ALBERT

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 September 2000

Description: This thesis investigates the physical mechanisms of air-water gas transfer through direct measurements of turbulence at the air--water interface. To enable this study, a new approach to the particle image velocimetry (PIV) technique is developed in order to quantify free-surface flows. Two aspects of this work are innovative. First, the use of a three-dimensional laser light cone and optical filtering of the camera allow for the motion of fluorescent flow tracers at the water surface to be isolated and measured. Validation experiments indicate that this measurement reflects the fluid motion within the upper few hundred microns. A key benefit to this approach is the ability to deal with deforming surfaces, provided the amplitudes are not prohibitively large. This feature was used in this thesis to explore the surface flow induced by mechanically generated waves. Second, a new hybrid PIV image processing algorithm was developed that provides high accuracy velocity estimation with improved computational efficiency. This algorithm combines the concepts of dynamic Fourier-domain cross-correlation with a localized direct multiplicative correlation. In order to explore relationships between free-surface hydrodynamics and air-water gas transfer, an oscillating grid-stirred tank was constructed. By its design, this tank can be managed for chemical cleanliness, offers an unobstructed free surface, and is suited for turbulent mixing and air--water gas-exchange studies. A series of acoustic Doppler velocimeter, PIV, and infrared imaging experiments are presented that characterize the flow for the grid forcing conditions studied. Results indicate that the flows are stationary and reasonably repeatable. In addition, the flows exhibit near-isotropic turbulence and are quasi-homogeneous in horizontal planes. Secondary circulations are revealed and investigated. Finally, PIV measurements of free-surface turbulence are performed with concurrent measurements of gas transfer in the grid tank for a range of turbulent mixing and surface conditions. Surface turbulence, vorticity, and divergence are all affected by the presence of a surface film, with significant effects realized for relatively small surface pressures. Results show that while a relationship between surface turbulence and the gas-transfer velocity is an obvious improvement over that found using an estimate of the bulk flow turbulence, this relationship is dependent on the flow regime. This is revealed through additional surface wave studies. However, the data from both the wave experiments and the grid turbulence experiments can be reconciled by a single relationship between the gas-transfer velocity and the 1/2-power of the surface divergence, which agrees with previous conceptual models. These results (1) further our understanding of interfacial transport processes, (2) demonstrate the important role of surface divergence in air-water gas exchange, and (3) relate, in a physically meaningful way, the interactions between surface renewal, surfactants, and gas transfer.

Description: I was supported as an Office of Naval Research Graduate Fellow. This assistance was the impetus for my pursuit of a doctoral degree and is gratefully acknowledged. Very special thanks also to the WHOI Ocean Ventures Fund Program, Mr. F. Thomas Westcott, and the WHOI Education Department for generous financial support over the past several years.

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 2000

Description: The thesis develops and demonstrates methods of classifying ocean processes using an underwater moving platform such as an Autonomous Underwater Vehicle (AUV). The "mingled spectrum principle" is established which concisely relates observations from a moving platform to the frequency-wavenumber spectrum of the ocean process. It clearly reveals the role of the AUV speed in mingling temporal and spatial information. For classifying different processes, an AUV is not only able to jointly utilize the time-space information, but also at a tunable proportion by adjusting its cruise speed. In this respect, AUVs are advantageous compared with traditional oceanographic platforms. Based on the mingled spectrum principle, a parametric tool for designing an AUVbased spectral classifier is developed. An AUV's controllable speed tunes the separability between the mingled spectra of different processes. This property is the key to optimizing the classifier's performance. As a case study, AUV-based classification is applied to distinguish ocean convection from internal waves. The mingled spectrum templates are derived from the MIT Ocean Convection Model and the Garrett-Munk internal wave spectrum model. To allow for mismatch between modeled templates and real measurements, the AUVbased classifier is designed to be robust to parameter uncertainties. By simulation tests on the classifier, it is demonstrated that at a higher AUV speed, convection's distinct spatial feature is highlighted to the advantage of classification. Experimental data are used to test the AUV-based classifier. An AUV-borne flow measurement system is designed and built, using an Acoustic Doppler Velocimeter (ADV). The system is calibrated in a high-precision tow tank. In February 1998, the AUV acquired field data of flow velocity in the Labrador Sea Convection Experiment. The Earth-referenced vertical flow velocity is extracted from the raw measurements. The classification test result detects convection's occurrence, a finding supported by more traditional oceanographic analyses and observations. The thesis work provides an important foundation for future work in autonomous detection and sampling of oceanographic processes.

