Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982. / Microfiche copy available in Archives and Science / Bibliography: leaves 194-196. / by Joshua Michael Aaron Ryder Wurman. / M.S.

Located at the northern edge of the Northern Gulf of Alaska (NGOA), Prince William Sound(PWS) is a small, two-strait semi-enclosed sea. The general ocean circulation pattern inside PWS is significantly affected by external forcings, for instance, the large-scale circulation in NGOA, atmospheric pressure and surface winds, surface heating/cooling, runoff, and tides. Motivated by multi-year experience with a well-validated, quasi-operational ocean circulation nowcast/forecast system for PWS (viz., Extended PWS Nowcast/Forecast System (EPWS/NFS)), the present study addresses some aspects of the PWS response to various external forcings, via numerical simulations. Based on the Princeton Ocean Model (POM), four numerical implementations have been examined, viz., PWS-POM, Extended PWS-POM (EPWS-POM), Idealized PWS-POM (IPWS-POM), and a 2-D tidal model. These implementations are used to simulate physical processes with various spatial and temporal scales in PWS. A series of numerical simulations are conducted, driven by various external forcings ranging from large scale and mesoscale circulation in NGOA represented by the Global Navy Coastal Ocean Model (NCOM), to atmospheric pressure observed by National Data Buoy Center (NDBC) buoys and mesoscale winds predicted by Regional Atmospheric Modeling System (RAMS), and to tides simulated by the 2-D tidal model. These simulations, along with analysis from a Helmholtz resonance model, demonstrate and help interpret some phenomena in PWS; for instance, barotropic Helmholtz resonance in coastal sea levels, and volume transports through the two PWS straits, and a dominant cyclonic gyre in the Central Sound in August and September. The simulation results are used to study a wide range of oceanic phenomena in PWS; e.g., two-layer/three-layer baroclinic transports through the straits, a "transition band" in the coherence pattern between volume transports through the two straits, mesoscale circulation in the Central Sound, the deep water circulation, and the annual tidal energy budget.

<p>This dissertation identifies and quantifies the impact of processes driving two biogeochemical phenomena of interest by considering them in the context of the large-scale circulation in which they occur. Both processes occur in the North Pacific transition zone (NPTZ), a basin-wide region near the subpolar-subtropical gyre boundary. </p><p>First, this work investigates the physical forcing behind the large seasonal variability in the location of the Transition zone chlorophyll front (TZCF). The TZCF is a persistent gradient in sea surface color that separates high chlorophyll waters to the north from low chlorophyll waters to the south. The chlorophyll front shifts seasonally by about 1000 km, oscillating between its southernmost winter latitude and its northernmost summer latitude. The forcing behind this seasonal signal is of interest because a number of migratory marine animals, both commercial and endangered, appear to track it.</p><p>This first study finds that vertical processes, traditionally viewed as controlling the dynamical supply of nutrients to surface waters, are insufficient to explain seasonal variations in nutrient supply to the transition zone. Instead, the wind-driven horizontal Ekman transport of nutrients, moving southward from the subpolar gyre into the northern reaches of the subtropical gyre, drives the southward migration of the TZCF. Such lateral transport of nitrate supports up to 40% of new primary productivity in the region annually, and nearly all of new primary productivity in the winter. </p><p>Second, this work investigates why the North Pacific transition zone waters are a notably strong sink for atmospheric carbon dioxide (CO2) on a mean annual basis, while seasonally they vary from a sink in the winter to a neutral to weak source in the summer. As the partial pressure of carbon dioxide (pCO2) in the surface mixed layer exerts primary control on the direction and magnitude of air-sea carbon exchange, this study quantifies the impact of processes regulating seawater pCO2: temperature, salinity, advection of dissolved inorganic carbon (DIC) and alkalinity (ALK), mixing of DIC and ALK, biology, and air-sea carbon flux.</p><p>Seasonal controls on pCO2 in the North Pacific transition zone differ from annual controls. Temperature effects dominate the seasonal signal, but are partially countered throughout the year by opposing processes. In spring and summer, biological drawdown partially offsets the increase in pCO2 due to warming waters; in fall and winter, the vertical entrainment of carbon moderates the decrease in pCO2 due to cooling waters. On a mean annual basis, air-sea carbon flux, biology, mixing, and advection all have a net impact on seawater pCO2. Though important seasonally, temperature has a small impact on pCO2 and air-sea carbon flux annually, accounting for only about 15-20% of oceanic carbon uptake through temperature-driven solubility changes.</p><p>This second study again finds an important role for lateral processes to play in regulating biogeochemical phenomena in the North Pacific transition zone. The ability of the region to uptake atmospheric carbon year after year is maintained by those processes exporting carbon from its surface waters: the vertical export of organic carbon to depth, and the lateral geostrophic advection of carbon out of the region. This lateral advection alone determines the location of the sink region: of the processes impacting seawater pCO2 on a mean annual basis, only the geostrophic divergence of DIC disproportionately lowers pCO2 in the transition zone latitudes, supporting greater atmospheric CO2 uptake here than in surrounding regions.</p><p>This dissertation identifies and quantifies processes driving biogeochemical features in the North Pacific transition zone, finding the large-scale circulation in the region plays a significant role in regulating these processes. The unique physical oceanographic characteristics of the NPTZ, and in particular the lateral transport, support biological and chemical attributes notably distinct from adjacent waters.</p> / Dissertation

