ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March, 1980

Description: The Southern Ocean as defined here is the body of water between the Antarctic Continent and the Antarctic Polar Front, (APF). This ocean is considered important in the global thermodynamic balance of the ocean-atmosphere system because large planetary heat losses are believed to occur at high latitudes. The ocean and atmosphere must transport heat poleward to balance these losses. In the Southern Hemisphere, the oceanic contribution to this flux involves a southward transport of heat across the APF into the Southern Ocean where it is given up to the atmosphere through air-sea interactions. In Part I, the air-sea interactions and structure of the near surface waters of the Southern Ocean are investigated with a three dimensional time dependent numerical model. The surface waters in this region in summer are characterized by a relatively warm surface mixed layer with low salinity. Below this layer, a cold temperature extremum is usually observed in vertical profiles which is believed to be the remnant of a deep surface mixed layer produced in winter. The characteristics of this layer, the surface mixed layer and the observed distribution of wintertime sea ice are reproduced well by this model. Unlike some other sea-ice models the air-sea heat exchange is a free variable. Model estimates of the annual heat loss by the Southern Ocean exhibit the observed meridional variation of heat gained by the ocean along the APF with heat lost further south. The model's area average heat loss is much smaller than that estimated with direct observations. While several model parameterizations were made which could be in error, the model results suggest that the Southern Ocean does give up vast amounts of heat to the atmosphere away from the continental margins. The model results and direct calculations of air-sea exchanges suggest a southward heat flux must occur across the APF. The lateral water mass transition across the front is not discontinuous but occurs over a finite sized zone of fluid which is dominated by intrusive finestructure. The characteristics and dynamics of these features are investigated in Part II to try and assess their importance in the meridional heat budget. Observations made on two cruises to the APF are presented and the space-time scales of the features and thermohaline characteristics are discussed. It is suggested that double diffusive processes dominated by salt fingering are active within the intrusions. An extension of Stern's (1967) model of the stability of a thermohaline front to intrusive finestructure driven by saltfingering where small scale viscous processes are included, is presented to explain why intrusions are observed in frontal zones. The model successfully predicts vertical scales of intrusions observed in the ocean and the observed dependence of the intrusions' slopes across density surfaces on the vertical scale. Since the fastest growing intrusion is not strongly determined by the model, though, it is likely that finite amplitude effects determine the dominant scale of interleaving in the ocean. The analysis predicts that intrusions transport heat, salt and density down the mean gradients of the front. For the APF, this heat flux is poleward which is the direction required by the global heat budget. This model does not describe intrusions at finite amplitude or in steady state and so cannot be used to estimate the magnitude of the poleward heat flux due to intrusions in the APF.

Description: The research reported on here, and my support as a graduate student was provided by the National Science Foundation through grants OCE 75 14056. OCE 76 82036 and OCE 77 28355.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2003

Description: Oceanic observations indicate that abyssal mixing is localized in regions of rough topography. How locally mixed fluid interacts with the ambient fluid is an open question. Laboratory experiments explore the interaction of mechanically induced boundary mixing and an interior body of linearly stratified rotating fluid. A single oscillating bar produces a small region of turbulence along the wall at middepth. Mixed fluid quickly reaches a steady state height set by a turbulent-buoyant balance, independent of rotation. Initially, the bar is exposed on three sides. Mixed fluid intrudes directly into the interior rather than forming a boundary current. The circulation patterns suggest a model of unmixed fluid being laterally entrained into the turbulent zone. In accord with the model, observed outflux is constant, independent of stratification and restricted by rotation. Later the bar is laterally confines between two walls, which form a channel opening into the basin. A small percentage of mixed fluid enters a boundary current, which exits the channel. The bulk forms a cyclonic circulation in front of the bar, which blocks the channel and restricts horizontal entrainment. In the confined case, the volume flux of mixed fluid decays with time.

Description: This work was supported by the Ocean Ventures Fund, the Westcott Fund and the WHOI Education Office. Financial support was also provided by the National Science Foundation through grant OCE-9616949.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2009

