Michael Gregg - Academia.edu (original) (raw)

Papers by Michael Gregg

Research paper thumbnail of Tidally Generated Turbulence over the Knight Inlet Sill

Journal of Physical Oceanography, 2004

Very high turbulent dissipation rates (above ϭ 10 Ϫ4 W kg Ϫ1 ) were observed in the nonlinear int... more Very high turbulent dissipation rates (above ϭ 10 Ϫ4 W kg Ϫ1 ) 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 ( ϭ 10 Ϫ7 to ϭ 10 Ϫ8 W kg Ϫ1 ). Energy removed from the barotropic tide was estimated using a pair of tide gauges; a peak of 20 MW occurred during spring tide. Approximately twothirds 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.

Research paper thumbnail of An Estimate of Tidal Energy Lost to Turbulence at the Hawaiian Ridge

Journal of Physical Oceanography, 2006

An integrated analysis of turbulence observations from four unique instrument platforms obtained ... more 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 ϳ E 1Ϯ0.5 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 Ϯ 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.

Research paper thumbnail of Detail and scaling of turbulent overturns in the Pacific Equatorial Undercurrent

Journal of Geophysical Research, 1995

We analyze turbulent overturns in the high-shear, low Richardson number flow of the upper 350 m a... more We analyze turbulent overturns in the high-shear, low Richardson number flow of the upper 350 m at 0ø/140øW. Profiles of shear and stratification combined from fine-and microscale sensors resolve the vertical wavenumber spectrum from large-scale to dissipation scales. We compute a turbulent length scale l from Thorpe-sorting potential temperature, while using potential density to avoid thermohaline intrusions. Pragmatically, we consider scales smaller than l turbulent and scales large than I nonturbulent. From local fine-scale velocity spectra, we extract the horizontal turbulent kinetic energy at scales smaller than I and thus estimate the turbulent velocity q, a parameter characteristic of the energetic eddies. The independently observed viscous dissipation rate ½, q, and l follow Taylor scaling, e -Cqq3/l, with Cq • 4. Similarly, the measured thermal dissipation rate X and the turbulent temperature fluctuation T •, also estimated by spectral extraction at scales smaller than l, follow similarity scaling, X = ct T•2q/l, with ct • 7. From q, l, buoyancy frequency N, and kinematic viscosity y, we estimate turbulent Reynolds numbers Ret -q l/•, and turbulent Froude numbers Frt -q/(Nl). The more energetic overturns of vertical thickness exceeding I m have 0.1 • Frt • 3 and 250 • Ret • 10 •. Ozmidov scales follow overturning scales as described by Dillon (1982) only on average, but not in individual overturns. Richardson numbers Ri of overturns show large scatter around a median of Ri -0.23 and virtually no correlation with e. The mixing efficiency shows a weak increase with increasing Ri and a weak decrease with increasing Frt. Turbulence parameters are briefly compared with formulations in turbulence closure models. A few individual mixing events are analyzed in detail, with focus on possible forcing mechanisms. In such identifiable events, enhanced turbulence is paralleled by enhanced fine-scale variance of velocity, shear, and temperature. One mixing event shows signatures of critical layer absorption. A very large overturn in the nighttime turbulent layer near the surface surprisingly shows dissipation rates indistinguishable from the surroundings. 18,349 18,350 PETERS ET AL.: EQUATORIAL TURBULENT OVERTURNS ters et al., this issue], we show that vertical overturning scales l provide a natural scale of separation; our findings are briefly summarized in section 2.3. On the basis of l, we are able to extract turbulent velocity and temperature fluctuations directly from high-resolution profiles of velqcity q and temperature T •, and we can show that these fluctuations follow similarity scaling. With q and l we estimate turbulent Reynolds numbers Ret and Froude numbers Frt, basic descriptors of the dynamics of the turbulent motions. Hence we introduce and analyze parameters characteristic of the energetic scale of the turbulence in addition to and independently from the dissipation-scale parameters usually employed in oceanic microstructure research. Because of the rather large number of variables and parameters used herein, we summarize their definitions and explanations in the notation section. Overbars indicate averages throughout. This paper treats only the upper 350 m; deeper layers will be addressed separately. We include the upper 50 m containing the diurnal cycle of mixing, which has been analyzed differently and in detail by Peters et al. [1994]. This paper complements analyses of the large-scale, fine-scale, and turbulent motions and their mutual relationship by Peters et al. [1991, this issue]. While the latter paper analyzes ensemble-averaged properties of shear and mixing, turbulent overturns are discussed here in examples of individual mixing events and in their ensemble scaling. Our 1987 Tropic Heat observations are henceforth referred to as "TH2." Their uncertainties and methods of their processing and analysis are discussed by Peters et al. [1991, 1994, this issue]. In order to make this study self-contained, we briefly introduce the observations in section 2 and outline some relevant limitations. Parallel to our own measurements, investigators from Oregon State University obtained extensive microstructure observations in the eastern tropical Pacific [H•bert at al., 1991a,b; Mourn et al., 1992a,b; H•bert et al., 1992]. In addition to outlining the observations, section 2 summarizes patterns of flow and mixing. Vertical turbulent length scales and overturns are introduced in section 3, and the average properties of the ensemble of overturns are analyzed. Section 4 contains detailed studies of a few individual mixing events. Summary and conclusions follow. 2. Observations, Flow, and Mixing Patterns 2.1. Summary of the Observations At 0ø/140øW, from April 4 to 15, we obtained 69 profiles of stratification and horizontal velocity to 1100 m, which were taken at 4-hour intervals as part of the TH2 experiment. These measurements with the multiscale profiler (MSP) range from vertical scales larger than , 300 m to below I cm from a combination of fine-and microscale sensors.

