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), 2002. / Includes bibliographical references (p. 75-77). / Mixing induced by gravity currents on a sloping bottom was studied through laboratory experiments in a rotating fluid. The dense fluid on the sloping bottom formed a gravity current that could be in regimes where the flow was laminar or had waves. The mixing on a sloping bottom for gravity currents in the laminar and wave regimes was studied both qualitatively and quantitatively. The laboratory experiments were conducted on rotating tables in a tank with homogeneous ambient fluid. The slope angle, rotation rate, reduced gravity, and flow rate of the dense source water were changed for the experiments. The mixing was quantized by measuring the density of the ambient fluid, dense source water, and the bottom water collected at the end of the bottom slope and calculating the ratio of the source water in the bottom water. Comparing the mixing in the laminar regime and the wave regime by changing the slope angle and rotation rate showed that the waves in the gravity current increased the mixing due to the waves. Analysis of the ratio of source water based on the internal Froude number, the Ekman number, and the timescale of the experiments showed that diffusion was not the main mechanism for mixing. The Ekman layer solution was validated by the observation of a streak left by a grain of dye in the dense water layer. The values for the entrainment parameter for the laboratory experiments bracketed those calculated for the Denmark Strait overflow and the Mediterranean outflow, and the values based on observations in the ocean and those from the laboratory were similar for a nondimensional parameter defined using variables used in the laboratory experiments. This shows that the results from the experiments could be used to discuss the mixing in the ocean due to gravity currents along a slope in the ocean and that the waves observed in the laboratory might also be observed in the ocean. / by Mitchihiro Ohiwa. / S.M.

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), 2009. / Includes bibliographical references (p. 257-264). / This thesis examines the nature of eddy-mean flow interactions in western boundary current jets and recirculation gyre dynamics from both theoretical and observational perspectives. It includes theoretical studies of eddy-mean flow interactions in idealized configurations relevant to western boundary current jet systems, namely (i) a study of the mechanism by which eddies generated from a localized forcing drive mean recirculation gyres through the process of nonlinear rectification; and (ii) a study of the role of eddies in the downstream evolution of a baroclinic jet subject to mixed instabilities. It also includes an observational analysis to characterize eddy-mean flow interactions in the Kuroshio Extension using data from the downstream location of maximum eddy kinetic energy in the jet. New insights are presented into a rectification mechanism by which eddies drive the recirculation gyres observed in western boundary current systems. Via this mechanism, eddies drive the recirculations by an up-gradient eddy potential vorticity flux inside a localized region of eddy activity. The effectiveness of the process depends on the properties of the energy radiation from the region, which in turn depends on the population of waves excited. In the zonally-evolving western boundary current jet, eddies also act to stabilize the unstable jet through down-gradient potential vorticity fluxes. In this configuration, the role of eddies depends critically on their downstream location relative to where the unstable time-mean jet first becomes stabilized by the eddy activity. The zonal advection of eddy activity from upstream of this location is fundamental to the mechanism permitting the eddies to drive the mean flows. / (cont.) Observational results are presented that provide the first clear evidence of a northern recirculation gyre in the Kuroshio Extension, as well as support for the hypothesis that the recirculations are, at least partially, eddy-driven. Support for the idealized studies' relevance to the oceanic regime is provided both by indications that various model simplifications are appropriate to the observed system, as well as by demonstrated consistencies between model predictions and observational results in the downstream development of time-mean and eddy properties. / by Stephanie N. Waterman. / Ph.D.

