13 research outputs found
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Tidally Forced Internal Waves and Overturns Observed on a Slope: Results from HOME
Tidal mixing over a slope was explored using moored time series observations on Kaena Ridge extending northwest from Oahu, Hawaii, during the Survey component of the Hawaii Ocean Mixing Experiment (HOME). A mooring was instrumented to sample the velocity and density field of the lower 500 m of the water column to look for indirect evidence of tidally induced mixing and was deployed on a slope in 1453-m water depth for 2 months beginning in November 2000. The semidiurnal barotropic tidal currents at this site have a significant cross-ridge component, favorable for exciting an internal tidal response. A large-amplitude response is expected, given that the slope of the topography (4.5°) is nearly the same as the slope of the internal wave group velocity at semidiurnal frequency. Density overturns were inferred from temperature profiles measured every 2 min. The number and strength of the overturns are greater in the 200 m nearest the bottom, with overturns exceeding 24 m present at any depth nearly 10% of the time. Estimates of turbulent dissipation rate Δ were made for each overturn by associating the measured Thorpe scale with the Ozmidov scale. The average Δ between 1300 and 1450 m for the entire experiment is about 10â»âž mÂČ sâ»Âł, corresponding to an average K[subscript]Ï of 10â»Âł mÂČ sâ»1. Both Δ and K[subscript]Ï decrease by about an order of magnitude by 1200 m. The occurrence of overturns and the magnitude of Δ are both highly correlated with the tide: both with the springâneap cycle as well as the phase of the semidiurnal tide itself. Dissipation rate varies by at least an order of magnitude over the springâneap cycle. It appears that tidal frequency vertical shear within 200 m of the boundary leads to significant strain (vertical divergence). Most of the overturns occur during the few hours when the vertical strain is greatest. The buoyancy frequency N calculated from reordering these overturns is a factor of 3 lower than the background N[with line above]. This is consistent with the following kinematic description: the internal tide first strains the mean density field, leading to regions of low N that subsequently overturn. Less regularly, overturns also occur when the internal tide strain has created relatively high stratification within 200 m of the bottom
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Submarine-based hydrographic observations of the Arctic Ocean, March-May 1995 : SCICEX-95
This report documents observations of temperature and salinity made in the Arctic Ocean during the 1995 cruise of the submarine USS Cavalla. This cruise was the second civilian scientific cruise to the Arctic Ocean aboard a U.S. Navy Sturgeon-class submarine, and the first of five annual SCICEX cruises.
The SCICEX-95 cruise began on March 8 with a transit from Pearl Harbor, Hawaii through Bering Strait to the Arctic Ocean data sampling area, defined to exclude non-U.S. EEZs. The Cavalla entered the sampling area on March 26, covered approximately 10,800 nautical miles within that area while collecting data over the next 44 days, and exited the sampling area on May 8 (Figure 1). Following a second passage through Bering Strait, the scientific party departed the submarine in Victoria, B.C., Canada on May 24
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Mooring observations from the Mid-Atlantic Bight, July-September 1996 : synthetic aperture sonar primer and coastal mixing & optics programs
This report documents the observations of velocity, temperature and conductivity made in the
Mid-Atlantic Bight region of the NW Atlantic Ocean during the Synthetic Aperture Sonar (SAS)
Primer experiment. The primary data were obtained from instruments moored in 70 in of water
near 40° 30' N, 70° 30' W, from July through September 1996 (Figure 1). Vertical profiles of
conductivity, temperature, light transmission and fluorescence were also made during
deployment and recovery cruises.
The overall goals of this PRIMER program are to assess the feasibility of the operation of an
SAS on the continental shelf. A study of acoustic propagation was conducted during the period
the mooring was deployed, from August 20 through August 27, by F. Henyey, T. Ewart, and K.
Williams (Applied Physics Laboratory / University of Washington). The evaluation of the
synthetic aperture sonar itself was performed by S. Stanic and R. Meredith (Naval Research
Laboratory). This project was carried out in close cooperation with the ONR-sponsored Coastal
Mixing & Optics ARE. In addition to shared logistical planning, we anticipate joint analysis and
sharing of data.
The specific goals of this project are to describe the internal wave field and associated sound
speed fluctuations on the shelf--both statistically and by events. The sampling scheme was
designed to resolve the many components of the wavefield including: near-inertial waves,
internal tide, background continuum and internal solitary waves.
This report is divided into two sections. The first section contains descriptions of the
instrumentation deployed on the PRIMER moorings including locations, sampling rates, and
calibrations. It also includes a brief description of the CTD sampling and sample profiles from
the CTD stations occupied. Copies of the deployment and recovery cruise logs are appended to
the first section. The second section contains plots from the mooring deployment. Several views
of the time series recorded by the moorings are presented. Time series of vertically separated
temperature, salinity, and velocity measurements are shown for the main, subsurface mooring.
Temperature observations from sensors at the same depth on horizontally separated mooring are
also shown. These data are presented as both low-pass filtered and unfiltered time series. Time
is given as day of year 1996 in all of the time series plots; conversion to calendar date is provided
in Table 8. The data displayed in these figures are a subset of the data collected during the
mooring deployment, all of which are available on CD-ROM from the authors
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Mooring observations from the Hawaiian ridge : November 2000 - January 2002 : a component of the Hawaii Ocean Mixing Experiment (HOME) Survey Program
This report documents the observations of velocity, temperature and conductivity made from two moorings deployed in the Kauai Channel on the ridge extending NW from Oahu, Hawaii. The moorings were deployed in November 2000 for two months as part of the Hawaii Ocean Mixing Experiment (HOME) Survey Program
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Mooring observations from the Oregon continental shelf : April-September 1999 : a component of the prediction of wind-driven coastal circulation project
This report documents oceanographic and meteorological measurements made from instruments deployed on four moorings over the continental shelf west of Oregon, from spring through
summer, 1999. These moorings were a component of an observational and numerical modeling program to study the response of the coastal ocean to wind forcing.