Description: This thesis research has been funded by the Office of Naval Research (ONR) under Grants NOOOl4-95-1-1316, NOO0l4-97-1-0470, and by the MIT Sea Grant College Program under Grant NA46RG0434.

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 18, 1988

Description: We have contrived a regional model Φ(K, ω, η, φ, λ) for the distribution of low frequency variability energy in horizontal wavenumber, frequency, vertical mode and geography. We assume horizontal isotropy, Φ(K, ω, η, φ, λ) = 2πKψ(k, l, ω, η, φ, λ), with K designating the amplitude of total horizontal wavenumber. The parameters of Φ(K, ω, η, φ, λ) can be derived from observations: (i) satellite altimetry measurements yield the surface eddy kinetic energy wavenumber and frequency spectra and the geographic distribution of surface eddy kinetic energy magnitude, (ii) XBT measurements yield the temperature wavenumber spectra, (iii) current meter and thermistor measurements yield the frequency spectra of kinetic energy and temperature, (iv) tomographic measurements yield the frequency spectra of range— and depth—averaged temperature, and (v) the combination of satellite altimetry and current meter measurements yields the vertical partitioning of kinetic energy among dynamical modes. We assume the form of the geography—independent part of our model Φ(K, ω, η) ∝Kpωq. The observed kinetic energy and temperature wavenumber spectra suggest p = 3/2 at K 〈 K0 and p = —2 at K 〉 K0 for the barotropic mode, and p = —1/2 at K 〈 K0 and p = —3 at K 〉 K0 for the baroclinic mods, where K0 is the transitional wavenumber of the wavenumber spectra. The observed frequency spectra of temperature and kinetic energy suggest that q = —1/2 for ω 〈 ω0 and q = —2 for ω 〉 ω0, where ω0 is the transitional frequency of the frequency spectra. The combination of satellite altimetry and current meter measurements suggests the vertical structure of the low frequency variability is governed by the first few modes. The geography—dependent part of our model is the energy magnitude. Although we have shown analytically that the tomographic measurements behave as a low—pass filter, it is impossible to identify this filtering effect in the real data due to the strong geographic variability of the energy magnitude and the vertical gradient of the mean temperature. The model wavenumber spectrum is appropriate only where the statistical properties are relatively homogeneous in space.

Description: My first year in the Joint Program was supported by the National Science Foundation under grant OCE 92-16628, then were supported by the National Science Foundation under grant OCE 95-29545.

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 1996

Description: This thesis addresses the question of how a highly energetic eddy field could be generated in the interior of the ocean away from the swift boundary currents. The energy radiation due to the temporal growth of non-trapped (radiating) disturbances in such a boundary current is thought to be one of the main sources for the described variability. The problem of stability of an energetic current, such as the Gulf Stream, is formulated. The study then focuses on the ability of the current to support radiating instabilities capable of significant penetration into the far-field and their development with time. The conventional model of the Gulf Stream as a zonal current is extended to allow the jet axis to make an angle to a latitude circle. The linear stability of such a nonzonal flow, uniform in the along-jet direction on a beta-plane, is first studied. The stability computations are performed for piece-wise constant and continuous velocity profiles. New stability properties of nonzonal jets are discussed. In particular, the destabilizing effect of the meridional tilt of the jet axis is demonstrated. The radiating properties of nonzonal currents are found to be very different from those of zonal currents. In particular, purely zonal flows do not support radiating instabilities, whereas flows with a meridional component are capable of radiating long and slowly growing waves. The nonlinear terms are then included in the consideration and the effects of the nonlinear interactions on the radiating properties of the solution are studied in detail. For these purposes, the efficient numerical code for solving equation for the QG potential vorticity with open boundary conditions of Orlanski's type is constructed. The results show that even fast growing linear solutions, which are trapped during the linear stage of developement, can radiate energy in the nonlinear regime if the basic current is nonzonal. The radiation starts as soon as the initial fast exponential growth significantly slows. The initial trapping of those solutions is caused by their fast temporal growth. The new mechanism for radiation is related to the nonzonality of a current.