Research paper thumbnail of Persistent Near-Diurnal Internal Waves Observed above a Site of M 2 Barotropic-to-Baroclinic Conversion

Journal of Physical Oceanography, 2006

... Internal tides provide a narrow band of strong energy not described by the Garrett–Munk model... more ... Internal tides provide a narrow band of strong energy not described by the Garrett–Munk model and, therefore, could provide distinct triad ... The authors are grateful to Matthew Alford, Paul Aguilar, Steve Bayer, Earl Krause, John Mickett, Jack Miller, Avery Synder, Maya Whitmont ...

Research paper thumbnail of Intense, Variable Mixing near the Head of Monterey Submarine Canyon

Journal of Physical Oceanography, 2002

... mouth decaying nonmonotonically to 1 kW m −1 . Consistent with our observations, only some ..... more ... mouth decaying nonmonotonically to 1 kW m −1 . Consistent with our observations, only some ... flux, Δr is the upcanyon separation distance, Γ is the mixing efficiency (0.2 ... vertically integrated flux convergences and divergences with vertically integrated turbulence production rate. ...

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

Journal of Geophysical Research, 2001

We report direct, quantitative measurements of mixing associated with three cycles of a single, e... more 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 6.5øS, 128øE 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, log•0 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 too = 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 2950 true. Ratios of shear to strain and of parallel to transverse shear suggest that the wave's intrinsic frequency to• m 1.18f•ff. This indicates that background vorticity • has lowered the effective Coriolis frequency, f•ff = fo q-•/2, relative to its planetary value, fo . Ray tracing suggests that the wave was generated near 6.9øS, 130.6øE, •20 days prior to the cruise, coincident with the end of high winds associated with the SE monsoon. A slab mixed layer model , 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. time or space, an accurate atlas must encompass the resultant spatiotemporal patterns in mixing.

Research paper thumbnail of Internal Waves in Monterey Submarine Canyon

Journal of Physical Oceanography, 2002

... 1989, 1996; Allen 1996; Chen and Allen 1996) have shown that strong upwelling occurs on ... w... more ... 1989, 1996; Allen 1996; Chen and Allen 1996) have shown that strong upwelling occurs on ... winds (CG01) in August 1997, the internal wave and turbulence fields in Monterey Submarine Canyon ... roughly half the total variance, (iii) the presence of semidiurnal and diurnal (nM 2 ...

Research paper thumbnail of Internal waves, solitary-like waves, and mixing on the Monterey Bay shelf

Continental Shelf Research, 2005

Microstructure measurements taken on the Monterey Bay continental shelf, within 4 km of the shelf... more 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 ð ¼ 7:4255:8 Â 10 À9 W kg À1 Þ and diapycnal diffusivities ðK r ¼ 6:1237:8 Â 10 À5 m 2 s À1 Þ 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 solitarylike 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, 0278-4343/$ -see front matter r (R.-C. Lien). their average dissipation increased 10-fold to ¼ 2:6 Â 10 À6 W kg À1 , and accounted for nearly half the dissipation in the upper 150 m. r

Research paper thumbnail of Reduced mixing from the breaking of internal waves in equatorial waters

Nature, 2003

In the oceans, heat, salt and nutrients are redistributed much more easily within water masses of... more 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 degrees N and 2 degrees 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.

Research paper thumbnail of From Tides to Mixing Along the Hawaiian Ridge

Science, 2003

The following resources related to this article are available online at

Research paper thumbnail of Structure, Propagation, and Mixing of Energetic Baroclinic Tides in Mamala Bay, Oahu, Hawaii

Journal of Physical Oceanography, 2006

... Matthew H. Alford and Michael C. Gregg ... During September 2002, the authors sought to chara... more ... Matthew H. Alford and Michael C. Gregg ... 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. ...

