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 August 1987

Description: Several problems connected by the theme of thermal forcing are addressed herein. The main topic is the stratification and flow field resulting from imposing a specified heat flux on a fluid that is otherwise confined to a rigid insulating basin. In addition to the traditional eddy viscosity and diffusivity, turbulent processes are also included by a convective overturning adjustment at locations where the local density field is unstable. Two classes of problems are treated. The first is the large scale meridional pattern of a fluid in an annulus. The detailed treatment is carried out in two steps. In the beginning (chapter 2) it is assumed that the fluid is very diffusive, hence, to first approximation no flow field is present. It is found that the convective overturning adjustment changes the character of the stratification in all the regions that are cooled from the top, resulting in a temperature field that is nearly depth independent in the northernmost latitudes. The response to a seasonal cycle in the forcing, and the differences between averaging the results from the end of each season compared to driving the fluid by a mean forcing are analyzed. In particular, the resulting sea surface temperature is warmer in the former procedure. This observation is important in models where the heat flux is sensitive to the gradient of air to sea surface temperatures. The analysis of the problem continues in chapter 5 where the contribution of the flow field is included in the same configuration. The dimensionless parameter controlling the circulation is now the Rayleigh number, which is a measure of the relative importance of gravitational and viscous forces. The effects of the convective overturning adjustment is investigated at different Rayleigh numbers. It is shown that not only is the stratification now always stable, but also that the vigorous vertical mixing reduces the effective Rayleigh number; thereby the flow field is more moderate, the thermocline deepens, and the horizontal surface temperature gradients are weaker. The interior of the fluid is colder compared to cases without convective overturning, and, because the amount of heat in the system is assumed to be fixed, the surface temperature is warmer. The fluid is not only forced by a mean heat flux, or a seasonally varying one, but its behavior under permanent winter and summer conditions is also investigated. A steady state for the experiments where the net heat flux does not vanish is defined as that state where the flow field and temperature structure are not changing with time except for an almost uniform temperature decrease or increase everywhere. It is found that when winter conditions prevail the circulation is very strong, while it is rather weak for continuous summer forcing. In contrast to those results, if a yearly cycle is imposed, the circulation tends to reach a minimum in the winter time and a maximum in the summer. This suggests that, depending on the Rayleigh number, there is a phase leg of several months between the response of the ocean and the imposed forcing. Differences between the two averaging procedures mentioned before are also observed when the flow field is present, especially for large Rayleigh numbers. The circulation is found to be weaker and the sea surface temperature colder in the mean of the seasonal realizations compared to the steady state derived by the mean forcing. As an extension to the numerical results, an analytic model is presented in chapter 4 for a similar annular configuration. The assumed dynamics is a bit different, with a mixed layer on top of a potential vorticity conserving interior. It is demonstrated that the addition of the thermal wind balance to the conservation of potential vorticity in the axially symmetric problem leads to the result that typical fluid trajectories in the interior are straight lines pointing downward going north to south. The passage of information in the system is surprisingly in the opposite sense to the clockwise direction of the flow. A model for water mass formation by buoyancy loss in the absence of a flow field is introduced in chapter 3. The idea behind it is to use the turbulent mixing parameterization to generate chimney-like structures in open water, followed by along-isopycnal advection and diffusion. This model can be applied to many observations of mode water. In particular, in this work it is related to the chimneys observed by the MEDOC Group (1970), and the Levantine Intermediate Water in the Eastern Mediterranean Basin. An analytic prediction of the depth of the water mass is derived and depends on the forcing and initial stratification. It suggests that the depth of shallow mode water like the 18°C water or the Levantine Intermediate Water would not be very sensitive to reasonable changes in atmospheric forcing. Similar conclusions were also reached by Warren (1972) by assuming that the temperature in the thermocline decreases linearly with depth, and by approximating the energy balance in a water column by a Newtonian cooling law.

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 2010

Description: 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.

Description: This work was supported by National Science Foundation grants OCE-0241354 and OCE-0424865, as well as the Woods Hole Oceanographic Institution's Ocean and Cli- mate Change Institute. Funding to initiate the McLane Moored Pro ler observations at Line W were provided by grants from the G. Unger Vetlesen Foundation and the Comer Charitable Fund to the Woods Hole Oceanographic Institutions Ocean and Climate Change Institute.

Description: Author Posting. © American Meteorological Society, 2010. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 40 (2010): 789-801, doi:10.1175/2009JPO4039.1.

