3,313 research outputs found
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A Modification of the GarrettâMunk Internal Wave Spectrum
The GarrettâMunk (GM) spectrum continues to be a useful description of the oceanic internal wave field. However, there are several inconsistencies and ambiguities that make it difficult to use in comparing internal wave fields at different latitudes, stratifications, and water depths. A modified spectral formulation is presented that treats three problems with the GarrettâMunk formulation: the normalization of the energy spectrum as a function of frequency bandwidth, the energy distribution at frequencies below the semidiurnal tide, and the treatment of vertical boundaries and turning points. Addressing these problems leads to a substitution of the GM parameters E (nondimensional energy), b (vertical length scale), and Nâ (buoyancy frequency scale) with two new dimensional scales: E[subscript]ref, the energy per unit mass, and D(Ï), the WentzelâKramersâBrillouin (WKB)-scaled thickness of the vertical waveguide. The advantages of the modified spectrum are illustrated by comparing with observations from the equator and the continental shelf
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Internal waves under the Arctic pack ice during the Arctic Internal Wave Experiment: The coherence structure
The spectral composition of internal gravity waves under the Arctic pack ice during the Arctic Internal Wave Experiment (AIWEX) was found to be strikingly different from observations at lower latitudes. Time series of vertical displacement were inferred from horizontal and vertical arrays of temperature and conductivity sensors. Frequency spectra indicate a whiter spectrum (spectral slope near â1) and a less energetic wave field (by a factor of 0.02) than observations at lower latitude. The analysis of vertical and horizontal coherences revealed a horizontally isotropic wave field that is consistent with assumptions of a random field of linear internal waves. The wavenumber bandwidth of the wave field is about a factor of 10 wider than found at lower latitude
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The upper ocean internal wave field: Influence of the surface and mixed layer
A model is developed to calculate the upper ocean internal wave spectrum as modified by the surface boundary and mixed layer. The GarrettâMunk spectrum is assumed to describe the deep ocean wave field. The main effect of the surface and mixed layer is to align the vertical structure of the waves forming vertically standing waves locally; this contrasts with the assumption of random alignment in the GarrettâMunk model. Model spectra and coherences are calculated for idealized buoyancy frequency profiles and compared with the GarrettâMunk model. Measurements in the upper 200 m from the Mixed Layer Dynamics Experiment in the northeast Pacific in 1983 are also compared with the model results. The most striking success of the model is predicting the observed high coherence and 180° phase difference across the mixed layer of horizontal velocity
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Internal waves in the ocean: A review
This review documents the advances in our knowledge of the oceanic internal wave field during the past quadrennium. Emphasis is placed on studies that deal most directly with the measurement and modeling of internal waves as they exist in the ocean. Progress has come by realizing that specific physical processes might behave differently when embedded in the complex, omnipresent sea of internal waves. To understand fully the dynamics of the internal wave field requires knowledge of the simultaneous interactions of the internal waves with other oceanic phenomena as well as with themselves.Copyrighted by American Geophysical Union
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Extracting the internal tide from data: Methods and observations from the Mixed Layer Dynamics Experiment
Several methods are developed for analyzing data containing a highly variable internal tide. In particular, the methods are aimed at the analysis of moored observations with relatively few measurements in the vertical. The analysis depends upon an "elliptical decomposition" that is a generalization of the familiar "rotary decomposition." The technique is applied to velocity and temperature observations in the upper ocean made during the Mixed Layer Dynamics Experiment (MILDEX) in the northeast Pacific Ocean, about 700 km west of Santa Barbara, California, during October-November 1983. The observed propagation direction and amplitude of the internal tide was highly variable in time. It was anticipated that the wave could be propagating from the continental shelf where it is presumed to be generated. However, most of the time the internal tide appears to be propagating parallel to the coast. This result suggests the importance of density and velocity structure at mesoscale and frontal scale in affecting the propagation of the internal tide
Dispersion in the open ocean seasonal pycnocline at scales of 1-10 km and 1-6 days
Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(2), (2020): 415-437, doi:10.1175/JPO-D-19-0019.1.Results are presented from two dye release experiments conducted in the seasonal thermocline of the Sargasso Sea, one in a region of low horizontal strain rate (~10â6 sâ1), the second in a region of intermediate horizontal strain rate (~10â5 sâ1). Both experiments lasted ~6 days, covering spatial scales of 1â10 and 1â50 km for the low and intermediate strain rate regimes, respectively. Diapycnal diffusivities estimated from the two experiments were Îșz = (2â5) Ă 10â6 m2 sâ1, while isopycnal diffusivities were ÎșH = (0.2â3) m2 sâ1, with the range in ÎșH being less a reflection of site-to-site variability, and more due to uncertainties in the background strain rate acting on the patch combined with uncertain time dependence. The Site I (low strain) experiment exhibited minimal stretching, elongating to approximately 10 km over 6 days while maintaining a width of ~5 km, and with a notable vertical tilt in the meridional direction. By