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.

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), 1998. / Includes bibliographical references (leaves 155-161). / A detailed examination of the development of a deep convection event observed in the Greenland Sea in 1988-89 is carried out through a combination of modeling, scale estimates, and data analysis. We develop a prognostic one-dimensional mixed layer model which is coupled to a thermodynamic ice model. Our model contains a representation of the lowest order boundary layer dynamics and adjustable coupling strengths between the mixed layer, ice, and atmosphere. We find that the model evolution is not very sensitive to the strength of the coupling between the ice and the mixed layer sufficiently far away from the limits of zero and infinite coupling; we interpret this result in physical terms. Further, we derive an analytical expression which provides a scale estimate of the rate of salinification of the mixed layer during the ice-covered preconditioning period as a function of the rate of ice advection. We also derive an estimate for the rate of the mixed layer deepening which includes ice effects. Based on these scale estimates and model simulations, we confirm that brine rejection and advection of ice out of the convection area were essential ingredients during the preconditioning process. We also demonstrate that an observed rise in the air temperature starting in late December 1988 followed by a period of moderately cold ~ -10*C temperatures was key to the development of the observed convection event. Finally, we show that haline driven deep convection underneath an ice cover is possible, but unlikely to occur in the Greenland Sea. On the basis of these results, we develop a coherent picture of the evolution of the convection process which is more detailed than that presented in any previous work. We also comment on the likelihood that deep convection occurred in the Greenland Sea in the past two decades from an examination of historical data, and relate these findings to what is known about the inter-annual variability of convective activity in the Greenland Sea / by Vikas Bhushan. / 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), February 1999. / Includes bibliographical references (leaves 220-224). / This thesis studies the problems of generation and maintenance of recirculations by Gulf Stream instabilities. Observations show that the horizontal structure of the jet and its recirculations suffer significant changes in time. Here, the role of internal dynamics of the jet is isolated as one of the possible sources of such variability, and the differences between barotropic and baroclinic instabilities are investigated. The problem of recirculation development is considered in a framework of a free spin down of the 2-layer and the lI-layer, zonally symmetric, quasi-geostrophic jets. Linear stability analysis shows that in strongly baroclinic basic flows, eddies are capable of driving recirculations in the lower layer through the residual meridional circulation. In strongly barotropic jets, the linearly most unstable wave simply diffuses the jet. Nonlinear stability analysis indicates that recirculations are robust features of the 2-layer model. The strength of recirculations is a function of the model's parameters. It increases with a decrease in the value of the nondimensional # due to potential vorticity homogenization constrained by enstrophy conservation. The recirculation strength is a non-monotonic function of the baroclinic velocity parameter; it is the strongest for strongly baroclinic basic flows, weakest for flows with intermediate baroclinic structure and of medium strength for strongly barotropic basic flows. Such non-monotonic behavior is the result of two different processes responsible for the recirculation development: linear eddy-mean flow interactions for strongly baroclinic basic flows and strongly nonlinear eddy-eddy and eddy-mean flow interaction for strongly barotropic flows. In the case of the reduced-gravity model, recirculations develop only for infinite deformation raduis. Basic flows with finite deformation radius are only weakly supercritical and therefore produced negligible recirculations after equilibration. The problem of maintenance of the recirculations is coupled to the questions of existence of low frequency variability and of multiple dynamical regimes of a system consisting of a quasi-geostrophic jet and its recirculations. The problem is studied in a framework of a 2-layer or a reduced-gravity colliding jets model which has no windforcing. Instead, it is forced by inflows and outflows through the open boundaries. Only the western boundary of the domain is closed, and a free slip boundary condition is used there. The results of the numerical experiments show that when only the mechanism of barotropic instability is present, the model has two energy states for a wide range of interfacial friction coefficients. The high energy state is characterized by well-developed recirculations and displays strong variability associated with either large recirculating gyres and a weak eddy field or small recirculations and a strong eddy field. The low energy state is characterized by large meridional excursions in the separation point and large amplitude, westward propagating meanders that produce strong rings after interacting with the western wall. For physically relevant bottom friction values, the presence of baroclinic instability in the recirculation regions of the 2-layer model allows for a unique dynamical regime characterized by well-developed recirculations in both layers. The low-frequency variability associated with the regime is weak and is related to meridional shifts in the position of the jet, to wrapping of the recirculations around each other, and to pulsations in their zonal extent. / by Natalia Yurievna Beliakova. / Ph.D.

Thesis (Sc. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), February 2000. / Includes bibliographical references (p. 250-266). / A new, global inversion is used to estimate the large scale oceanic circulation based on the World Ocean Circulation Experiment and Java Australia Dynamic Experiment hydrographic data. A linear inverse "box" model is used to combine consistently the transoceanic sections. The circulation is geostrophic with an Ekman layer at the surface and oceanic layers defined by neutral surfaces. Near-conservation of mass, salt and top-to-bottom silica is required and, in addition, heat and the phosphate-oxygen combination (170[PO 4]+[0 2]) are conserved in layers that are not in contact with the surface. A globally-consistent solution is obtained for a depth-independent adjustment to the thermal wind field, freshwater flux divergences, the Ekman transport, and the advective and diffusive dianeutral fluxes between layers. A detailed error budget permits calculation of statistical uncertainties, taking into account both the non-resolved part of the solution and the systematic errors due to the temporal oceanic variability. The estimated water mass transports during the WOCE period (1985-1996) are generally similar to previous published estimates. However, important differences are found. In particular, the inflow of bottom waters into the Pacific Ocean is smaller than in most previous estimates. Utilization of property anomaly conservation constraints allows the estimation of significant dianeutral diffusivities in deep layers, with a global average of 3 ± 1cm 2 s-1 north of 30'S. Dianeutral transfers indicate that about 20 Sv of bottom water is formed in the Southern Ocean. Significant oceanatmosphere heat fluxes are found, with a global heating of 2.3 ± 0.4 PW in the tropical band and a corresponding cooling at high latitudes. The signature of a large-scale average export production is found for nutrients in several temperate regions. Despite the large uncertainties, the production magnitudes are consistent with independent measurements from sediment traps and isotopic data. Net nutrient sources or sinks are found in several regions, suggesting either transport of dissolved organic matter or a seasonal alias. Oxygen indicates large exchanges with the atmosphere, with intake at high latitudes and outgassing/remineralization at low latitudes. / by Alexandre Similien Ganachaud. / Sc.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. / by James Maxwell Pringle. / Ph.D.

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.

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.

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.

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.