Description: This work was supported by NSF Grant OCE 9301845.

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 April, 1977

Description: A total of four moorings from POLYMODE Array I and II were analyzed in an investigation of internal wavefield-mean flow interactions. In particular, evidence for wave-mean flow interaction was sought by searching for time correlations between the wavefield vertically-acting Reynolds stress (estimated using the temperature and velocity records), and the mean shear. No significant stress-shear correlations were found at the less energetic moorings, indicating that the magnitude of the eddy viscosity was under 200 cm2/sec, with the sign of the energy transfer uncertain. This is considerably below the 0(4500 cm2/sec) predicted by Müller (1976). An extensive error analysis indicates that the large wave stress predicted by the theory should have been clearly observable under the conditions of measurement. Theoretical computations indicate that the wavefield "basic state" may not be independent of the mean flow as assumed by Müller, but can actually be modified by large-scale vertical shear and still remain in equilibrium. In that case, the wavefield does not exchange momentum with a large-scale vertical shear flow, and, excepting critical layer effects, a small vertical eddy viscosity is to be expected. Using the Garrett-Munk (1975) model internal wave spectrum, estimates were made of the maximum momentum flux (stress) expected to be lost to critical layer absorption. Stress was found to increase almost linearly with the velocity difference across the shear zone, corresponding to a vertical eddy viscosity of -100 cm2 s -1. Stresses indicative of this effect were not observed in the data. The only significantly non-zero stress correlations were found at the more energetic moorings. Associated with the 600 m mean velocity and the shear at the thermocline were a positively correlated stress at 600 m, and a negatively correlated stress at 1000 m. These stress correlations were most clearly observable in the frequency range corresponding to 1 to 8 hour wave periods. The internal wavefield kinetic and potential energy were modulated by the mean flow at both levels, increasing by a factor of two with a factor of ten in the mean flow. The observed stress correlations and energy level changes were found to be inconsistent with ideas of a strictly local eddy viscosity, in which the spectrum of waves is only slightly modified by the shear. When Doppler effects in the temperature equation used to estimate vertical velocity were considered, the observations of stress and energy changes were found to be consistent with generation of short (0.4 to 3 km) internal waves at the level of maximum shear, about 800 m. The intensity of the generated waves increases with the shear, resulting in an effective vertical eddy viscosity (based on the main thermocline shear) of about +100 cm2 s-1 The stresses were not observable at the 1500 m level, indicating that the waves were absorbed within 500 m of vertical travel. The tendency for internal wave currents to be horizontally anisotropic in the presence of a mean current was investigated. Using the Garrett- Munk (1975) model internal wave spectrum, it was found that critical layer absorption cannot induce anisotropies as large as observed. A mechanical noise problem was found to be the cause of large anisotropies measured with Geodyne 850 current meters. It could not be decided, however, whether or not the A.M.F. Vector Averaging Current Meter is able to satisfactorily remove the noise with its averaging scheme.

Description: The research reported here was provided by Office of Naval Research Contract Numer N00014-76-C-0197 NR 083-400.