Description: Observations and inverse models suggest that small-scale turbulent mixing is enhanced in the Southern Ocean in regions above rough topography. The enhancement extends 1 km above the topography suggesting that mixing is supported by breaking of gravity waves radiated from the ocean bottom. In other regions, gravity wave radiation by bottom topography has been primarily associated with the barotropic tide. In this study, we explore the alternative hypothesis that the enhanced mixing in the Southern Ocean is sustained by internal waves generated by geostrophic motions flowing over bottom topography. Weakly-nonlinear theory is used to describe the internal wave generation and the feedback of the waves on the zonally averaged flow. A major finding is that the waves generated at the ocean bottom at finite inverse Froude numbers drive vigorous inertial oscillations. The wave radiation and dissipation at equilibrium is therefore the result of both geostrophic flow and inertial oscillations and differs substantially from the classical lee wave problem. The theoretical predictions are tested versus two-dimensional and three-dimensional high resolution numerical simulations with parameters representative of the Drake Passage region. Theory and fully nonlinear numerical simulations are used to estimate internal wave radiation from LADCP, CTD and topography data from two regions in the Southern Ocean: Drake Passage and the Southeast Pacific. The results show that radiation and dissipation of internal waves generated by geostrophic motions reproduce the magnitude and distribution of dissipation measured in the region.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1998

Description: A freshwater plume often forms when a river or an estuary discharges water onto the continental shelf. Freshwater plumes are ubiquitous features of the coastal ocean and usually leave a striking signature in the coastal hydrography. The present study combines both hydrographic data and idealized numerical simulations to examine how ambient currents and winds influence the transport and mixing of plume waters. The first portion of the thesis considers the alongshore transport of freshwater using idealized numerical simulations. In the absence of any ambient current, the downstream coastal current only carries a fraction of the discharged fresh water; the remaining fraction recirculates in a continually growing "bulge" of fresh water in the vicinity of the river mouth. The fraction of fresh water transported in the coastal current is dependent on the source conditions at the river mouth. The presence of an ambient current augments the transport in the plume so that its freshwater transport matches the freshwater source. For any ambient current in the same direction as the geostrophic coastal current, the plume will evolve to a steady-state width. A key result is that an external forcing agent is required in order for the entire freshwater volume discharged by a river to be transported as a coastal current. The next section of the thesis addresses the wind-induced advection of a river plume, using hydrographic data collected in the western Gulf of Maine. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to a few Rossby radii. Near-surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate an approximate Ekman balance within the plume. A significant correlation between alongshore currents and alongshore wind stress suggests that interfacial drag may be important. The final section of the thesis is an investigation of the advection and mixing of a surface-trapped river plume in the presence of an upwelling favorable wind stress, using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the crossshore Ekman transport. The thinned plume is susceptible to significant mixing due to the vertically sheared horizontal currents. The first order plume response is explained by Ekman dynamics and a Richardson number mixing criterion. Under a sustained wind event, the plume evolves to a quasi-steady, uniform thickness. The rate of mixing slowly decreases for longer times as the stratification in the plume weakens, but mixing persists under a sustained upwelling wind until the plume is destroyed. Mixing is most intense at the seaward plume front due to an Ekman straining mechanism in which the advection of cross-shore salinity gradients balances vertical mixing. The mean mixing rate observed in the plume is consistent with the mixing power law suggested by previous studies of I-D mixing, in spite of the two-dimensional dynamics driving the mixing in the plume.

Description: This research was funded by a National Science Foundation graduate fellowship, and Gulf of Maine Regional Marine Research Program grants UM-S227 and UM-S276.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1999

Description: Today, deep waters produced in the North Atlantic are exported through the western South Atlantic. Antarctic intermediate water AAJW also enters the Atlantic in this region. Circumpolar deep water (CDW) fills the depths below AAIW and above and below northern source waters. A depth transect of cores from 1567-3909 m water depth in the western South Atlantic are ideally located to monitor inter-ocean exchange of deep water, and variations in the relative strength of northern versus southern source water production. Last glacial maximum (LGM) Cd/Ca and δ13C data indicate a nutrient-depleted intermediate-depth water mass. In the mid-depth western South Atlantic, a simple conversion of LGM δ13C data suggests significantly less nutrient enrichment than LGM Cd/Ca ratios, but Cd/Ca and δ13C data can be reconciled when plotted in CdW/δ13C space. Paired LGM Cd/Ca and δ13C data from mid-depth cores suggest increasingly nutrient rich waters below 2000 m, but do not require an increase in Southern Ocean water contribution relative to today. Cd/Ca data suggest no glacial-interglacial change in the hydrography of the deepest waters ofthe region. To maintain relatively low Cd/Ca ratios low nutrients in the deepest western South Atlantic waters, and in CDW in general, during the LGM requires an increased supply ofnutrient-depleted glacial North Atlantic intermediate water (GNA1W) and/or nutrient-depleted glacial Subantarctic surface waters to CDW to balance reduced NADW contribution to CDW. LGM Cd/Ca and δ13C data suggest strong GNA1W influence in the western South Atlantic which in turn implies export of GNAIW from the Atlantic, and entrainment of GNA1W into the Antarctic Circumpolar current.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2018