<p>The timing of the spring phytoplankton bloom in the subpolar North Atlantic Ocean has important consequences for the marine carbon cycle and ecosystems. There are currently several proposed mechanisms to explain the timing of this bloom. The conventional theory holds that the bloom begins when the ocean warms and the seasonal mixed layer shoals in the spring, decreasing the depth to which phytoplankton are mixed and increasing the light available to the population. Recent work has attributed the beginning of the bloom to decreases in turbulence within the upper ocean, driven by the onset of positive heat fluxes or decreases in the strength of local winds. Other studies have focused on the increase in the seasonal mixed layer in the winter as a driver of changes in ecosystem interactions and a control on the spring bloom. Finally, submesoscale eddies, occurring as a result of lateral density gradients, have been proposed as a stratification mechanism that can create phytoplankton blooms prior to the onset of ocean surface warming.</p><p>This dissertation critically examines and compares the proposed theories for the initiation of the spring bloom and draws on these theories to propose a new framework: that blooms begin when the active mixing depth shoals, a process generally driven by a weakening of surface heat fluxes and consequent shift from convective mixing to wind-driven mixing. Using surface forcing data, we develop a parameterization for the active mixing depth from estimates of the largest energy-containing eddies in the upper ocean. </p><p>Using in situ records of turbulent mixing and biomass, we find that the spring phytoplankton bloom occurs after mixing shifts from being driven by convection to being driven by wind, and that biomass increases as the active mixing depth shoals. Using remote sensing data, we examine patterns of bloom initiation in the North Atlantic at the basin scale, compare current theories of bloom initiation, and find that the shoaling of the active mixing depth better predicts the onset of the bloom across the North Atlantic subpolar basin and over multiple years than do other current theories. Additionally, using a process study model, we evaluate the importance of submesoscale eddy-driven stratification as a control on the initiation of the spring bloom, determining that this mechanism has a relatively minor effect on alleviation of phytoplankton light limitation. Finally, we describe potential techniques and tools to examine whether interannual variability in the active mixing depth acts as a control on variability in the timing of the spring bloom.</p> / Dissertation

The spatial and temporal structure of the flow, vorticity and stress over equilibrium orbital-scale sand ripples are investigated at turbulence-resolving scales with a wide-band coherent Doppler profiler (MFDop) in an oscillating tray apparatus. The oscillation period and horizontal excursion were 10 s and 0.5 m. Velocity profiles were acquired with 3 mm vertical resolution and at a 42 Hz sampling rate. Ripple wavelength and amplitude were 25 cm and 2.2 cm. The MFDop measurements are used to investigate the development of the lee vortex as a function of phase, and the co-evolution of turbulent kinetic energy, Reynolds stress and turbulence production. Shear stress is determined from the vertically-integrated vorticity equation and using the double-averaging approach. Friction factors obtained from the two methods are comparable and range from 0.1 to 0.2.

