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Abstract
During May of 1980 an internal-wave-measurement experiment was conducted from the Research Platform FLIP off the California coast. This paper discusses an 18-day sequence of velocity profiles obtained during the experiment using a pair of Doppler sonars. The sonars Profile to a depth of 700 m, with approximately 20 m depth resolution. Plots of the velocity and shear field indicate the dominance of near-inertial motions. Much of the near-inertial variance can be ascribed to a few identifiable wave groups. The progress of these groups can he tracked for many days. The shear at the base of the mixed layer is often dominated by near-inertial motions propagating vertically through the thermocline rather than wind-forced motions in the mixed layer itself. Power-spectral analysis suggests that the low-frequency component of the wave field is dominated by the near-inertial and tidal peaks and their harmonics. The wisdom in modeling the low-frequency wave field as an “equivalent continuum” is questioned.
Abstract
During May of 1980 an internal-wave-measurement experiment was conducted from the Research Platform FLIP off the California coast. This paper discusses an 18-day sequence of velocity profiles obtained during the experiment using a pair of Doppler sonars. The sonars Profile to a depth of 700 m, with approximately 20 m depth resolution. Plots of the velocity and shear field indicate the dominance of near-inertial motions. Much of the near-inertial variance can be ascribed to a few identifiable wave groups. The progress of these groups can he tracked for many days. The shear at the base of the mixed layer is often dominated by near-inertial motions propagating vertically through the thermocline rather than wind-forced motions in the mixed layer itself. Power-spectral analysis suggests that the low-frequency component of the wave field is dominated by the near-inertial and tidal peaks and their harmonics. The wisdom in modeling the low-frequency wave field as an “equivalent continuum” is questioned.
Abstract
In May 1980 an 18-day sequence of velocity profiles of the top 600 m of the sea was collected off the coast of Southern California. The measurements were obtained using a pair of Doppler sonars mounted on the Research Platform FLIP. From these data, estimates of the wavenumber-frequency spectrum of the oceanic internal wavefield are obtained. The spectra are characterized by a series of ridges, which occur at near-internal and tidal frequencies as well as higher harmonics and sums of these fundamentals. The ridges run parallel to the wavenumber axis. There is a pronounced near-inertial spectral peak. The near-inertial motions are dominated by a few identifiable wave groups. There is a net downward energy propagation in the near-inertial frequency band. The vertical-wavenumber dependence of a the spectrum is decidedly asymmetric in this region. The asymmetry extends to five times the inertial frequency, making much of the so-called continuum asymmetric. A high-wavenumber cutoff at approximately 60 m vertical wavelength extends from the inertial frequency to approximately 5 cycles per day (cpd). The changing form of the wavenumber dependence of the spectrum. The total variance of the downward propagating motions exceeds that of the upward, primarily because of an excess of downward near-inertial energy. Surprisingly, the net energy transport of the wavefield is upward, of the order 0.003 W m−2. The upward flux results from an excess of high-frequency (5–60 cpd) upward propagating waves. Although these have much less variance than the downward propagating near-inertial waves, they have a far greater vertical group velocity.
Abstract
In May 1980 an 18-day sequence of velocity profiles of the top 600 m of the sea was collected off the coast of Southern California. The measurements were obtained using a pair of Doppler sonars mounted on the Research Platform FLIP. From these data, estimates of the wavenumber-frequency spectrum of the oceanic internal wavefield are obtained. The spectra are characterized by a series of ridges, which occur at near-internal and tidal frequencies as well as higher harmonics and sums of these fundamentals. The ridges run parallel to the wavenumber axis. There is a pronounced near-inertial spectral peak. The near-inertial motions are dominated by a few identifiable wave groups. There is a net downward energy propagation in the near-inertial frequency band. The vertical-wavenumber dependence of a the spectrum is decidedly asymmetric in this region. The asymmetry extends to five times the inertial frequency, making much of the so-called continuum asymmetric. A high-wavenumber cutoff at approximately 60 m vertical wavelength extends from the inertial frequency to approximately 5 cycles per day (cpd). The changing form of the wavenumber dependence of the spectrum. The total variance of the downward propagating motions exceeds that of the upward, primarily because of an excess of downward near-inertial energy. Surprisingly, the net energy transport of the wavefield is upward, of the order 0.003 W m−2. The upward flux results from an excess of high-frequency (5–60 cpd) upward propagating waves. Although these have much less variance than the downward propagating near-inertial waves, they have a far greater vertical group velocity.