contrast, the Site II (intermediate strain) experiment exhibited significant stretching, elongating to more than 50 km in length and advecting more than 150 km while still maintaining a width of order 3â5 km. Early surveys from both experiments showed patchy distributions indicative of small-scale stirring at scales of order a few hundred meters. Later surveys show relatively smooth, coherent distributions with only occasional patchiness, suggestive of a diffusive rather than stirring process at the scales of the now larger patches. Together the two experiments provide important clues as to the rates and underlying processes driving diapycnal and isopycnal mixing at these scales.Results are presented from two dye release experiments conducted in the seasonal thermocline of the Sargasso Sea, one in a region of low horizontal strain rate (~10â6 sâ1), the second in a region of intermediate horizontal strain rate (~10â5 sâ1). Both experiments lasted ~6 days, covering spatial scales of 1â10 and 1â50 km for the low and intermediate strain rate regimes, respectively. Diapycnal diffusivities estimated from the two experiments were Îșz = (2â5) Ă 10â6 m2 sâ1, while isopycnal diffusivities were ÎșH = (0.2â3) m2 sâ1, with the range in ÎșH being less a reflection of site-to-site variability, and more due to uncertainties in the background strain rate acting on the patch combined with uncertain time dependence. The Site I (low strain) experiment exhibited minimal stretching, elongating to approximately 10 km over 6 days while maintaining a width of ~5 km, and with a notable vertical tilt in the meridional direction. By contrast, the Site II (intermediate strain) experiment exhibited significant stretching, elongating to more than 50 km in length and advecting more than 150 km while still maintaining a width of order 3â5 km. Early surveys from both experiments showed patchy distributions indicative of small-scale stirring at scales of order a few hundred meters. Later surveys show relatively smooth, coherent distributions with only occasional patchiness, suggestive of a diffusive rather than stirring process at the scales of the now larger patches. Together the two experiments provide important clues as to the rates and underlying processes driving diapycnal and isopycnal mixing at these scales.2020-08-0
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Fine structure, microstructure, and vertical mixing processes in the upper ocean in the western Weddell Sea
The upward flux of heat from the subsurface core of Warm Deep Water
(WDW) to the perennially ice-covered sea surface over the continental slope in the
western Weddell Sea is estimated using data obtained during February-June 1992
from a drifting ice station. Through the permanent pycnocline the diapycnal heat
flux is estimated to be about 3 W mâ»ÂČ, predominantly because of double-diffusive
convection. There is no evidence that shear-driven mixing is important in the
pycnocline. The estimated mean rate of heat transfer from the mixed layer to the
ice is 1.7 W mâ»ÂČ, although peak heat fluxes of up to 15 W mâ»ÂČ are found during
storms. It is hypothesized that isopycnal mixing along sloping intrusions also
contributes to the loss of heat from the WDW in this region; however, we are unable
to quantify the fluxes associated with this process. Intrusions occur intermittently
throughout this experiment but are most commonly found near the boundary of
the warm-core current and the shelf-modified water to the east. These heat fluxes
are significantly lower than the basin-averaged value of 19 W mâ»ÂČ (Fahrbach et
al., 1994) that is required to balance the heat budget of the Weddell Gyre. Other
studies suggest that shelf processes to the west of the ice station drift track and
more energetic double-diffusive convection in the midgyre to the east could account
for the difference between our flux estimates for this region and those based on the
basin-scale heat budget
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A Correction to the Baroclinic Pressure Gradient Term in the Princeton Ocean Model
An error in the calculation of the baroclinic pressure gradient term in the Princeton Ocean Model (POM) was identified while modeling the Mâ tidal current near its critical latitude in the southern Weddell Sea. The error arises from the present calculation of density, which involves the subtraction of a background density profile from the density field calculated at each internal time step. The small displacement of sigma surface depths relative to the surface, as surface elevation changes, causes a slight error in the calculation of the vertical and horizontal gradients of potential density. The error is largest at the seabed over rapidly changing bathymetry such as the continental slope. The baroclinic pressure gradient error is typically much smaller than the Coriolis term in the momentum equations and, therefore, usually unimportant. Close to the critical latitude, however, near-resonance between the error and Coriolis terms can cause an energetic and spatially complex spurious inertial mode to develop. The error is significant when modeling tides near their critical latitudes, and will contribute to the error in the baroclinic pressure gradient in other simulations. Two methods were suggested for fixing this problem. The preferred method was tested by applying the new form of POM to the southern Weddell Sea. The new results are consistent with both current meter data and predictions of linear internal wave theory
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Moored temperature and conductivity observations during AIWEX
This report presents observations from moorings of temperature, conductivity
and pressure, made during the Arctic Internal Wave Experiment (AIWEX)
in March-April 1985.
The purpose of the temperature and conductivity measurements was to
provide time series from which inferences could be made about the vertical
displacement of the internal waves
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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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