The Dynamics and Prediction of Wind-Driven Coastal Circulation was funded by the National Oceanographic Partnership Program (NOPP) with the principal goal to develop nowcast and
forecast systems for wind-driven coastal flow fields. The observational program was designed to provide measurements that would allow testing and improvement of the modeling capability.
See http://www.oce.orst.edu/po/research/nopp/, http://diana.OCE.ORST.EDU/cmoweb/nopp/,
and Austin et al. (2000) for description of the modeling program and a description of other
aspects of the observational program.
This report is divided into two sections. The first section contains descriptions of the
instrumentation deployed on the NOPP moorings including locations, sampling rates, and
calibrations. The second section contains plots of the observations. Several views of the time
series recorded by the moorings are presented. Time series of vertically separated velocity,
temperature, and salinity are shown for each mooring. Velocity and temperature observations from the same depth on horizontally separated moorings are also shown. These data are
presented as both 40-hour low-pass filtered and 1-hour low-pass filtered time series. Time is
given as day of year 1999 in all of the time series plots; conversion to calendar date is provided in Table 5
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Observations from moorings on the Oregon continental shelf, January - March 2003 : a component of the Coastal Ocean Advances in Shelf Transport (COAST) experiment
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Distant effect of assimilation of moored currents into a model of coastal wind-driven circulation off Oregon
An optimal interpolation (OI) sequential algorithm is implemented for a three-dimensional primitive equation model to assimilate current measurements from acoustic Doppler profilers moored on the Oregon shelf as a part of the Coastal Ocean Advances in Shelf Transport (COAST) upwelling experiment (MayâAugust 2001). A stationary estimate of the forecast error covariance required by the OI is computed based on the error covariance in the model solution not constrained by data assimilation. Lagged model error covariances are used to account for the effect of previously assimilated data. The forecast error covariance has a shorter alongshore spatial scale than the model error covariance unconstrained by the data, as an effect of propagating dynamical modes. Assimilation of currents from one or two of the moorings located on the path of the upwelling jet helps to improve the model data rms error and correlation at the mooring sites located at an alongshore distance of 90 km, south or north from the assimilation sites. The coastal jet is deflected offshore over Heceta Bank, and assimilation of data from an inner-shelf mooring in the jet separation zone does not help to improve prediction in the far field. Larger improvements are obtained for the first part of the study period (yeardays 146â190). In the second part (days 191â237) the geometry of our limited area model possibly limits
prediction accuracy. In numerical experiments involving assimilation of data from only one mooring the actual and expected rms error improvements are compared, providing a consistency test for the forecast error covariance.Keywords: upwelling, coastal ocean prediction, data assimilationKeywords: upwelling, coastal ocean prediction, data assimilatio
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Atmospheric forcing of the Oregon coastal ocean during the 2001 upwelling season
Meteorological conditions during an intensive oceanographic observational program
in May through August 2001 along the central Oregon coast are described and related to
larger-scale and longer-term conditions. Southward wind stresses of 0.05-0.1 N mâ»ÂČ
occurred roughly 75% of the time, with a sustained period of dominantly southward stress
from mid-June through July. Wind variations were correlated with variations in the large-scale
Aleutian Low and North Pacific High pressure centers; correlations with the
continental Thermal Low were small. Intraseasonal oscillations in alongshore wind stress
(periods near 20 days) correlated with the north-south position of the jet stream. These
stress oscillations drove 20 day oscillations in upper ocean temperature, with a lag of
roughly 5 days for maximum correlation and amplitudes near 4°C. The sum of sensible
and latent air-sea heat fluxes was generally into the atmosphere through June, then weakly
into the ocean thereafter, with fluctuations on synoptic timescales. Semidiurnal
fluctuations in surface air temperature were observed at two northern moorings, apparently
forced indirectly by nonlinear internal ocean tides. The diurnal cycle of wind stress
was similar for both southward and northward wind conditions, with the diurnal
alongshore fluctuation southward in the evening and northward in the morning. During
southward winds the marine atmospheric boundary layer (MABL) was typically defined
clearly by a strong temperature inversion, and a shallow stable internal boundary layer
often formed within the MABL over cool upwelled waters, with surface air temperature
roughly 1°C lower inshore than offshore. During northward winds, essentially no
low-level temperature stratification was observed.Keywords: coastal meteorology, airâsea interactions, coastal upwellin
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Energetics of Mâ Barotropic-to-Baroclinic Tidal Conversion at the Hawaiian Islands
A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (Mâ) 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°âhorizontal 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 modelâs sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The Mâ 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
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Introduction to special section on Recent Advances in the Study of Optical Variability in the Near-Surface and Upper Ocean
Optical variability occurs in the near-surface and upper ocean on very short time and space scales (e.g., milliseconds and millimeters and less) as well as greater scales. This variability is caused by solar, meteorological, and other physical forcing as well as biological and chemical processes that affect optical properties and their distributions, which in turn control the propagation of light across the air-sea interface and within the upper ocean. Recent developments in several technologies and modeling capabilities have enabled the investigation of a variety of fundamental and applied problems related to upper ocean physics, chemistry, and light propagation and utilization in the dynamic near-surface ocean. The purpose here is to provide background for and an introduction to a collection of papers devoted to new technologies and observational results as well as model simulations, which are facilitating new insights into optical variability and light propagation in the ocean as they are affected by changing atmospheric and oceanic conditions