Submarine canyons incising the continental shelf and slope are hot spots for topography-internal wave interactions, with elevated dissipation and mixing contributing to regional transport and biological productivity. At two Barkley Canyon sites (the continental slope below the shelf-break, and deep within the canyon), four overlapping years of horizontal velocity time-series data are used to examine the effects of irregular topography on the internal wave field. Mean currents are topographically guided at both sites, and in the canyon there is an inter-annually consistent, periodic (about a week) up-canyon flow (-700 to -900 m) above a near-bottom down-canyon layer. There is elevation of internal wave energy near topography, up to a factor of 10, 130 m above the slope, and up to a factor of 100, 230 m above the canyon bottom. All bands display weak inter-annual variability, but significant seasonality. Sub-diurnal and diurnal flows are presumably sub-inertially trapped along topography, and the diurnal band appears to be forced locally (barotropically). Both sites have high near-inertial energy. At the slope site, near-inertial energy is attenuated with depth, while in the canyon it is amplified near the bottom. Both sites show intermittent near-inertial forcing associated with wind events, downward propagation of high-mode internal waves, and the seasonal mixed-layer depth, though fewer events are observed in the canyon. Free semidiurnal internal tides are focused and reflected near critical shelf-break and canyon floor topography, and appear to experience both local and remote (baroclinic) forcing. The high-frequency internal wave continuum has enhanced energy near bottom at both sites (up to 7 times the open-ocean Garrett-Munk spectrum), and inferred dissipation rates increasing from a background of less than 10^-9 W kg^-1 and reaching 10^-7 W kg^-1 near topography. Dissipation is most strongly correlated with the semidiurnal (M2) constituent at both sites, with secondary contributions from the sub-diurnal (Sub_K1) band on the slope, and the near-inertial (NI) band in the canyon. Power laws for these dependencies are dissipation ~ M2^0.83 + Sub_K1^0.59 at the slope, and dissipation ~ M2^1.47 + NI^0.24 in the canyon. There is evidence in spectra of a near-buoyancy frequency build-up of energy correlated with high-frequency continuum variability, with a power law fit of 'shoulder' power ~ dissipation^0.34 that is independent of site topography. Though some general results are expected from observations at other slope and canyon sites, the greater temporal extent of these data provide a uniquely long-term evaluation of such processes. / Graduate

The accurate and efficient numerical simulation of geophysical flows is of great interest in numerical weather prediction and climate modeling as well as in numerous critical areas and industries, such as agriculture, construction, tourism, transportation, weather-related disaster management, and sustainable energy technologies. Oceanic and atmospheric flows display an enormous range of temporal and spatial scales, from seconds to decades and from centimeters to thousands of kilometers, respectively. Scale interactions, both spatial and temporal, are the dominant feature of all aspects of general circulation models in geophysical fluid dynamics. In this thesis, to decrease the cost for these geophysical flow computations, several types of multiscale methods were systematically developed and tested for a variety of physical settings including barotropic and stratified wind-driven large scale ocean circulation models, decaying and forced two-dimensional turbulence simulations, as well as several benchmark incompressible flow problems in two and three dimensions. The new models proposed here are based on two classes of modern multiscale methods: (i) interpolation based approaches in the context of the multigrid/multiresolution methodologies, and (ii) deconvolution based spatial filtering approaches in the context of large eddy simulation techniques. In the first case, we developed a coarse-grid projection method that uses simple interpolation schemes to go between the two components of the problem, in which the solution algorithms have different levels of complexity. In the second case, the use of approximate deconvolution closure modeling strategies was implemented for large eddy simulations of large-scale turbulent geophysical flows. The numerical assessment of these approaches showed that both the coarse-grid projection and approximate deconvolution methods could represent viable tools for computing more realistic turbulent geophysical flows that provide significant increases in accuracy and computational efficiency over conventional methods. / Ph. D.

This study reports on a new formulation of the spectral dissipation source term Sds for wind-wave modelling applications. This new form of Sds features a nonlinear dependence on the local wave spectrum, expressed in terms of the azimuthally integrated saturation parameter B(k)=k^4 F(k). The basic form of this saturation-dependent Sds is based on a new framework for the onset of deep-water wave breaking due to the nonlinear modulation of wave groups. The new form of Sds is succesfully validated through numerical experiments that include exact nonlinear computations of fetch-limited wind-wave evolution and hindcasts of two-dimensional wave fields made with an operational wind-wave model. The newly-proposed form of Sds generates integral spectral parameters that agree more closely with observations when compared to other dissipation source terms used in state-of-the-art wind-wave models. It also provides more flexibility in controlling properties of the wave spectrum within the high wavenumber range. Tests using a variety of wind speeds, three commonly-used wind input source functions and two alternative full-development evolution limits further demonstrate the robustness and flexibility of the new saturation-dependent dissipation source term. Finally, improved wave hindcasts obtained with an implementation of the new form of Sds in a version of the WAM model demonstrate its potential usefulness in operational wind-wave forecasting applications.

