Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society, 2010
Turbulent diapycnal mixing in the ocean controls the transport of heat, freshwater, dissolved gas... more Turbulent diapycnal mixing in the ocean controls the transport of heat, freshwater, dissolved gases, nutrients, and pollutants. Though many present generation climate models represent turbulent mixing with a simplistic diffusivity below the surface mixed layer, the last two decades of ocean mixing research have instead revealed dramatic spatial and temporal heterogeneity in ocean mixing. Climate models that do not appropriately represent the turbulent fluxes of heat, momentum, and CO2 across critical interfaces will not accurately represent the ocean's role in present or future climate. An accurate picture of the worldwide geography of mixing requires a vastly increased database of observations. Unfortunately, traditional microstructure estimates of turbulent mixing are expensive, difficult, and rare. A key development of the last decade has been the development of tools to estimate the turbulent mixing rate from finescale (order 10-50 meter resolution) measurements of internal-wave shear and vertical strain. Global arrays such as the Argo program provide an unprecedented and as yet underdeveloped opportunity to define the global internal wave climate, and in turn identify mixing patterns and hotspots.
Global patterns of internal wave variability from observations of full-depth rotary shear spectra... more Global patterns of internal wave variability from observations of full-depth rotary shear spectra Amy F Waterhouse 1 , Eric Kunze 2 , Jennifer A Mackinnon 1 , Harper Simmons 3 , Rob Pinkel 1 , and Maxim Nikurashin 4 Scripps Institution of Oceanography, UCSD San Diego, CA [email protected] Northwest Research Associates, Redmond, WA University of Alaska Fairbanks, Fairbanks, AK IMAS, University of Tasmania, Hobart, Tasmania, Australia Abstract Internal waves are ubiquitous throughout the world ocean and are generated by a combina- tion of processes including the tides and winds. The vertical directionality of low-frequency internal waves can be diagnosed by separating shear components into their rotary-with- depth components. Partitioning of energy in upward and downward propagation is helpful for understanding how energy is distributed and potentially lost in the global ocean en- ergy budget. Using observations of horizontal velocity from the GO-SHIP surveys and in- terpreting Nor...
The northeastward progression of the semidiurnal internal tide from French Frigate Shoals (FFS), ... more The northeastward progression of the semidiurnal internal tide from French Frigate Shoals (FFS), Hawaii, is studied with an array of six simultaneous profiling moorings spanning 25.58–37.18N (’1400 km) and 13-yr-long Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter data processed by a new technique. The moorings have excellent temporal and vertical resolutions, while the altimeter offers broad spatial coverage of the surface manifestation of the internal tide’s coherent portion. Together these two approaches provide a unique view of the internal tide’s long-range propagation in a complex ocean environment. The moored observations reveal a rich, time-variable, and multimodal internal tide field, with higher-mode mo-tions contributing significantly to velocity, displacement, and energy. In spite of these contributions, the coherent mode-1 internal tide dominates the northeastward energy flux, and is detectable in both moored and altimetric data over the entire array. Phase...
ABSTRACT We study the interactions of internal waves in a realistic ocean environment using ray t... more ABSTRACT We study the interactions of internal waves in a realistic ocean environment using ray theory and numerical simulations by following an initial spectrum of short waves as they propagate through near inertial waves. We also analyze observational data taken on the stationary Floating Instrument Platform over Kaena Ridge, Hawaii as a part of the Hawaiian Ocean Mixing Experiment. We are looking for signs that an interaction is occurring between small-scale, high-frequency waves and the inertial shear. Then we relate the observational conclusions to the results of the ray theory and numerical simulations. A strong coherence between the inertial shear and the strain rate field is found in all three methods of analysis, showing the short waves are being affected by the inertial wave. An analysis of the triple product of the Reynold's stress and inertial shear shows the short waves tend to have a net transfer of energy to the inertial shear. Calculating short wave overturning shows that when the short waves strongly refract in the inertial wave they may take enough energy from the inertial wave to break.
AbstractLow-mode internal tides, a dominant part of the internal wave spectrum, carry energy over... more AbstractLow-mode internal tides, a dominant part of the internal wave spectrum, carry energy over large distances, yet the ultimate fate of this energy is unknown. Internal tides in the Tasman Sea ...
