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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
251

The effects of double-diffusion on a baroclinic vortex / Baroclinic vortex, The effects of double-diffusion on a

Smith, Wendy Marie January 1987 (has links)
Thesis (M.S.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987. / Includes bibliographical references (leaves 57-59). / Laboratory experiments were performed to study the combined effects of double-diffusion and rotation on an oceanic intrusion. Intrusions are driven across density-compensated fronts by the divergence of the double-diffusive buoyancy flux. The increased momentum transport across a double-diffusive interface, however, acts to oppose the action of the buoyancy flux. Turbulent double-diffusive Ekman layers could be a means of redistributing momentum. A model of an intrusion was made by injecting salt or sugar solution at the surface of a denser layer of sugar or salt solution in a rotating tank to form a baroclinic vortex. The size and shape of the vortex and the velocity structure of the intrusion were measured as functions of time. The double-diffusive vortex spread more quickly and had slower azimuthal velocities than a non-double-diffusive one. This effect increased as the density ratio approached unity. These results indicate that momentum transport across a double-diffusive interface is larger than that across a non-double-diffusive one; thus, the parameterization of friction in an intrusion model should be considered carefully. / by Wendy Marie Smith. / M.S.
252

Constraining North Atlantic circulation with transient tracer observations

Li, Xingwen, 1968- January 2003 (has links)
Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, and the Woods Hole Oceanographic Institution), February 2003. / Includes bibliographical references (p. 187-197). / The capability of transient tracers to constrain the ocean circulation in the North Atlantic is explored. Study of an idealized tracer shows that inferences of circulation properties from transient state distributions are impacted by uncertainties in the time-varying boundary conditions and sparse data coverage. Comparison of CFC, tritium, temperature and salinity (T-S) observations with model results in the North Atlantic shows that regions of important model-data disagreements in the transient tracer fields can also be readily identified in the T-S distributions. In the model, excessive vertical penetration of convective adjustment, leads to problematic production and outflow of the NADW, again appearing in both transient tracer and T-S fields. Sensitivities of the model fields are determined using the adjoint model. In the dual solutions, CFC-11, CFC-11/CFC-12 ratio age, and T - (β/α)S (β and α are thermal and haline expansion coefficients, respectively) exhibit the major ventilation pathways and the associated timescales, in the model. High sensitivity fields are candidates for providing the most powerful constraints in the corresponding inverse problems. Assimilation of both CFC and tritium data, with different input histories, sampling distributions, and radioactive decay constants, shows that by adjusting only initial-boundary conditions of CFCs and tritium, a 1⁰ x 1⁰ offline model and the transient tracer data can be brought into near-consistency, in the domain between 4.5⁰S and 39.5⁰N of the North Atlantic. Constraining a GCM with transient tracers is thus fully practical. / (cont.) However, the large uncertainties in the time-varying boundary conditions of transient tracer concentrations, and in their interior distributions, renders the transient tracers less-effective in determining the circulation than are more conventional steady tracers, and known oceanic dynamics. / by Xingwen Li. / Ph.D.
253