Using a combination of hydrographic data and the trajectories and profiles of isobaric floats, this dissertation evaluates the connections between remote regions in the North Atlantic. First, I establish that the production and advection of the North Atlantic Subtropical Mode Water (STMW) introduces spatial and temporal variability in the subsurface nutrient reservoir of the subtropical gyre. As the mode water is formed, its nutrients are depleted by biological utilization. When the depleted water mass is exported to the gyre, it injects a wedge of low-nutrient water into the upper layers of the ocean. Contrary to intuition, cold winters that promote deep convective mixing and vigorous mode water formation may diminish downstream primary productivity by altering the subsurface delivery of nutrients. Next, the source of elevated nutrient concentrations in the Gulf Stream is assessed. The historical hydrographic data suggest that imported water advected into the Gulf Stream via the tropics supplies an important source of nutrients to the Gulf Stream. Because the high nutrients are likely imported from the tropics, diapycnal mixing need not be invoked to explain the Gulf Stream's high nutrient concentrations, as had been previously hypothesized. Furthermore, nutrients do not increase along the length of the Stream, as would be expected with strong diapycnal mixing.Finally, profiling float data are used to investigate how the Labrador Sea Water enters the Deep Western Boundary Current, one of the primary pathways by which it exits the subpolar gyre. With the trajectories and profiles of an extensive array of P-ALACE floats I evaluate three processes for their role in the entry of Labrador Sea Water in the Deep Western Boundary Current (DWBC): 1) LSW is formed directly in the DWBC, 2) Eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) A horizontally divergent mean flow advects LSW from the interior to the DWBC. Each of the three processes has the potential to remove heat from the boundary current, and both the formation of LSW directly in the boundary current and the eddy heat flux are possible sources of interannual variability in the exported LSW product. / Dissertation

Includes vita. / Title from PDF of title page. / Document formatted into pages; contains 119 pages. / Thesis (Ph.D.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: The air-sea interaction thermodynamics of the western equatorial Pacific, the Earth's largest region of warm SST, is a major component of the global climate system. Along the equator, warm pool thermodynamics and momentum are influenced by equatorial ocean visco-inertial boundary layer dynamics that occur within a few degrees of the equator because of the sign reversal of the Coriolis force. Designed to study this system, COARE Enhanced Monitoring Array (EMA) observations of temperature, salinity, velocity, and surface meteorology were centered at 0, 156°E from February 1992 through April 1994. They sampled variability on the equator over larger space/time-scales than the concurrent Intensive Flux Array (IFA) centered at 2°S, 156°E. The EMA data are examined within the context of the larger scale equatorial Pacific and the El Niño conditions that occurred at that time. / ABSTRACT: There is a structural change in the equatorial Pacific near the dateline resulting from the winds that are strong, steady, and easterly in the east and generally weak, punctuated by westerly wind bursts, in the west. East of the dateline the EUC's speed and transport increases downstream, while in the west it tends to be zonally uniform, consistent with the extra-tropical ocean interior water pathways that tend to converge on the equator east of the dateline. At 0°, 156°E in the western Pacific deep, seasonal upwelling (appearing stronger after the peak of the 1991/92 El Niño than during the following weaker El Niño year) occurs within the thermocline in boreal summer with magnitudes as large as upwelling in the eastern Pacific cold tongue. This large upwelling is associated with large downward turbulent heat flux and large turbulent shear stress. / ABSTRACT: While the inferred mixing is quantitatively inconclusive because of unresolved potential errors, it is consistent with the visco-inertial boundary layer concepts from early theory [e.g. Arthur 1960; Robinson 1960; Stommel 1960; and Charney and Spiegel 1971]. These findings suggest that the equatorial thermodynamics differ from those of the IFA. Further process experimentation is necessary to quantify these results. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

Tide-topography coupling is important for understanding surface-tide energy loss, the intermittency of internal tides, and the cascade of internal-tide energy from large to small scales. Although tide-topography coupling has been observed and modeled for 50 years, the identification of surface and internal tides over arbitrary topography has not been standardized. Here, we begin by examining five surface/internal-tide decompositions and find that only one is (i) consistent with the normal-mode description of tides over a flat bottom, (ii) produces a physically meaningful depth-structure of internal-tide energy flux, and (iii) results in an established expression for internal-tide generation. Next, we examine the expression for internal-tide generation and identify how it is influenced by remotely-generated shoaling internal tides. We show that internal-tide generation is subject to both resonance and intermittency, and can not always be predicted from isolated regional models. Lastly, we quantify internal-tide generation and scattering on the Oregon Continental slope. First, we derive a previously unpublished expression for inter-modal energy conversion. Then we evaluate it using observations and numerical simulations. We find that the surface tide generates internal tides, which propagate offshore; while at the same time, low-mode internal tides shoal on the slope, scatter, and drive turbulent mixing. These results suggest that internal tides are unlikely to survive reflection from continental slopes, and that continental margins play an important role in deep-ocean tidal-energy dissipation. / Graduation date: 2011