Mode-1 internal tides can propagate far away from their generation sites, but how and where their... more Mode-1 internal tides can propagate far away from their generation sites, but how and where their energy is dissipated is not well understood. One example is the semidiurnal internal tide generated south of New Zealand, which propagates over a thousand kilometers before impinging on the continental slope of Tasmania. In situ observations and model results from a recent process-study experiment are used to characterize the spatial and temporal variability of the internal tide on the southeastern Tasman slope, where previous studies have quantified large reflectivity. As expected, a standing wave pattern broadly explains the cross-slope and vertical structure of the observed internal tide. However, model and observations highlight several additional features of the internal tide on the continental slope. The standing wave pattern on the sloping bottom as well as small-scale bathymetric corrugations lead to bottom-enhanced tidal energy. Over the corrugations, larger tidal currents and ...
The M2 internal tide in the Tasman Sea is investigated using sea surface height measurements made... more The M2 internal tide in the Tasman Sea is investigated using sea surface height measurements made by multiple altimeter missions from 1992 to 2012. Internal tidal waves are extracted by two-dimensional plane wave fits in 180 km by 180 km windows. The results show that the Macquarie Ridge radiates three internal tidal beams into the Tasman Sea. The northern and southern beams propagate respectively into the East Australian Current and the Antarctic Circumpolar Current and become undetectable to satellite altimetry. The central beam propagates across the Tasman Sea, impinges on the Tasmanian continental slope, and partially reflects. The observed propagation speeds agree well with theoretical values determined from climatological ocean stratification. Both the northern and central beams refract about 15° toward the equator because of the beta effect. Following a concave submarine ridge in the source region, the central beam first converges around 45.5°S, 155.5°E and then diverges beyond the focal region. The satellite results reveal two reflected internal tidal beams off the Tasmanian slope, consistent with previous numerical simulations and glider measurements. The total energy flux from the Macquarie Ridge into the Tasman Sea is about 2.2 GW, of which about half is contributed by the central beam. The central beam loses little energy in its first 1000-km propagation, for which the likely reasons include flat bottom topography and weak mesoscale eddies.
Turbulent mixing rates are inferred from measurements spanning 25-37◦ N in the Pacific. The obser... more Turbulent mixing rates are inferred from measurements spanning 25-37◦ N in the Pacific. The obser-vations were made as part of the Internal Waves Across the Pacific experiment, designed to investigate the long-range fate of the low-mode internal tide propagating north from ...
In the early 1970s, the possibility of remotely measuring oceanic and atmospheric flows was becom... more In the early 1970s, the possibility of remotely measuring oceanic and atmospheric flows was becoming appreciated. By then, techniques for information extraction from sets of discrete targets were well advanced. Inferring fluid motion from the echo off a random cloud of independent scatterers was a different issue, as illustrated by the abject failure of contemporary matched-filter technology. Conceptual as well as hardware challenges were addressed over time, leading to the variety of Doppler sonars and sodars available commercially today. In this talk, I focus on systems developed at the Marine Physical Laboratory of Scripps over the past 42 years. Both pulse-to-pulse coherent and incoherent backscatter systems are reviewed with an emphasis on real-world problems encountered. Presently, under-exploited techniques based on sea-surface scattering or bi-static geometries are discussed. While many quality Doppler sonar manufacturers are now active, a large gap exists between what is technically possible and ...
ABSTRACT The purpose of this calibration was to examine the farfield beam pattern of an acoustic ... more ABSTRACT The purpose of this calibration was to examine the farfield beam pattern of an acoustic array which weighed 1,836 lbs in air. Calculations showed that a source to receiver distance of at least 77 m was required to provide the farfield output of the measured array. This range requirement and weight burden demanded new methods compared to the established calibration methods used by MPL to date.
Lateral submesoscale processes and their influence on vertical stratification at shallow salinity... more Lateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a salinity-controlled density front, embedded in a zone of moderate mesoscale strain (0.15 times the Coriolis parameter) and forced by winds with a downfront orientation. Below a thin mixed layer, often ≤10 m, the analysis shows several dynamical signatures indicative of submesoscale processes: (i) negative Ertel potential vorticity (PV); (ii) low-PV anomalies with O(1–10) km lateral extent, where the vorticity estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to yield low PV; (iii) flow conditions susceptible to forced symmetric instability (FSI) or bearing the imprint of earlier FSI events; (iv) negative lateral gradients in the absolute momentum field (in...