Near-inertial and thermal to atmospheric forcing in the North Atlantic Ocean

Silverthorne, Katherine E January 2010 (has links)
Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 127-133). / Observational and modeling techniques are employed to investigate the thermal and inertial upper ocean response to wind and buoyancy forcing in the North Atlantic Ocean. First, the seasonal kinetic energy variability of near-inertial motions observed with a moored profiler is described. Observed wintertime enhancement and surface intensification of near-inertial kinetic energy support previous work suggesting that near-inertial motions are predominantly driven by surface forcing. The wind energy input into surface ocean near-inertial motions is estimated using the Price-Weller- Pinkel (PWP) one-dimensional mixed layer model. A localized depth-integrated model consisting of a wind forcing term and a dissipation parameterization is developed and shown to have skill capturing the seasonal cycle and order of magnitude of the near-inertial kinetic energy. Focusing in on wintertime storm passage, velocity and density records from drifting profiling floats (EM-APEX) and a meteorological spar buoy/tethered profiler system (ASIS/FILIS) deployed in the Gulf Stream in February 2007 as part of the CLIvar MOde water Dynamics Experiment (CLIMODE) were analyzed. Despite large surface heat loss during cold air outbreaks and the drifting nature of the instruments, changes in the upper ocean heat content were found in a mixed layer heat balance to be controlled primarily by the relative advection of temperature associated with the strong vertical shear of the Gulf Stream. Velocity records from the Gulf Stream exhibited energetic near-inertial oscillations with frequency that was shifted below the local resting inertial frequency. This depression of frequency was linked to the presence of the negative vorticity of the background horizontal current shear, implying the potential for near-inertial wave trapping in the Gulf Stream region through the mechanism described by Kunze and Sanford (1984). Three-dimensional PWP model simulations show evidence of near-inertial wave trapping in the Gulf Stream jet, and are used to quantify the resulting mixing and the effect on the stratification in the Eighteen Degree Water formation region. / by Katherine E. Silverthorne / Ph.D.
254

Energy pathways and structures of oceanic eddies from the ECCO2 State Estimate and Simplified Models

Chen, Ru, Ph. D. Massachusetts Institute of Technology January 2013 (has links)
Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2013. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (p. 193-206). / Studying oceanic eddies is important for understanding and predicting ocean circulation and climate variability. The central focus of this dissertation is the energy exchange between eddies and mean ow and banded structures in the low-frequency component of the eddy eld. A combination of a realistic eddy-permitting ocean state estimate and simplied theoretical models is used to address the following speci c questions. (1) What are the major spatial characteristics of eddy-mean ow interaction from an energy perspective? Is eddy-mean ow interaction a local process in most ocean regions? (2) The banded structures in the low-frequency eddy eld are termed striations. How much oceanic variability is associated with striations? How does the time-mean circulation, for example a subtropical gyre or constant mean ow, inuence the origin and characteristics of striations? How much do striations contribute to the energy budget and tracer mixing? / by Ru Chen. / Ph.D.
255

Moored observations of shelfbreak processes at the inflow to and outflow from the Arctic Ocean

Von Appen, Wilken-Jon January 2013 (has links)
Thesis (Ph. D. in Physical Oceanography)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 259-275). / Two high-resolution mooring arrays extending from the outer shelf to the mid continental slope are used to elucidate shelf-basin exchange at the inflow to and the outflow from the Arctic Ocean. Pacific Water entering the Arctic Ocean forms the Western Arctic shelfbreak current along the Beaufort Sea slope. Data from the mooring array at 152°W-approximately 150 km east of Pt. Barrow, AK-reveals that this current has two distinct states in summer depending on the water mass it transports. When advecting Alaskan Coastal Water it is surface-intensified and both baroclinically and barotropically unstable. This configuration lasts about a month with an average transport of 0.7 Sv. When advecting Chukchi Summer Water the current is bottom-intensified and is only baroclinically unstable. This state also exists for approximately a month with an average transport of 0.6 Sv. The strong mean-to-eddy energy conversion causes both configurations of the current to spin down over a distance of a few hundred kilometers, suggesting that warm Pacific Water does not enter the Canadian Arctic Archipelago via this route. Dense water formed in the Nordic Seas overflows the Denmark Strait and undergoes vortex stretching, forming intense cyclones that propagate along the East Greenland slope. Data from the mooring array at 65°N-roughly 300 km downstream of Denmark Strait-was used to determine the full water column structure of the cyclones. On average a cyclone passes the array every other day in the vicinity of the 900 m isobath, although the depth range of individual cyclones ranges between the 500 m and 1600 m isobaths. The cyclones self-propagate at 0.45 m/s and are also advected by the mean flow of 0.27 m/s, resulting in a total propagation speed of 0.72 m/s. They have a peak azimuthal speed of 0.22 m/s at a radius of 7.8 km and contain overflow water in their core. In the absence of the cyclones, the background flow is dominated by the East Greenland Spill Jet. This is shown to be a year-round feature transporting 2-4 Sv of dense water equatorward along the upper continental slope. / by Wilken-Jon von Appen. / Ph.D.in Physical Oceanography
256