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), 2019 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 137-153). / Each year, surface ocean ecosystems export sinking particles containing gigatons of carbon into the ocean's interior. This particle flux connects the entire ocean microbiome and constitutes a fundamental aspect of marine microbial ecology and biogeochemical cycles. Particle flux is also variable and intricately complex, impeding its mechanistic or quantitative description. In this thesis we pair compilations of available data with novel mathematical models to explore the relationships between particle flux and other key variables - temperature, net primary production, and depth. Particular use is made of (probability) distributional descriptions of quantities that are known to vary appreciably. First, using established thermodynamic dependencies for primary production and respiration, a simple mechanistic model is developed relating export efficiency (i.e. the fraction of primary production that is exported out of the surface ocean via particle flux) to temperature. / The model accounts for the observed variability in export efficiency due to temperature without idealizing out the remaining variability that evinces particle flux's complexity. This model is then used to estimate the metabolically-driven change in average export efficiency over the era of long-term global sea surface temperature records, and it is shown that the underlying mechanism may help explain glacial-interglacial atmospheric carbon dioxide drawdown. The relationship between particle flux and net primary production is then explored. Given that these are inextricable but highly variable and measured on different effective scales, it is hypothesized that a quantitative relationship emerges between collections of the two measurements - i.e. that they can be related not measurement-by-measurement but rather via their probability distributions. / It is shown that on large spatial or temporal scales both are consistent with lognormal distributions, as expected if each is considered as the collective result of many subprocesses. A relationship is then derived between the log-moments of their distributions and agreement is found between independent estimates of this relationship, suggesting that upper ocean particle flux is predictable from net primary production on large spatiotemporal scales. Finally, the attenuation of particle flux with depth is explored. It is shown that while several particle flux-versus-depth models capture observations equivalently, these carry very different implications mechanistically and for magnitudes of export out of the surface ocean. A model is then proposed for this relationship that accounts for measurements of both the flux profile and of the settling velocity distribution of particulate matter, and is thus more consistent with and constrained by empirical knowledge. / Possible future applications of these models are discussed, as well as how they could be tested and/or constrained observationally. / by Brendan Barry. / Ph. D. / Ph.D. Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution)

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), 2008. / Includes bibliographical references (p. [221]-232). / Marine ecosystems reflect the physical structure of their environment and the biological processes they carry out. This leads to spatial heterogeneity and temporal variability, some of which is imposed externally and some of which emerges from the ecological mechanisms themselves. The main focus of this thesis is on the formation of spatial patterns in the distribution of zooplankton arising from social interactions between individuals. In the Southern Ocean, krill often assemble in swarms and schools, the dynamics of which have important ecological consequences. Mathematical and numerical models are employed to study the interplay of biological and physical processes that contribute to the observed patchiness. The evolution of social behavior is simulated in a theoretical framework that includes zooplankton population dynamics, swimming behavior, and some aspects of the variability inherent to fluid environments. First, I formulate a model of resource utilization by a stage-structured predator population with density-dependent reproduction. Second, I incorporate the predator-prey dynamics into a spatially-explicit model, in which aggregations develop spontaneously as a result of linear instability of the uniform distribution. In this idealized ecosystem, benefits related to the local abundance of mates are offset by the cost of having to share resources with other group members. Third, I derive a weakly nonlinear approximation for the steady-state distributions of predator and prey biomass that captures the spatial patterns driven by social tendencies. Fourth, I simulate the schooling behavior of zooplankton in a variable environment; when turbulent flows generate patchiness in the resource field, schools can forage more efficiently than individuals. / (cont.) Taken together, these chapters demonstrate that aggregation/ schooling can indeed be the favored behavior when (i) reproduction (or other survival measures) increases with density in part of the range and (ii) mixing of prey into patches is rapid enough to offset the depletion. In the final two chapters, I consider sources of temporal variability in marine ecosystems. External perturbations amplified by nonlinear ecological interactions induce transient ex-cursions away from equilibrium; in predator-prey dynamics the amplitude and duration of these transients are controlled by biological processes such as growth and mortality. In the Southern Ocean, large-scale winds associated with ENSO and the Southern Annular Mode cause convective mixing, which in turn drives air-sea fluxes of carbon dioxide and oxygen. Whether driven by stochastic fluctuations or by climatic phenomena, variability of the biogeochemical/physical environment has implications for ecosystem dynamics. / by Ariane Verdy. / Ph.D.