Mixed layers are defined to have homogeneous density, temperature, and salinity. However, bio-opt... more Mixed layers are defined to have homogeneous density, temperature, and salinity. However, bio-optical profiles may not always be fully homogenized within the mixed layer. The relative timescales of mixing and biological processes determine whether bio-optical gradients can form within a uniform density mixed layer. Vertical profiles of bio-optical measurements from biogeochemical Argo floats and elephant seal tags in the Southern Ocean are used to assess biological structure in the upper ocean. Within the hydrographically defined mixed layer, the profiles show significant vertical variance in chlorophyll-a (Chl-a) fluorescence and particle optical backscatter. Biological structure is assessed by fitting Chl-a fluorescence and particle backscatter profiles to functional forms (i.e., Gaussian, sigmoid, exponential, and their combinations). In the Southern Ocean, which characteristically has deep mixed layers, only 40% of nighttime bio-optical profiles were characterized by a sigmoid, indicating a well-mixed surface layer. Of the remaining 60% that showed structure, ∼40% had a deep fluorescence maximum below 20-m depth that correlated with particle backscatter. Furthermore, a significant fraction of these deep fluorescence maxima were found within the mixed layer (20-80%, depending on mixed-layer depth definition and season). Results suggest that the timescale between mixing events that homogenize the surface layer is often longer than biological timescales of restratification. We hypothesize that periods of quiescence between synoptic storms, which we estimate to be ∼3-5 days (depending on season), allow bio-optical gradients to develop within mixed layers that remain homogeneous in density. Plain Language Summary Storms influence high-latitude oceans by stirring the upper ocean nearly continuously. This wind mixing is usually expected to homogenize properties within the upper layer of the ocean, known as the mixed layer. New water column observations from floats and elephant seal tag confirm homogenization of hydrographic properties that determine density of seawater (e.g., temperature and salinity); however, biogeochemical properties are not necessarily homogenized. Most of the time optical measurements of biological properties within the mixed layer show vertical structure, which is indicative of phytoplankton biomass. These vertical inhomogenities are ubiquitous throughout the Southern Ocean and may occur in all seasons, often close to the base of the mixed layer. Within the mixed layer, observations suggest that biological processes create inhomogenities faster than mixing can homogenize. We hypothesize that 3-to 5-day periods of quiescence between storm events are long enough to allow bio-optical structure to develop without perturbing the mixed layers' uniform density. This may imply that phytoplankton in the Southern Ocean are better adapted to the harsh environmental conditions than commonly thought.
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188),
Public reporting burden for the collection of information is estimated to average 1 hour per resp... more Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society, 2010
Turbulent diapycnal mixing in the ocean controls the transport of heat, freshwater, dissolved gas... more Turbulent diapycnal mixing in the ocean controls the transport of heat, freshwater, dissolved gases, nutrients, and pollutants. Though many present generation climate models represent turbulent mixing with a simplistic diffusivity below the surface mixed layer, the last two decades of ocean mixing research have instead revealed dramatic spatial and temporal heterogeneity in ocean mixing. Climate models that do not appropriately represent the turbulent fluxes of heat, momentum, and CO2 across critical interfaces will not accurately represent the ocean's role in present or future climate. An accurate picture of the worldwide geography of mixing requires a vastly increased database of observations. Unfortunately, traditional microstructure estimates of turbulent mixing are expensive, difficult, and rare. A key development of the last decade has been the development of tools to estimate the turbulent mixing rate from finescale (order 10-50 meter resolution) measurements of internal-wave shear and vertical strain. Global arrays such as the Argo program provide an unprecedented and as yet underdeveloped opportunity to define the global internal wave climate, and in turn identify mixing patterns and hotspots.
Global patterns of internal wave variability from observations of full-depth rotary shear spectra... more Global patterns of internal wave variability from observations of full-depth rotary shear spectra Amy F Waterhouse 1 , Eric Kunze 2 , Jennifer A Mackinnon 1 , Harper Simmons 3 , Rob Pinkel 1 , and Maxim Nikurashin 4 Scripps Institution of Oceanography, UCSD San Diego, CA [email protected] Northwest Research Associates, Redmond, WA University of Alaska Fairbanks, Fairbanks, AK IMAS, University of Tasmania, Hobart, Tasmania, Australia Abstract Internal waves are ubiquitous throughout the world ocean and are generated by a combina- tion of processes including the tides and winds. The vertical directionality of low-frequency internal waves can be diagnosed by separating shear components into their rotary-with- depth components. Partitioning of energy in upward and downward propagation is helpful for understanding how energy is distributed and potentially lost in the global ocean en- ergy budget. Using observations of horizontal velocity from the GO-SHIP surveys and in- terpreting Nor...