Description: The issue of internal wave–mesoscale eddy interactions is revisited. Previous observational work identified the mesoscale eddy field as a possible source of internal wave energy. Characterization of the coupling as a viscous process provides a smaller horizontal transfer coefficient than previously obtained, with vh 50 m2 s−1 in contrast to νh 200–400 m2 s−1, and a vertical transfer coefficient bounded away from zero, with νυ + (f2/N2)Kh 2.5 ± 0.3 × 10−3 m2 s−1 in contrast to νυ + (f2/N2)Kh = 0 ± 2 × 10−2 m2 s−1. Current meter data from the Local Dynamics Experiment of the PolyMode field program indicate mesoscale eddy–internal wave coupling through horizontal interactions (i) is a significant sink of eddy energy and (ii) plays an O(1) role in the energy budget of the internal wave field.

Description: Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 39 (2009): 2910-2925, doi:10.1175/2009JPO4139.1.

Description: The propagation of Rossby waves on a midlatitude β plane is investigated in the presence of density diffusion with the aid of linear hydrostatic theory. The search for wave solutions in a vertically bounded medium subject to horizontal (vertical) diffusion leads to an eigenvalue problem of second (fourth) order. Exact solutions of the problem are obtained for uniform background stratification (N), and approximate solutions are constructed for variable N using the Wentzel–Kramers–Brillouin method. Roots of the eigenvalue relations for free waves are found and discussed. The barotropic wave of adiabatic theory is also a solution of the eigenvalue problem as this is augmented with density diffusion in the horizontal or vertical direction. The barotropic wave is undamped as fluid parcels in the wave move only horizontally and are therefore insensitive to the vortex stretching induced by mixing. On the other hand, density diffusion modifies the properties of baroclinic waves of adiabatic theory. In the presence of horizontal diffusion the baroclinic modes are damped but their vertical structure remains unaltered. The ability of horizontal diffusion to damp baroclinic waves stems from its tendency to counteract the deformation of isopycnal surfaces caused by the passage of these waves. The damping rate increases (i) linearly with horizontal diffusivity and (ii) nonlinearly with horizontal wavenumber and mode number. In the presence of vertical diffusion the baroclinic waves suffer both damping and a change in vertical structure. In the long-wave limit the damping is critical (wave decay rate numerically equal to wave frequency) and increases as the square roots of vertical diffusivity and zonal wavenumber. Density diffusion in the horizontal or vertical direction reduces the amplitude of the phase speed of westward-propagating waves. Observational estimates of eddy diffusivities suggest that horizontal and vertical mixing strongly attenuates baroclinic waves in the ocean but that vertical mixing is too weak to notably modify the vertical structure of the gravest modes.

Description: This work was supported by the U.S. National Science Foundation.

Description: Author Posting. © American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 38 (2008): 1091-1106, doi:10.1175/2007JPO3805.1.

Description: A model of deep ocean circulation driven by turbulent mixing is produced in a long, rectangular laboratory tank. The salinity difference is substituted for the thermal difference between tropical and polar regions. Freshwater gently flows in at the top of one end, dense water enters at the same rate at the top of the other end, and an overflow in the middle removes the same amount of surface water as is pumped in. Mixing is provided by a rod extending from top to bottom of the tank and traveling back and forth at constant speed with Reynolds numbers 〉500. A stratified upper layer (“thermocline”) deepens from the mixing and spreads across the entire tank. Simultaneously, a turbulent plume (“deep ocean overflow”) from a dense-water source descends through the layer and supplies bottom water, which spreads over the entire tank floor and rises into the upper layer to arrest the upper-layer deepening. Data are taken over a wide range of parameters and compared to scaling theory, energetic considerations, and simple models of turbulently mixed fluid. There is approximate agreement with a simple theory for Reynolds number 〉1000 in experiments with a tank depth less than the thermocline depth. A simple argument shows that mixing and plume potential energy flux rates are equal in magnitude, and it is suggested that the same is approximately true for the ocean.

Description: The research was supported by the Ocean Climate Change Institute of Woods Hole Oceanographic Institution.

Description: Author Posting. © American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 38 (2008): 2556-2574, doi:10.1175/2008JPO3666.1.

Description: Vertical profiles of horizontal velocity obtained during the Mid-Ocean Dynamics Experiment (MODE) provided the first published estimates of the high vertical wavenumber structure of horizontal velocity. The data were interpreted as being representative of the background internal wave field, and thus, despite some evidence of excess downward energy propagation associated with coherent near-inertial features that was interpreted in terms of atmospheric generation, these data provided the basis for a revision to the Garrett and Munk spectral model. These data are reinterpreted through the lens of 30 years of research. Rather than representing the background wave field, atmospheric generation, or even near-inertial wave trapping, the coherent high wavenumber features are characteristic of internal wave capture in a mesoscale strain field. Wave capture represents a generalization of critical layer events for flows lacking the spatial symmetry inherent in a parallel shear flow or isolated vortex.

Description: Salary support for this analysis was provided by Woods Hole Oceanographic Institution bridge support funds.

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: Author Posting. © American Meteorological Society, 2010. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 40 (2010): 2605–2623, doi:10.1175/2010JPO4132.1.