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 August, 1977

Description: Stimulated by new evidence from both "in situ" oceanic observations and results from numerical modelling, a laboratory study of quasigeostrophic flow and turbulence in a rotating homogeneous fluid has been undertaken. Two dimensional turbulence driven by a uniform distribution of sources and sinks which oscillate in time, can be fairly well reproduced in this context. Inertial time scales are about ten times smaller than Ekman spinup time, and typical Reynolds numbers read 2000. The observations emphasize the spectral tendency of the energy containing eddies. The case of no topography is first discussed. In steadily forced turbulence, it is observed that the energy containing scale is significantly larger than the forcing scale. In the decaying stage the red cascade is observed and rates of interaction are measured. Theoretical arguments for both behaviors are presented; the former concerning the forced turbulence case is believed to be new. The forcing is next applied over various large scale topographies, modelling the geophysical beta effect. The polar beta plane geometry preserves the above spectral characteristics but at the same time introduces anisotropy into the flow pattern. A broad westward mean flow develops in the north and is surrounded by a belt of cyclones lying on its southward side. The calculated second-order Eulerian mean flows induced by steadily and uniformly forced Rossby waves in a long zonal channel, exhibit much of the same momentum distribution in the inertial regime. In contrast, the "sliced cylinder" geometry which possesses no closed geostrophic contours drastically modifies the above picture. Both mean flow production and a large scale tendency for the eddies are inhibited. The geographical distribution of the eddy intensities and scales is now wildly inhomogeneous. The second aspect of this work is a study of the interaction of Rossby waves with mean flows. A zonally traveling, forced wave is generated near the southern boundary of a polar beta plane. Due to energy radiation in the free interior and (or) potential vorticity mixing by the finite amplitude waves, a westward zonal flow develops. The effect of the mean flow upon the forced steady waves is to weaken the anticyclones and intensify the cyclones. Pressure time series reveal a growth of harmonics and general spectral broadening as the waves travel freely inwards, suggesting active nonlinear interactions. An experimental test of Rhines' (1977) potential vorticity mixing theory is also presented at free latitudes. The decay period when the driving is suppressed shows that a net transfer from the waves to the mean flow kinetic energy occurs. Connection with hydrodynamic stability theory is discussed. Interaction of Rossby waves with an externally generated westward mean flow allows one to make a controlled study of the critical layer problem. For small amplitude waves, the mean flow is accelerated in the entire region between the forcing and the critical latitude which acts as a wall for mean wave momentum. In nonlinear runs the steady profile of the westward flow indicates that an accelerating force is acting everywhere, revealing the increasing transmission of wave momentum through the critical layer. At the same time, pressure measurements near the critical point show considerable fine structure developing over a long time scale. The third part deals with steady isolated source-sink flows in the sliced cylinder geometry. The response of the fluid to a meridionally oriented steady dipole extends exclusively westward of the forcing. The viscously balanced solutions are discussed and relevance to oceanic abyssal circulation is emphasized. With strong driving, the combination of a cyclone to the north and an anticyclone to the south is absolutely stable although the reverse configuration is not. A connection with a certain class of free, steady, isolated, inertial solutions developed recently by Stern (1976) is made.

Description: The DGRST . (FRACE) and the Joint Program in Oceanography, Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution offered a fellowship for the first two years. The National Science Foundation under Grant OCE75-2l 674 and the Office of Naval Research under Contract N00014-74-C0262-NR-083-004 supported this study for the final two years.

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 August 1984

Description: Intermittent, shoreward propagating packets of high frequency first mode internal waves are common on the continental shelf when the water column is stratified and may induce large fluctuations in near bottom velocity. Simple theoretical considerations here lead to an approximate method for estimating those quantities of most interest for the bottom boundary layer interaction problem. Examination of data from the pilot Coastal Ocean Dynamics Experiment (CODE I) shows that near bottom velocity fluctuations in the high frequency internal wave band were dominated by shoreward propagating, intermittent mode 1 internal events. Predictions of CODE I internal wave characteristics using the above approximate method are shown to be good. A boundary layer model is developed, which allows for the nonlinear interaction of surface waves, internal waves, and a steady current over a rough bottom. Modeling results suggest that internal waves will significantly enhance the stress felt by the steady current, and can increase the variability and decrease the reliability of boundary layer measurements by the "log profile" technique, when the waves are present. Theoretical dissipation of internal wave energy in the bottom boundary layer is found to be significantly enhanced in the presence of surface waves and currents, and may be important to the overall internal wave energy balance on the shelf.

Description: My doctoral work was supported for the first three years by an NSF Graduate Fellowship and has been supported since under NSF grant OCE-8014938.

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.