Research paper thumbnail of Tidally Generated Turbulence over the Knight Inlet Sill

Journal of Physical Oceanography, 2004

Very high turbulent dissipation rates (above ϭ 10 Ϫ4 W kg Ϫ1 ) were observed in the nonlinear int... more Very high turbulent dissipation rates (above ϭ 10 Ϫ4 W kg Ϫ1 ) 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 ( ϭ 10 Ϫ7 to ϭ 10 Ϫ8 W kg Ϫ1 ). Energy removed from the barotropic tide was estimated using a pair of tide gauges; a peak of 20 MW occurred during spring tide. Approximately twothirds 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.

Research paper thumbnail of An Estimate of Tidal Energy Lost to Turbulence at the Hawaiian Ridge

Journal of Physical Oceanography, 2006

An integrated analysis of turbulence observations from four unique instrument platforms obtained ... more 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 ϳ E 1Ϯ0.5 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 Ϯ 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.

Research paper thumbnail of Detail and scaling of turbulent overturns in the Pacific Equatorial Undercurrent

Journal of Geophysical Research, 1995

We analyze turbulent overturns in the high-shear, low Richardson number flow of the upper 350 m a... more We analyze turbulent overturns in the high-shear, low Richardson number flow of the upper 350 m at 0ø/140øW. Profiles of shear and stratification combined from fine-and microscale sensors resolve the vertical wavenumber spectrum from large-scale to dissipation scales. We compute a turbulent length scale l from Thorpe-sorting potential temperature, while using potential density to avoid thermohaline intrusions. Pragmatically, we consider scales smaller than l turbulent and scales large than I nonturbulent. From local fine-scale velocity spectra, we extract the horizontal turbulent kinetic energy at scales smaller than I and thus estimate the turbulent velocity q, a parameter characteristic of the energetic eddies. The independently observed viscous dissipation rate ½, q, and l follow Taylor scaling, e -Cqq3/l, with Cq • 4. Similarly, the measured thermal dissipation rate X and the turbulent temperature fluctuation T •, also estimated by spectral extraction at scales smaller than l, follow similarity scaling, X = ct T•2q/l, with ct • 7. From q, l, buoyancy frequency N, and kinematic viscosity y, we estimate turbulent Reynolds numbers Ret -q l/•, and turbulent Froude numbers Frt -q/(Nl). The more energetic overturns of vertical thickness exceeding I m have 0.1 • Frt • 3 and 250 • Ret • 10 •. Ozmidov scales follow overturning scales as described by Dillon (1982) only on average, but not in individual overturns. Richardson numbers Ri of overturns show large scatter around a median of Ri -0.23 and virtually no correlation with e. The mixing efficiency shows a weak increase with increasing Ri and a weak decrease with increasing Frt. Turbulence parameters are briefly compared with formulations in turbulence closure models. A few individual mixing events are analyzed in detail, with focus on possible forcing mechanisms. In such identifiable events, enhanced turbulence is paralleled by enhanced fine-scale variance of velocity, shear, and temperature. One mixing event shows signatures of critical layer absorption. A very large overturn in the nighttime turbulent layer near the surface surprisingly shows dissipation rates indistinguishable from the surroundings. 18,349 18,350 PETERS ET AL.: EQUATORIAL TURBULENT OVERTURNS ters et al., this issue], we show that vertical overturning scales l provide a natural scale of separation; our findings are briefly summarized in section 2.3. On the basis of l, we are able to extract turbulent velocity and temperature fluctuations directly from high-resolution profiles of velqcity q and temperature T •, and we can show that these fluctuations follow similarity scaling. With q and l we estimate turbulent Reynolds numbers Ret and Froude numbers Frt, basic descriptors of the dynamics of the turbulent motions. Hence we introduce and analyze parameters characteristic of the energetic scale of the turbulence in addition to and independently from the dissipation-scale parameters usually employed in oceanic microstructure research. Because of the rather large number of variables and parameters used herein, we summarize their definitions and explanations in the notation section. Overbars indicate averages throughout. This paper treats only the upper 350 m; deeper layers will be addressed separately. We include the upper 50 m containing the diurnal cycle of mixing, which has been analyzed differently and in detail by Peters et al. [1994]. This paper complements analyses of the large-scale, fine-scale, and turbulent motions and their mutual relationship by Peters et al. [1991, this issue]. While the latter paper analyzes ensemble-averaged properties of shear and mixing, turbulent overturns are discussed here in examples of individual mixing events and in their ensemble scaling. Our 1987 Tropic Heat observations are henceforth referred to as "TH2." Their uncertainties and methods of their processing and analysis are discussed by Peters et al. [1991, 1994, this issue]. In order to make this study self-contained, we briefly introduce the observations in section 2 and outline some relevant limitations. Parallel to our own measurements, investigators from Oregon State University obtained extensive microstructure observations in the eastern tropical Pacific [H•bert at al., 1991a,b; Mourn et al., 1992a,b; H•bert et al., 1992]. In addition to outlining the observations, section 2 summarizes patterns of flow and mixing. Vertical turbulent length scales and overturns are introduced in section 3, and the average properties of the ensemble of overturns are analyzed. Section 4 contains detailed studies of a few individual mixing events. Summary and conclusions follow. 2. Observations, Flow, and Mixing Patterns 2.1. Summary of the Observations At 0ø/140øW, from April 4 to 15, we obtained 69 profiles of stratification and horizontal velocity to 1100 m, which were taken at 4-hour intervals as part of the TH2 experiment. These measurements with the multiscale profiler (MSP) range from vertical scales larger than , 300 m to below I cm from a combination of fine-and microscale sensors.