Description: Many chemical constituents are removed from the ocean by attachment to settling particles, a process referred to as “scavenging.” Radioisotopes of thorium, a highly particle-reactive element, have been used extensively to study scavenging in the ocean. However, this process is complicated by the highly variable chemical composition and concentration of particles in oceanic waters. This thesis focuses on understanding the cycling of thorium as affected by particle concentration and particle composition in the North Atlantic. This objective is addressed using (i) the distributions 228,230,234Th, their radioactive parents, particle composition, and bulk particle concentration, as measured or estimated along the GEOTRACES North Atlantic Transect (GA03) and (ii) a model for the reversible exchange of thorium with particles. Model parameters are either estimated by inversion (chapter 2-4), or prescribed in order to simulate 230Th in a circulation model (chapter 5). The major findings of this thesis follow. In chapters 2 and 3, I find that the rate parameters of the reversible exchange model show systematic variations along GA03. In particular, 𝑘1, the apparent first-order rate "constant" of Th adsorption onto particles, generally presents maxima in the mesopelagic zone and minima below. A positive correlation between 𝑘1 and bulk particle concentration is found, consistent with the notion that the specific rate at which a metal in solution attaches to particles increases with the number of surface sites available for adsorption. In chapter 4, I show that Mn (oxyhydr)oxides and biogenic particles most strongly influence 𝑘1 west of the Mauritanian upwelling, but that biogenic particles dominate 𝑘1 in this region. In chapter 5, I find that dissolved 230Th data are best represented by a model that assumes enhanced values of 𝑘1 near the seafloor. Collectively, my findings suggest that spatial variations in Th radioisotope activities observed in the North Atlantic reflect at least partly variations in the rate at which Th is removed from the water column.

Description: This work was supported by the US National Science Foundation. Two US NSF grants have supported the research in this thesis (OCE-1232578 and OCE-155644).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1977

Description: Millimeter scale fluctuations in refractive index recorded with a freely sinking shadowgraph system are correlated with finestructure profiles of temperature, salinity and density and compared to models of ocean turbulence. Images with vertically aligned periodic structure, called bands, are identified as salt fingers, while others with chaotic structure are turbulent. Images are found on interfaces that are 1-10m thick and have gradients at least several times the mean. From 6 profiles in the Mediterranean Outflow region of the eastern North Atlantic between 1.0 and 1.9 km depth, 398 interfaces have been identified and a significant fraction (about 1/3) of these have detectable images. High contrast images, including bands, are most often found below warm, saline intrusions and within stepped structure where there is a regular sequence of homogeneous mixed layers separated by interfaces. As the interfacial salinity gradient increases in the sense that allows salt finger convection, the fraction of interfaces with images increases. The horizontal spacing of bands (~5 mm) is consistent with calculated salt finger diameter. The calculated and observed length of ocean salt fingers (10-20 cm) is a small fraction of the interface thickness. High levels of small scale variability in the shadowgraphs is reflected in high levels of variance in the finestructure band of the temperature spectra. The temperature gradient spectra have a slope of -1, indicative of turbulence affected by buoyancy forces, and there is a relative peak at a wavelength near the observed salt finger length. The high contrast images are found at interfaces within the enhanced mean salinity gradient below saline intrusions. For very strong salinity gradients there is a solitary interface with intense images, but for weaker mean gradients the convection takes the form of stepped structure. The steps may evolve from the solitary interface as the salinity gradient is run down by salt finger convection. This study identifies parts of the ocean where salt finger convection is prevalent and includes the first comprehensive description of salt fingers in the ocean. Existing models of salt fingers are evaluated in light of ocean observations, and models of ocean turbulence are compared to measurements.

Description: Support from ONR contract N00014-66-C0241 NR. 083-004 is also acknowledged.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1999

Description: The bottom boundary layer is an important dynamical region of shallow water flows. In this thesis, the problem of turbulent mixing in the coastal bottom boundary layer is investigated with a unique set of field measurements of velocity and sound speed that span a significant fraction of the boundary layer obtained over a six-week long period in the late summer of 1996 on the New England shelf. The energetics of the turbulent fluctuations are investigated by testing simplified budgets for turbulent kinetic energy and scalar variance. The turbulent kinetic energy budget is locally balanced while the scalar variance budget is not, probably due to turbulent diffusion. The direct effects of stratification are consistently significant only in the outer part of the boundary layer, where the flux Richardson number is approximately equal to a critical value of 0.2. Turbulence closure is investigated in terms of non-dimensional profiles of velocity and sound speed. Close to the bottom, the results are consistent with Monin-Obukhov similarity theory, while in the outer part of the boundary layer other scales including the height of the boundary layer are important for setting the turbulent length scale.