Description: Steady scale-invariant solutions of a kinetic equation describing the statistics of oceanic internal gravity waves based on wave turbulence theory are investigated. It is shown in the nonrotating scale-invariant limit that the collision integral in the kinetic equation diverges for almost all spectral power-law exponents. These divergences come from resonant interactions with the smallest horizontal wavenumbers and/or the largest horizontal wavenumbers with extreme scale separations. A small domain is identified in which the scale-invariant collision integral converges and numerically find a convergent power-law solution. This numerical solution is close to the Garrett–Munk spectrum. Power-law exponents that potentially permit a balance between the infrared and ultraviolet divergences are investigated. The balanced exponents are generalizations of an exact solution of the scale-invariant kinetic equation, the Pelinovsky–Raevsky spectrum. A small but finite Coriolis parameter representing the effects of rotation is introduced into the kinetic equation to determine solutions over the divergent part of the domain using rigorous asymptotic arguments. This gives rise to the induced diffusion regime. The derivation of the kinetic equation is based on an assumption of weak nonlinearity. Dominance of the nonlocal interactions puts the self-consistency of the kinetic equation at risk. However, these weakly nonlinear stationary states are consistent with much of the observational evidence.

Description: This research is supported by NSF CMG Grants 0417724, 0417732 and 0417466. YL is also supported by NSF DMS Grant 0807871 and ONR Award N00014-09-1-0515.

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 1997

Description: In order to constrain the processes controlling the cycles of biogeochemically important gases such as 02 and C02, and thereby infer rates of biological activity in the upper ocean or the uptake of radiatively important "greenhouse" gases, the noble gases are used to characterize and quantify the physical processes affecting the dissolved gases in aquatic environments. The processes of vertical mixing, gas exchange, air injection, and radiative heating are investigated using a 2 year time-series of the noble gases, temperature, and meteorological data from Station S near Bermuda, coupled with a 1- dimensional upper ocean mixing model to simulate the physical processes in the upper ocean. The rate of vertical mixing that best simulates the thermal cycle is 1.1±0.1 x104 m The gas exchange rate required to simulate the data is consistent with the formulation of Wanninkhof (1992) to ± 40%, while the formulation of Liss and Merlivat 1986 must be increased by a factor of 1.7± 0.6. The air injection rate is consistent with the formulation of Monahan and Torgersen (1991) using an air entrainment velocity of 3±1 cm s1. Gas flux from bubbles is dominated on yearly time-scales by larger bubbles that do not dissolve completely, while the bubble flux is dominated by complete dissolution of bubbles in the winter at Bermuda. In order to obtain a high-frequency time-series of the noble gases to better parameterize the gas flux from bubbles, a moorable, sequential noble gas sampler was developed. Preliminary results indicate that the sampler is capable of obtaining the necessary data. Dissolved gas concentrations can be significantly modified by ice formation and melting, and due to the solubility of He and Ne in ice, the noble gases are shown to be unique tracers of these interactions. A three-phase equilibrium partitioning model was constructed to quantify these interactions in perennially ice-covered Lake Fryxell, and this work was extended to oceanic environments. Preliminary surveys indicate that the noble gases may provide useful and unique information about interactions between water and ice.

Description: This work has been supported by the National Science Foundation - OCE 9302812 and DPP 9118363, the Ocean Ventures Fund Ditty Bag Fund and Westcott Fund and the WHOI Education Office.

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 September 2011

Description: Remote sensing and in situ observations are used to investigate the ocean response to the Tokar Wind Jet in the Red Sea. The wind jet blows down the pressure gradient through the Tokar Gap on the Sudanese coast, at about 18°N, during the summer monsoon season. It disturbs the prevailing along-sea (southeastward) winds with strong cross-sea (northeastward) winds that can last from days to weeks and reach amplitudes of 20-25 m/s. By comparing scatterometer winds with along-track and gridded sea level anomaly observations, it is shown that an intense dipolar eddy spins up in less than seven days in response to the wind jet. The eddy pair has a horizontal scale of 140 km. Maximum ocean surface velocities can reach 1 m/s and eddy currents extend at least 200 m into the water column. The eddy currents appear to cover the width of the sea, providing a pathway for rapid transport of marine organisms and other drifting material from one coast to the other. Interannual variability in the strength of the dipole is closely matched with variability in the strength of the wind jet. The dipole is observed to be quasi-stationary, although there is some evidence for slow eastward propagation—simulation of the dipole in an idealized high-resolution numerical model suggests that this is the result of self-advection. These and other recent in situ observations in the Red Sea show that the upper ocean currents are dominated by mesoscale eddies rather than by a slow overturning circulation.

Description: This work is supported by Award Nos. USA 00002, KSA 00011 and KSA 00011/02 made by King Abdullah University of Science and Technology (KAUST).