The northeastward progression of the semidiurnal internal tide from French Frigate Shoals (FFS), ... more The northeastward progression of the semidiurnal internal tide from French Frigate Shoals (FFS), Hawaii, is studied with an array of six simultaneous profiling moorings spanning 25.58–37.18N (’1400 km) and 13-yr-long Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter data processed by a new technique. The moorings have excellent temporal and vertical resolutions, while the altimeter offers broad spatial coverage of the surface manifestation of the internal tide’s coherent portion. Together these two approaches provide a unique view of the internal tide’s long-range propagation in a complex ocean environment. The moored observations reveal a rich, time-variable, and multimodal internal tide field, with higher-mode mo-tions contributing significantly to velocity, displacement, and energy. In spite of these contributions, the coherent mode-1 internal tide dominates the northeastward energy flux, and is detectable in both moored and altimetric data over the entire array. Phase...
ABSTRACT We study the interactions of internal waves in a realistic ocean environment using ray t... more ABSTRACT We study the interactions of internal waves in a realistic ocean environment using ray theory and numerical simulations by following an initial spectrum of short waves as they propagate through near inertial waves. We also analyze observational data taken on the stationary Floating Instrument Platform over Kaena Ridge, Hawaii as a part of the Hawaiian Ocean Mixing Experiment. We are looking for signs that an interaction is occurring between small-scale, high-frequency waves and the inertial shear. Then we relate the observational conclusions to the results of the ray theory and numerical simulations. A strong coherence between the inertial shear and the strain rate field is found in all three methods of analysis, showing the short waves are being affected by the inertial wave. An analysis of the triple product of the Reynold's stress and inertial shear shows the short waves tend to have a net transfer of energy to the inertial shear. Calculating short wave overturning shows that when the short waves strongly refract in the inertial wave they may take enough energy from the inertial wave to break.
AbstractLow-mode internal tides, a dominant part of the internal wave spectrum, carry energy over... more AbstractLow-mode internal tides, a dominant part of the internal wave spectrum, carry energy over large distances, yet the ultimate fate of this energy is unknown. Internal tides in the Tasman Sea ...
Mode-1 internal tides can propagate far away from their generation sites, but how and where their... more Mode-1 internal tides can propagate far away from their generation sites, but how and where their energy is dissipated is not well understood. One example is the semidiurnal internal tide generated south of New Zealand, which propagates over a thousand kilometers before impinging on the continental slope of Tasmania. In situ observations and model results from a recent process-study experiment are used to characterize the spatial and temporal variability of the internal tide on the southeastern Tasman slope, where previous studies have quantified large reflectivity. As expected, a standing wave pattern broadly explains the cross-slope and vertical structure of the observed internal tide. However, model and observations highlight several additional features of the internal tide on the continental slope. The standing wave pattern on the sloping bottom as well as small-scale bathymetric corrugations lead to bottom-enhanced tidal energy. Over the corrugations, larger tidal currents and ...
The M2 internal tide in the Tasman Sea is investigated using sea surface height measurements made... more The M2 internal tide in the Tasman Sea is investigated using sea surface height measurements made by multiple altimeter missions from 1992 to 2012. Internal tidal waves are extracted by two-dimensional plane wave fits in 180 km by 180 km windows. The results show that the Macquarie Ridge radiates three internal tidal beams into the Tasman Sea. The northern and southern beams propagate respectively into the East Australian Current and the Antarctic Circumpolar Current and become undetectable to satellite altimetry. The central beam propagates across the Tasman Sea, impinges on the Tasmanian continental slope, and partially reflects. The observed propagation speeds agree well with theoretical values determined from climatological ocean stratification. Both the northern and central beams refract about 15° toward the equator because of the beta effect. Following a concave submarine ridge in the source region, the central beam first converges around 45.5°S, 155.5°E and then diverges beyond the focal region. The satellite results reveal two reflected internal tidal beams off the Tasmanian slope, consistent with previous numerical simulations and glider measurements. The total energy flux from the Macquarie Ridge into the Tasman Sea is about 2.2 GW, of which about half is contributed by the central beam. The central beam loses little energy in its first 1000-km propagation, for which the likely reasons include flat bottom topography and weak mesoscale eddies.
Turbulent mixing rates are inferred from measurements spanning 25-37◦ N in the Pacific. The obser... more Turbulent mixing rates are inferred from measurements spanning 25-37◦ N in the Pacific. The obser-vations were made as part of the Internal Waves Across the Pacific experiment, designed to investigate the long-range fate of the low-mode internal tide propagating north from ...
