Abstract
Measurements of ocean bottom pressure, particularly on the continental slope, make an efficient means of monitoring large-scale integrals of the ocean circulation. However, direct pressure ...measurements are limited to monitoring relatively short time scales (compared to the deployment period) because of problems with sensor drift. Measurements are used from the northwest Atlantic continental slope, as part of the Rapid Climate Change (RAPID)–West Atlantic Variability Experiment, to demonstrate that the drift problem can be overcome by using near-boundary measurements of density and velocity to reconstruct bottom pressure differences with accuracy better than 1 cm of water (100 Pa). This accuracy permits the measurement of changes in the zonally integrated flow, below and relative to 1100 m, to an accuracy of 1 Sv (1 Sv ≡ 106 m3 s−1) or better. The technique employs the “stepping method,” a generalization of hydrostatic balance for sloping paths that uses geostrophic current measurements to reconstruct the horizontal component of the pressure gradient.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
A dataset of sea surface temperature (SST) estimates is generated from the temperature observations of surface drifting buoys of NOAA's Global Drifter Program. Estimates of SST at regular hourly time ...steps along drifter trajectories are obtained by fitting to observations a mathematical model representing simultaneously SST diurnal variability with three harmonics of the daily frequency, and SST low-frequency variability with a first degree polynomial. Subsequent estimates of non-diurnal SST, diurnal SST anomalies, and total SST as their sum, are provided with their respective standard uncertainties. This Lagrangian SST dataset has been developed to match the existing hourly dataset of position and velocity from the Global Drifter Program.
Velocity time series from surface drifter data are exploited in a novel way to study the Southern Ocean surface circulation response to wind forcing. The ageostrophic component of the drifter ...velocities at 15 m is approximated by subtracting altimeter-derived geostrophic velocities from the drifter velocities. The resultant ageostrophic velocity time series are studied in the frequency domain jointly with contemporaneous time series of local wind stress from atmospheric reanalysis data. Rotary spectral analysis indicates that both wind stresses and ocean velocities are predominantly anticyclonic. Cross-spectral analysis shows that the upper ocean responds preferentially to anticyclonic winds not only at the inertial frequency but also at subinertial frequencies. The phase of the cross-spectra which is interpreted as a geometric angle indicates that the component of velocity that is coherent with the wind stress is to the left of the wind at subinertial frequencies and to the right at supra-inertial frequencies, and is seen as evidence of Ekman-type currents. A first order closure of the oceanic vertical turbulence, where the oceanic stress is equal to a viscosity coefficient K times the velocity vertical shear, is used to interpret the cross-spectrum. In this framework, the real part of the cross-spectrum of the wind stress and ocean surface ageostrophic velocity is shown to be a measure of the wind energy input rate to the Ekman layer. This energy input is therefore estimated across the Southern Ocean. The observed transfer function, which is the cross-spectrum divided by the auto-spectrum of the wind stress, is compared to the theoretical transfer functions arising from 10 different Ekman-type boundary layer models. These models differ in the dependence of K on the vertical coordinate and in the bottom boundary condition. The most dynamically consistent model has a vertical viscosity that is finite at the surface and increases linearly to the bottom of the boundary layer depth. Results of the comparison to models provide in situ seasonal estimates of zonally averaged near-surface viscosities and boundary layer depths across the Southern Ocean.
The geographical variability, frequency content, and vertical structure of near-surface oceanic kinetic energy (KE) are important for air-sea interaction, marine ecosystems, operational oceanography, ...pollutant tracking, and interpreting remotely sensed velocity measurements. Here, KE in high-resolution global simulations (HYbrid Coordinate Ocean Model; HYCOM, and Massachusetts Institute of Technology general circulation model; MITgcm), at the sea surface (0 m) and 15 m, are respectively compared with KE from undrogued and drogued surface drifters. Global maps and zonal averages are computed for low-frequency (\(<\) 0.5 cpd), near-inertial, diurnal, and semi-diurnal bands. Both models exhibit low-frequency equatorial KE that is low relative to drifter values. HYCOM near-inertial KE is higher than in MITgcm, and closer to drifter values, probably due to more frequently updated atmospheric forcing. HYCOM semi-diurnal KE is lower than in MITgcm, and closer to drifter values, likely due to inclusion of a parameterized topographic internal wave drag. A concurrent tidal harmonic analysis in the diurnal band demonstrates that much of the diurnal flow is non-tidal. We compute a simple proxy of near-surface vertical structure, the ratio of 0 m KE to 0 m KE plus 15 m KE in model outputs, and undrogued KE to undrogued KE plus drogued KE in drifter observations. Over most latitudes and frequency bands, model ratios track the drifter ratios to within error bars. Values of this ratio demonstrate significant vertical structure in all frequency bands except the semidiurnal band. Latitudinal dependence in the ratio is greatest in diurnal and low-frequency bands.
Thesis (Ph. D.)--University of California, San Diego, 2006.
Title from first page of PDF file (viewed December 7, 2006). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical ...references (p. 119-126).
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