Strong wind events across Greenland's coast and their influence on the ice sheet, sea ice and ocean

Oltmanns, Marilena January 2012 (has links)
Thesis: Ph. D., Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 129-143). / In winter, Greenland's coastline adjacent to the subpolar North Atlantic and Nordic Seas is characterized by a large land-sea temperature contrast. Therefore, winds across the coast advect air across a horizontal temperature gradient and can result in significant surface heat fluxes both over the ice sheet (during onshore winds) and over the ocean (during offshore winds). Despite their importance, these winds have not been investigated in detail, and this thesis includes the first comprehensive study of their characteristics, dynamics and impacts. Using an atmospheric reanalysis, observations from local weather stations, and remote sensing data, it is suggested that high-speed wind events across the coast are triggered by the superposition of an upper level potential vorticity anomaly on a stationary topographic Rossby wave over Greenland, and that they intensify through baroclinic instability. Onshore winds across Greenland's coast can result in increased melting, and offshore winds drive large heat losses over major ocean convection sites. Strong offshore winds across the southeast coast are unique over Greenland, because the flow is funneled from the vast ice sheet inland into the narrow valley of Ammassalik at the coast, where it can reach hurricane intensity. In this region, the cold air, which formed over the northern ice sheet, is suddenly released during intense downslope wind events and spills over the Irminger Sea where the cold and strong winds can drive heat fluxes of up to 1000 W m-2, with potential implications for deep water formation. Moreover, the winds advect sea ice away from the coast and out of a major glacial fjord. Simulations of these wind events in Ammassalik with the atmospheric Weather Research and Forecast Model show that mountain wave dynamics contribute to the acceleration of the downslope flow. In order to capture these dynamics, a high model resolution with a detailed topography is needed. The effects of using a different resolution locally in the valley extend far downstream over the Irminger Sea, which has implications for the evolution and distribution of the heat fluxes. / by Marilena Oltmanns. / Ph. D.
257

When an eddy encounters shelf-slope topography

Cherian, Deepak Abraham January 2016 (has links)
Thesis: Ph. D., Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 129-135). / Eddies in the ocean move westwards. Those shed by western boundary currents must then interact with continental shelf-slope topography at the western boundary. The presence of other eddies and mean flows complicates this simple picture, yet satellite images show that mesoscale eddies translating near the shelfbreak routinely affect the continental shelves of the Mid-Atlantic Bight, the Gulf of Mexico etc. The consequent cross-shelfbreak transports are currently of unknown importance to shelf budgets of heat, salt and volume. Thus motivated, this thesis uses idealized continuously stratified numerical experiments to explore eddy-slope interactions under four questions: 1. Can the continental slope prevent an eddy from reaching the shelfbreak? 2. What is the structure of the eddy-driven offshore flow? 3. How is the continental shelf affected by an eddy at the shelfbreak? 4. Given surface observations, can one estimate the volume of water transported across the shelfbreak? The experiments show that the efficiency of Rossby wave radiation from the eddy controls whether it can cross isobaths: by radiating energy the eddy becomes shallow enough to move into shallower water. For wide continental slopes, relative to an eddy diameter, a slope can prevent an anticyclone from reaching the shelfbreak by shutting down such radiation. For narrow continental slopes, the interaction repeatedly produces dipoles, whose cyclonic halves contain shelf-slope water stacked over eddy water. The formation of such cyclones is explained. Then, the structure of shelf flows forced by the eddy are studied: their vertical structures are rationalized and scalings derived for their cross-isobath scales; for example, the extent to which the eddy influences the shelf. A recipe for estimating cross-isobath transports based on eddy surface properties is put forward. Finally, the findings are tested against observations in the Middle Atlantic Bight off the northeastern United States. / by Deepak Abraham Cherian. / Ph. D.
258