Research paper thumbnail of Persistent Near-Diurnal Internal Waves Observed above a Site of M 2 Barotropic-to-Baroclinic Conversion

Journal of Physical Oceanography, 2006

... Internal tides provide a narrow band of strong energy not described by the Garrett–Munk model... more ... Internal tides provide a narrow band of strong energy not described by the Garrett–Munk model and, therefore, could provide distinct triad ... The authors are grateful to Matthew Alford, Paul Aguilar, Steve Bayer, Earl Krause, John Mickett, Jack Miller, Avery Synder, Maya Whitmont ...

Research paper thumbnail of Intense, Variable Mixing near the Head of Monterey Submarine Canyon

Journal of Physical Oceanography, 2002

... mouth decaying nonmonotonically to 1 kW m −1 . Consistent with our observations, only some ..... more ... mouth decaying nonmonotonically to 1 kW m −1 . Consistent with our observations, only some ... flux, Δr is the upcanyon separation distance, Γ is the mixing efficiency (0.2 ... vertically integrated flux convergences and divergences with vertically integrated turbulence production rate. ...

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

Journal of Geophysical Research, 2001

We report direct, quantitative measurements of mixing associated with three cycles of a single, e... more 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 6.5øS, 128øE 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, log•0 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 too = 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 2950 true. Ratios of shear to strain and of parallel to transverse shear suggest that the wave's intrinsic frequency to• m 1.18f•ff. This indicates that background vorticity • has lowered the effective Coriolis frequency, f•ff = fo q-•/2, relative to its planetary value, fo . Ray tracing suggests that the wave was generated near 6.9øS, 130.6øE, •20 days prior to the cruise, coincident with the end of high winds associated with the SE monsoon. A slab mixed layer model , 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. time or space, an accurate atlas must encompass the resultant spatiotemporal patterns in mixing.

Research paper thumbnail of Internal Waves in Monterey Submarine Canyon

Journal of Physical Oceanography, 2002

... 1989, 1996; Allen 1996; Chen and Allen 1996) have shown that strong upwelling occurs on ... w... more ... 1989, 1996; Allen 1996; Chen and Allen 1996) have shown that strong upwelling occurs on ... winds (CG01) in August 1997, the internal wave and turbulence fields in Monterey Submarine Canyon ... roughly half the total variance, (iii) the presence of semidiurnal and diurnal (nM 2 ...

Research paper thumbnail of Internal waves, solitary-like waves, and mixing on the Monterey Bay shelf

Continental Shelf Research, 2005

Microstructure measurements taken on the Monterey Bay continental shelf, within 4 km of the shelf... more 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 ð ¼ 7:4255:8 Â 10 À9 W kg À1 Þ and diapycnal diffusivities ðK r ¼ 6:1237:8 Â 10 À5 m 2 s À1 Þ 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 solitarylike 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, 0278-4343/$ -see front matter r (R.-C. Lien). their average dissipation increased 10-fold to ¼ 2:6 Â 10 À6 W kg À1 , and accounted for nearly half the dissipation in the upper 150 m. r

Research paper thumbnail of Reduced mixing from the breaking of internal waves in equatorial waters

Nature, 2003

In the oceans, heat, salt and nutrients are redistributed much more easily within water masses of... more 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 degrees N and 2 degrees 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.

Research paper thumbnail of From Tides to Mixing Along the Hawaiian Ridge

Science, 2003

The following resources related to this article are available online at

Research paper thumbnail of Structure, Propagation, and Mixing of Energetic Baroclinic Tides in Mamala Bay, Oahu, Hawaii

Journal of Physical Oceanography, 2006

... Matthew H. Alford and Michael C. Gregg ... During September 2002, the authors sought to chara... more ... Matthew H. Alford and Michael C. Gregg ... 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. ...