<p>In response to the differential heating of the earth, atmospheric and oceanic flows constantly act to carry surplus energy from low to high latitudes. In the ocean, this poleward energy flux occurs as part of the large scale meridional overturning circulation: warm, shallow waters are transported to high latitudes where they cool and sink, then follow subsurface pathways equatorward until they are once again upwelled to the surface and reheated. In the North Atlantic, the upper limb of this circulation has always been explained in simplistic terms: the Gulf Stream/North Atlantic Current system carries surface waters directly to high latitudes, resulting in elevated sea surface temperatures in the eastern subpolar gyre, and, because the prevailing winds sweeping across the Atlantic are warmed by these waters, anomalously warm temperatures in Western Europe. This view has long been supported by Eulerian measurements of North Atlantic sea surface temperature and surface velocities, which imply a direct and continuous transport of surface waters between the two gyres. However, though the importance of this redistribution of heat from low to high latitudes has been broadly recognized, few studies have focused on this transport within the Lagrangian frame. </p><p>The three studies included in this dissertation use data from the observational record and from a high resolution model of ocean circulation to re-examine our understanding of upper limb transport between the subtropical and subpolar gyres. Specifically, each chapter explores intergyre Lagrangian pathways and investigates the impact of those pathways on subpolar property fields. The findings from the studies suggest that intergyre transport pathways are primarily located beneath the surface and that subtropical surface waters are largely absent from the intergyre exchange process, a very different image of intergyre transport than that compiled from Eulerian data alone. As such, these studies also highlight the importance of including 3d Lagrangian information in examinations of transport pathways.</p> / Dissertation

A combination of in situ sampling and real-time ocean observations was used to investigate the processes responsible for the formation and the areal extent of Texas coastal hypoxia from 2002 to 2011. In situ sampling, real-time mooring and buoy observations, and multivariate statistical modeling were used to investigate the physical processes driving hypoxia formation. Geostatistical interpolation (ordinary kriging) models were tested to compare the differences in annual hypoxia area on the Texas shelf. Results from these two sections were integrated into recommendations for improving federal hypoxia monitoring and mitigation strategies in the northwestern Gulf of Mexico. Winds, currents, temperature, salinity, and dissolved oxygen records revealed the annual, seasonal, and daily variability of hypoxia formation on the Texas coast from 2009 to 2011. Hypoxic events occurred from late May to late October lasting from hours to weeks. Hypoxia formation was either the result of salinity stratification, associated with the freshening of surface waters by the advection of Mississippi-Atchafalaya River freshwater westward or the wind- and current-driven upcoast or downcoast flow of Brazos River discharge. Records from 2010 and 2011 showed the variability and frequency of stratification development differs on the north and south Texas shelf. Multivariate linear model results showed contributing factors on the north Texas shelf vary annually and that primary factors for hypoxia development are near-surface current speeds and salinity-driven stratification. Interpolation models resulted in three size categories for hypoxia area: small (100 – 1,000 km^2), moderate (1,001 – 3,000 km^2), and large (3,001+ km^2). Moderate years include 2002, 2004, and 2007 and a large year was 2008. There was no increase in hypoxic area from years 2002 to 2011, but years 2007 and 2008 resulted in a hypoxic area over 5,000 km^2, which is the federally mandated hypoxia reduction target for the northwestern Gulf of Mexico. Geostatistical interpolators represent and predict the structure and spatial extent of the hypoxic area on the Texas shelf by accounting for the anisotropy of physical processes on the Texas shelf. Geostatistical interpolation models are preferred to deterministic models for developing and improving federal hypoxia monitoring and mitigation strategies on the northwestern Gulf of Mexico shelf.