Abstract
Repeat-sequence coding is a robust method for improving the precision of velocity estimates from incoherent Doppler sounders. The method involves transmitting a number of repeats of a broadband “subcode.” The Doppler shift is estimated from the complex autocovariance of the return, evaluated at a time lag, equal to the subcode duration. The repeat-sequence code is an extension of the simple pulse-train concept developed in the early days of radar. By transmitting codes, rather than discrete pulses, the average transmitted power is increased. A model is developed here to predict performance enhancement for specified codes. The model is based on the sample error of the covariance estimates. It explicitly accounts for the lag used. Root-mean-square precision is enhanced roughly in proportion to the square root of the time-bandwidth product of the subcode. Coded pulse technology has been implemented on a variety of Doppler sonar systems at Scripps Institution of Oceanography and used in both open-ocean (volume-scattering) and shallow-water (surface-scattering) applications. Field measurements of sonar precision roughly agree with predictions of the model, although with some increase in error.
Abstract
Repeat-sequence coding is a robust method for improving the precision of velocity estimates from incoherent Doppler sounders. The method involves transmitting a number of repeats of a broadband “subcode.” The Doppler shift is estimated from the complex autocovariance of the return, evaluated at a time lag, equal to the subcode duration. The repeat-sequence code is an extension of the simple pulse-train concept developed in the early days of radar. By transmitting codes, rather than discrete pulses, the average transmitted power is increased. A model is developed here to predict performance enhancement for specified codes. The model is based on the sample error of the covariance estimates. It explicitly accounts for the lag used. Root-mean-square precision is enhanced roughly in proportion to the square root of the time-bandwidth product of the subcode. Coded pulse technology has been implemented on a variety of Doppler sonar systems at Scripps Institution of Oceanography and used in both open-ocean (volume-scattering) and shallow-water (surface-scattering) applications. Field measurements of sonar precision roughly agree with predictions of the model, although with some increase in error.
Abstract
In situ observations of tidally driven turbulence were obtained in a small channel that transects the crest of the Mendocino Ridge, a site of mixed (diurnal and semidiurnal) tides. Diurnal tides are subinertial at this latitude, and once per day a trapped tide leads to large flows through the channel giving rise to tidal excursion lengths comparable to the width of the ridge crest. During these times, energetic turbulence is observed in the channel, with overturns spanning almost half of the full water depth. A high-resolution, nonhydrostatic, 2.5-dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave that extends from the ridge crest to the surface and the subsequent development of a hydraulic jump on the flanks of the ridge. Modeled dissipation rates show that turbulence is strongest on the flanks of the ridge and that local dissipation accounts for 28% of the energy converted from the barotropic tide into baroclinic motion.
Abstract
In situ observations of tidally driven turbulence were obtained in a small channel that transects the crest of the Mendocino Ridge, a site of mixed (diurnal and semidiurnal) tides. Diurnal tides are subinertial at this latitude, and once per day a trapped tide leads to large flows through the channel giving rise to tidal excursion lengths comparable to the width of the ridge crest. During these times, energetic turbulence is observed in the channel, with overturns spanning almost half of the full water depth. A high-resolution, nonhydrostatic, 2.5-dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave that extends from the ridge crest to the surface and the subsequent development of a hydraulic jump on the flanks of the ridge. Modeled dissipation rates show that turbulence is strongest on the flanks of the ridge and that local dissipation accounts for 28% of the energy converted from the barotropic tide into baroclinic motion.
