by either dip-coating or spin-coating. Polycrystalline single-phase cubic spinel was obtained after annealing from 600 to 800°C in air. Smooth and dense thin-films, free from cracks, were formed on aluminosilicate glass, however, the thin-film surface was irregular on fused silica, suggesting that the coefficient of thermal expansion of the substrate plays a crucial role in thin-film morphology. Electrical properties are strongly influenced by the annealing temperature: the resistivity and hopping activation energy vary from ρ(298 K)=9.75 Ω cm and EH=0.139 eV for films annealed at 600°C to ρ(298 K)=1015 Ω cm and EH=0.247 eV for the films annealed at 800°C. The Seebeck coefficient was negative for the films annealed at 600 and 650°C and became positive for films annealed at higher temperatures. Electrical conduction was described in terms of small polaron hopping between Mn4+ and Mn3+ cations located in octahedral sites of the spinel structure. Changes of the electrical properties were attributed to the reduction of manganese Mn4+ → Mn3+ during annealing at high temperatures.

Observations of turbulence in a tidal beam and across a coastal ridge

Lien, R.-C., and M.C. Gregg, "Observations of turbulence in a tidal beam and across a coastal ridge," J. Geophys. Res., 106, 4575-4591, doi:10.1029/2000JC000351, 2001.

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15 Mar 2001

During a microstructure survey off California in Monterey Bay, we found a midwater beam of strong turbulence emanating from the shelf break along the ray path of the semidiurnal M2 internal tide. Within the 50-m-thick beam the turbulence kinetic energy dissipation rate ε exceeded 10-6 W kg-1, and the diapycnal eddy diffusivity Kρ was > 0.01 m2 s-1. The beam extended 4 km off the shelf break. Several factors suggest that this beam of strong turbulence resulted from the breaking of semidiurnal internal tides: the beam appeared to originate from the shelf break, which is a potential generation site for semidiurnal internal tides; the beam closely followed the ray path of the semidiurnal internal tide; the averageε off the shelf break varied by a factor of 100 with a semidiurnal tidal periodicity; the isopycnal displacement confirmed the presence of semidiurnal internal tides. Processes associated with the breaking of internal tides are intermittent and sporadic. At the same location we also observed equally intense turbulence in a ~100-m-thick layer of stratified water across the ridge of a sea fan. This layer of strong turbulence was separated from the bottom and was clearly not generated by bottom friction. Although less well resolved in time, the strong turbulence above the bottom seemed to vary with the semidiurnal tide and existed at the lee of the ridge, where the isopycnal surface dipped and rebounded in a pattern resembling that of internal hydraulic jumps. On the basis of the behavior of the density field, we believe that the deep mixing was most likely produced by the across-ridge current of internal tides. The breaking of internal tides at middepth, where the Richardson number is close to the critical value, is likely due to shear instability. The presence of the coastal ridge provides an alternative pathway for converting energy from internal tides to turbulence via internal hydraulics. Multiplying the average ε in the midwater beam by the length of the global coastline gives 31 GW, only a small fraction of the estimated 360 GW dissipated globally by M2 internal tides. Our observations suggest that either most internal tides are generated away from shelf breaks or most internal tides generated at shelf breaks propagate away from their generation sites, rather than dissipate locally, and eventually contribute to pelagic mixing.

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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