<p>Little work has been done to estimate turbulence characteristics in the hypolimnetic waters of large lakes, where the magnitude and vertical structure of turbulent parameters have important implications for nutrient cycling and benthic exchange. In this thesis, hypolimnetic mixing is investigated over the annual stratification cycle in a large lake using a series of experiments in Lake Michigan that utilize acoustic Doppler velocimeters, thermistors, and microstructure profilers to characterize mean flow and turbulence throughout the water column. More than 500 days of physical limnological data were collected and analyzed over the course of this study, creating the most comprehensive data set of its kind in the Laurentian Great Lakes. While we found that bottom boundary layer turbulence and mean flow follow law-of-the-wall predictions in the mean, individual estimates were shown to deviate significantly from canonical expectations, with deviations linked to weakly energetic flow conditions (i.e. low speeds) and seiche-scale flow unsteadiness. Bottom boundary layer characteristics, including the mean current speed (U₅₀=3 cm/s), drag coefficient (Cd₅₀=0.0052), and turbulent kinetic energy dissipation (ϵ₅₀ =10^-8 W/kg), showed very little seasonal variation, despite highly variable surface forcing (e.g. stratification, wind speeds). Full water column turbulence profiles measured during the stratified summer were largely buoyancy suppressed, with internal Poincaré waves driving enhanced turbulent kinetic energy dissipation (ϵ= 10^-7 W/kg) in the relatively compact thermocline and weak hypolimnetic mixing (turbulent scalar diffusivity: K_z=10^-6 m²/s) limiting benthic nutrient delivery. Although small temperature gradients drove strong mixing over the isothermal period (K_z=10^-3 m²/s), velocity shear was overwhelmed by weakly stable stratification (Richardson number:Ri≈0.2), limiting the development of the surface mixed layer and suppressing hypolimnetic turbulence (ϵ=10^-9 W/kg; K_z=10^-4 m²/s). When surface temperatures fell below the temperature of maximum density (T_MD≈ 4℃), radiative convection played a major role in driving vertical transport, with energetic full water column mixing throughout the day followed by surface cooling and restratification overnight. During this “convective winter” period, daily temperature instabilities were directly correlated with elevated turbulence levels (ϵ=10^-7 W/kg; K_z≈10^-1 m²/s), and overnight turbulence characteristics were similar to those observed over the isothermal spring. Near surface dissipation and diffusivity measurements followed similarity scaling arguments, with wind shear and surface fluxes dominating production in the surface mixed layer during all three seasons. Together, these results are used to model the influence of invasive dreissenids over each forcing period, providing insight into the annual variability of effective filtration rates in the calm, hypolimnetic waters of Lake Michigan.</p><p></p>

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), 1998. / Includes bibliographical references (leaves 156-158). / by François W. Primeau. / Ph.D.

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), 1998. / Includes bibliographical references (leaves 150-157). / Double-diffusive processes are studied and parameterized, and their impacts on the oceanic thermohaline circulation are investigated by single-hemisphere numerical models and scaling analysis. Scaling analysis on the thermohaline circulation has been done under three types of surface boundary conditions. (a) Under "relaxation" conditions, there is a two-thirds power law dependence of the meridional overturning rate (and the poleward heat transport) on the diapycnal diffusivity. For any given external forcing, there is only one equilibrium state for the thermohaline circulation. (b) Under "flux" boundary conditions, there is a half power law dependence of the meridional overturning rate on the diapycnal diffusivity. Only one mode is possible for given external forcing. (c) Under "mixed" boundary conditions, multiple equilibria become possible. For given thermal forcing, the existence of multiple equilibria depends on the relative contributions of diapycnal diffusivity and the hydrologic forcing. Numerical experiments are implemented to test the above scaling arguments. Consistent results have been obtained under the above three types of boundary conditions. These provide a basis for understanding how the thermohaline circulation depends on the diapycnal diffusivity, which we know is influenced by the double-diffusive processes of "salt fingering" and "diffusive layering" in some parts of the ocean. In order to examine this issue, the double-diffusive processes are parameterized by diapycnal eddy diffusivities for heat and salt that are different and depend on the local density ratio, ... A background diffusivity is applied to represent turbulent mixing in the stratified environment. The implementation of this double-diffusive - parameterization in numerical models has significant impacts on the thermohaline circulation. (a) Under "relaxation" boundary conditions, the meridional overturning rate and the poleward heat transport are reduced, and water mass properties are also changed. Similar results are obtained under "flux" boundary conditions. (b) Under "mixed" boundary conditions, the critical freshwater flux for the existence of the thermal mode becomes smaller with the double-diffusive parameterization. The extent to which the thermohaline circulation is affected by double-diffusive processes depends on the magnitude of the freshwater forcing. / by Jubao Zhang. / Ph.D.