Abstract
Shipboard measurements of velocity and density were obtained in the vicinity of a small channel in the Mendocino Ridge, where flows were predominantly tidal. Measured daily inequalities in transport are much greater than those predicted by a barotropic tide model, with the strongest transport associated with full depth flows and the weakest with shallow, surface-confined flows. A regional numerical model of the area finds that the subinertial K1 (diurnal) tidal constituent generates topographically trapped waves that propagate anticyclonically around the ridge and are associated with enhanced near-topographic K1 transports. The interaction of the baroclinic trapped waves with the surface tide produces a tidal flow whose northward transports alternate between being surface confined and full depth. Full depth flows are associated with the generation of a large-amplitude tidal lee wave on the northward face of the ridge, while surface-confined flows are largely nonturbulent. The regional model demonstrates that, consistent with field observations, near-topographic dissipation over the entire ridge is diurnally modulated, despite the semidiurnal tidal constituent having larger barotropic velocities. It is concluded that at this location it is the bottom-trapped subinertial internal tide that governs near-topographic dissipation and mixing. The effect of the trapped wave on regional energetics is to increase the fraction of converted barotropic–baroclinic tidal energy that dissipates locally.
Abstract
Shipboard measurements of velocity and density were obtained in the vicinity of a small channel in the Mendocino Ridge, where flows were predominantly tidal. Measured daily inequalities in transport are much greater than those predicted by a barotropic tide model, with the strongest transport associated with full depth flows and the weakest with shallow, surface-confined flows. A regional numerical model of the area finds that the subinertial K1 (diurnal) tidal constituent generates topographically trapped waves that propagate anticyclonically around the ridge and are associated with enhanced near-topographic K1 transports. The interaction of the baroclinic trapped waves with the surface tide produces a tidal flow whose northward transports alternate between being surface confined and full depth. Full depth flows are associated with the generation of a large-amplitude tidal lee wave on the northward face of the ridge, while surface-confined flows are largely nonturbulent. The regional model demonstrates that, consistent with field observations, near-topographic dissipation over the entire ridge is diurnally modulated, despite the semidiurnal tidal constituent having larger barotropic velocities. It is concluded that at this location it is the bottom-trapped subinertial internal tide that governs near-topographic dissipation and mixing. The effect of the trapped wave on regional energetics is to increase the fraction of converted barotropic–baroclinic tidal energy that dissipates locally.
Abstract
Ocean wave energy is used to drive a buoyant instrument platform down a wire suspended from a surface float. At the lower terminus of the profiling range, the cam that rectifies wave vertical motion is released and the package, termed the Wirewalker, free ascends. No electronic components are used in the profiler, and only a few moving parts are involved. The Wirewalker is tolerant of a broad range of payloads: the ballast is adjusted by adding discrete foam blocks. The Wirewalker profiles 1000–3000 km month−1, vertically, with typical missions lasting from days to months. A description of the profiler is presented along with a discussion of basic profiling dynamics.
Abstract
Ocean wave energy is used to drive a buoyant instrument platform down a wire suspended from a surface float. At the lower terminus of the profiling range, the cam that rectifies wave vertical motion is released and the package, termed the Wirewalker, free ascends. No electronic components are used in the profiler, and only a few moving parts are involved. The Wirewalker is tolerant of a broad range of payloads: the ballast is adjusted by adding discrete foam blocks. The Wirewalker profiles 1000–3000 km month−1, vertically, with typical missions lasting from days to months. A description of the profiler is presented along with a discussion of basic profiling dynamics.
Abstract
Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatiotemporal patterns of mixing are largely driven by the geography of generation, propagation, and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last 5 years and under the auspices of U.S. Climate Variability and Predictability Program (CLIVAR), a National Science Foundation (NSF)- and National Oceanic and Atmospheric Administration (NOAA)-supported Climate Process Team has been engaged in developing, implementing, and testing dynamics-based parameterizations for internal wave–driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here, we review recent progress, describe the tools developed, and discuss future directions.
Abstract
Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatiotemporal patterns of mixing are largely driven by the geography of generation, propagation, and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last 5 years and under the auspices of U.S. Climate Variability and Predictability Program (CLIVAR), a National Science Foundation (NSF)- and National Oceanic and Atmospheric Administration (NOAA)-supported Climate Process Team has been engaged in developing, implementing, and testing dynamics-based parameterizations for internal wave–driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here, we review recent progress, describe the tools developed, and discuss future directions.