Description: My doctoral work was supported by the Office of Naval Research under grants N000149S10373 and N0001496109S3.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013

Description: Large-scale thermal forcing and freshwater fluxes play an essential role in setting temperature and salinity in the ocean. A number of recent estimates of the global oceanic freshwater balance as well as the global oceanic surface net heat flux are used to investigate the effects of heat- and freshwater forcing at the ocean surface. Such forcing induces changes in both density and density-compensated temperature and salinity changes (’spice’). The ratio of the relative contributions of haline and thermal forcing in the mixed layer is maintained by large-scale surface fluxes, leading to important consequences for mixing in the ocean interior. In a stratified ocean, mixing processes can be either along lines of constant density (isopycnal) or across those lines (diapycnal). The contribution of these processes to the total mixing rate in the ocean can be estimated from the large-scale forcing by evaluating the production of thermal variance, salinity variance and temperature-salinity covariance. Here, I use new estimates of surface fluxes to evaluate these terms and combine them to generate estimates of the production of density and spice variance under the assumption of a linear equation of state. As a consequence, it is possible to estimate the relative importance of isopycnal and diapycnal mixing in the ocean. While isopycnal and diapycnal processes occur on very different length scales, I find that the surface-driven production of density and spice variance requires an approximate equipartition between isopycnal and diapycnal mixing in the ocean interior. In addition, consideration of the full nonlinear equation of state reveals that surface fluxes require an apparent buoyancy gain (expansion) of the ocean, which allows an estimate of the amount of contraction on mixing due to cabbeling in the ocean interior.

Description: The author would like to acknowledge support from the National Aeronautics and Space Administration, grant #NNX12AF59G and the National Science Foundation, grant #OCE-0647949.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2017

Description: This thesis documents the origin, distribution, and fate of methane and several of its isotopic forms on Earth. Using observational, experimental, and theoretical approaches, I illustrate how the relative abundances of 12CH4, 13CH4, 12CH3D, and 13CH3D record the formation, transport, and breakdown of methane in selected settings. Chapter 2 reports precise determinations of 13CH3D, a “clumped” isotopologue of methane, in samples collected from various settings representing many of the major sources and reservoirs of methane on Earth. The results show that the information encoded by the abundance of 13CH3D enables differentiation of methane generated by microbial, thermogenic, and abiogenic processes. A strong correlation between clumped- and hydrogen-isotope signatures in microbial methane is identified and quantitatively linked to the availability of H2 and the reversibility of microbially-mediated methanogenesis in the environment. Determination of 13CH3D in combination with hydrogen-isotope ratios of methane and water provides a sensitive indicator of the extent of C–H bond equilibration, enables fingerprinting of methane-generating mechanisms, and in some cases, supplies direct constraints for locating the waters from which migrated gases were sourced. Chapter 3 applies this concept to constrain the origin of methane in hydrothermal fluids from sediment-poor vent fields hosted in mafic and ultramafic rocks on slow- and ultraslow-spreading mid-ocean ridges. The data support a hypogene model whereby methane forms abiotically within plutonic rocks of the oceanic crust at temperatures above ca. 300 C during respeciation of magmatic volatiles, and is subsequently extracted during active, convective hydrothermal circulation. Chapter 4 presents the results of culture experiments in which methane is oxidized in the presence of O2 by the bacterium Methylococcus capsulatus strain Bath. The results show that the clumped isotopologue abundances of partially-oxidized methane can be predicted from knowledge of 13C/12C and D/H isotope fractionation factors alone.

Description: The research activities documented in this thesis were made possible by grants to my advisor from the U.S. National Science Foundation (NSF award EAR-1250394), the National Aeronautics and Space Administration (NASA) Astrobiology Institute (NAI, University of Colorado, Boulder, CAN 7 under Cooperative Agreement NNA15BB02A), the Department of Energy (DOE, Small Business Innovation Research program, contract DE-SC0004575), the Alfred P. Sloan Foundation via the Deep Carbon Observatory, and a Shell Graduate Fellowship through the MIT Energy Initiative. I completed the bulk of the work in this thesis while being supported by a National Defense Science and Engineering Graduate (NDSEG) Fellowship awarded through the Office of Naval Research of the U.S. Department of Defense. The StanleyW.Watson Fellowship Fund provided support during my first summer term at WHOI.The Charles M. Vest Presidential Fellowship at MIT supported me in the first year of my Ph.D. studies. I received additional support that year through NSF award EAR-1159318 (to S. Ono and T. Bosak) and theWalter & Adel Hohenstein Graduate Fellowship of Phi Kappa Phi. The MIT Earth Resources Laboratory and PAOC Houghton Fund funded my attendance at several conferences.