In the early 1970s, the possibility of remotely measuring oceanic and atmospheric flows was becom... more In the early 1970s, the possibility of remotely measuring oceanic and atmospheric flows was becoming appreciated. By then, techniques for information extraction from sets of discrete targets were well advanced. Inferring fluid motion from the echo off a random cloud of independent scatterers was a different issue, as illustrated by the abject failure of contemporary matched-filter technology. Conceptual as well as hardware challenges were addressed over time, leading to the variety of Doppler sonars and sodars available commercially today. In this talk, I focus on systems developed at the Marine Physical Laboratory of Scripps over the past 42 years. Both pulse-to-pulse coherent and incoherent backscatter systems are reviewed with an emphasis on real-world problems encountered. Presently, under-exploited techniques based on sea-surface scattering or bi-static geometries are discussed. While many quality Doppler sonar manufacturers are now active, a large gap exists between what is technically possible and ...
ABSTRACT The purpose of this calibration was to examine the farfield beam pattern of an acoustic ... more ABSTRACT The purpose of this calibration was to examine the farfield beam pattern of an acoustic array which weighed 1,836 lbs in air. Calculations showed that a source to receiver distance of at least 77 m was required to provide the farfield output of the measured array. This range requirement and weight burden demanded new methods compared to the established calibration methods used by MPL to date.
Lateral submesoscale processes and their influence on vertical stratification at shallow salinity... more Lateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a salinity-controlled density front, embedded in a zone of moderate mesoscale strain (0.15 times the Coriolis parameter) and forced by winds with a downfront orientation. Below a thin mixed layer, often ≤10 m, the analysis shows several dynamical signatures indicative of submesoscale processes: (i) negative Ertel potential vorticity (PV); (ii) low-PV anomalies with O(1–10) km lateral extent, where the vorticity estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to yield low PV; (iii) flow conditions susceptible to forced symmetric instability (FSI) or bearing the imprint of earlier FSI events; (iv) negative lateral gradients in the absolute momentum field (in...
Mixed layers are defined to have homogeneous density, temperature, and salinity. However, bio-opt... more Mixed layers are defined to have homogeneous density, temperature, and salinity. However, bio-optical profiles may not always be fully homogenized within the mixed layer. The relative timescales of mixing and biological processes determine whether bio-optical gradients can form within a uniform density mixed layer. Vertical profiles of bio-optical measurements from biogeochemical Argo floats and elephant seal tags in the Southern Ocean are used to assess biological structure in the upper ocean. Within the hydrographically defined mixed layer, the profiles show significant vertical variance in chlorophyll-a (Chl-a) fluorescence and particle optical backscatter. Biological structure is assessed by fitting Chl-a fluorescence and particle backscatter profiles to functional forms (i.e., Gaussian, sigmoid, exponential, and their combinations). In the Southern Ocean, which characteristically has deep mixed layers, only 40% of nighttime bio-optical profiles were characterized by a sigmoid, indicating a well-mixed surface layer. Of the remaining 60% that showed structure, ∼40% had a deep fluorescence maximum below 20-m depth that correlated with particle backscatter. Furthermore, a significant fraction of these deep fluorescence maxima were found within the mixed layer (20-80%, depending on mixed-layer depth definition and season). Results suggest that the timescale between mixing events that homogenize the surface layer is often longer than biological timescales of restratification. We hypothesize that periods of quiescence between synoptic storms, which we estimate to be ∼3-5 days (depending on season), allow bio-optical gradients to develop within mixed layers that remain homogeneous in density. Plain Language Summary Storms influence high-latitude oceans by stirring the upper ocean nearly continuously. This wind mixing is usually expected to homogenize properties within the upper layer of the ocean, known as the mixed layer. New water column observations from floats and elephant seal tag confirm homogenization of hydrographic properties that determine density of seawater (e.g., temperature and salinity); however, biogeochemical properties are not necessarily homogenized. Most of the time optical measurements of biological properties within the mixed layer show vertical structure, which is indicative of phytoplankton biomass. These vertical inhomogenities are ubiquitous throughout the Southern Ocean and may occur in all seasons, often close to the base of the mixed layer. Within the mixed layer, observations suggest that biological processes create inhomogenities faster than mixing can homogenize. We hypothesize that 3-to 5-day periods of quiescence between storm events are long enough to allow bio-optical structure to develop without perturbing the mixed layers' uniform density. This may imply that phytoplankton in the Southern Ocean are better adapted to the harsh environmental conditions than commonly thought.
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188),
Public reporting burden for the collection of information is estimated to average 1 hour per resp... more Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
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