Temperature and salinity variability in thermohaline staircase layers

Stuebe, David Allen January 2005 (has links)
Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2005. / Includes bibliographical references (p. 65-67). / A moored profiler record from the western tropical North Atlantic provides the first continuous time series of temperature, salinity and velocity profiles in a thermohaline staircase. Variations in the intensity of layering and the evolution of layer properties are well documented during the 4.3 month record. Such staircases are the result of strong salt fingering at the interfaces between the mixed layers, and these data provide unique insights into the dynamics of salt fingers. In particular, a striking linear correlation between the temperature and salinity of the layers may be interpreted as resulting from vertical salt finger flux divergences. Data from this record allow new interpretations of previous work on this topic by McDougall (1991). / by David Allen Stuebe. / S.M.
259

Infragravity waves over topography: generation, dissipation, and reflection

Thomson, James M. (James McArthur) January 2006 (has links)
Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2006. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references. / Ocean surface infragravity waves (periods from 20 to 200 s) observed along the southern California coast are shown to be sensitive to the bottom topography of the shelf region, where propagation is linear, and of the nearshore region, where nonlinearity is important. Infragravity waves exchange energy with swell and wind waves (periods from 5 to 200 s) via conservative nonlinear interactions that approach resonance with decreasing water depth. Consistent with previous results, it is shown here that as waves shoal into water less than a few meters deep, energy is transfered from swell to infragravity waves. In addition, it is shown here that the apparent dissipation of infragravity energy observed in the surfzone is the result of nonlinear energy transfers from infragravity waves back to swell and wind waves. The energy transfers are sensitive to the shallow water bottom topography. On nonplanar beach profiles the transfers, and thus the amount of infragravity energy available for reflection from the shoreline, change with the tide, resulting in the tidal modulation of infragravity energy observed in bottom-pressure records on the continental shelf. The observed wave propagation over the shelf topography is dominated by refraction, and the observed partial reflection from, and transmission across, a steep-walled submarine canyon is consistent with long-wave theory. A generalized regional model incorporating these results predicts the observed infragravity wave amplitudes over variable bottom topography. / by James M. Thomson. / Ph.D.
260

Shelf currents, ice and wind : a numerical modeling study

Russell, Sarah L. (Sarah Louise), 1973- January 2003 (has links)
Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, and the Woods Hole Oceanographic Institution), 2003. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references (p. 190-197). / In this thesis, the effects of sea ice, downwelling favorable winds and barotropic background currents on shelf fronts are examined using numerical models. The models are configured with the characteristics of the East Greenland Current, north of the Denmark Strait, in mind. While the models are heavily idealized, basic physical parameters match the observed ocean. The first part of the thesis uses a three dimensional, primitive equation model to examine the behavior of a shelf front under steady, along shelf winds and barotropic currents. The wind stress generates shoreward surface Ekman transport and the barotropic current generates an offshore bottom Ekman transport. In both cases, the Ekman transport causes the creation of mixed layers and a relationship describing the mixed layer thickness is derived relating the cross shelf flux of density to the along shelf flux of density. When there is a barotropic inflow, the cross shelf Ekman mass transport is balanced by a return flow of mass in the interior. When there is a wind stress, in the present model configuration, the influence of the offshore boundary obscures the effect of the Ekman layers. The second part of the thesis focuses on the ice-ocean interaction using a simple, two layer, one dimensional toy model. The interaction of sea-ice, geostrophic currents, and wind are examined. In the presence of a current and the absence of wind, the ice is transported downstream with the current. In the presence of wind and the absence of a current, the net ice-ocean transport is perpendicular to the wind, as is expected for Ekman layer theory. The two layer system acts like a poorly resolved Ekman spiral: the ice has down wind and shoreward transport while the ocean has up wind and shoreward transport. / by Sarah L. Russell. / Ph.D.

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