ALBERT

Description: We study the quasi-geostrophic merging dynamics of axisymmetric baroclinic vortices to understand how baroclinicity affects merging rates and the development of the nonlinear cascade of enstrophy. The initial vortices are taken to simulate closely the horizontal' and vertical structure of Gulf Stream rings. A quasigeostrophic model is set with a horizontal resolution of 9 km and 6 vertical levels to resolve the mean stratification of the Gulf Stream region. The results show that the baroclinic merging is slower than the purely barotropic process, The merging is shown to occur in two phases: the tirst, which produces clove-shaped vortices and diffusive mixing of vorticity contours; and the second, which consists of the sliding of the remaining vorticity cores with a second diffusive mixing of the intemal vorticity field. Comparison among Nof, Cushman-Roisin, Polvani et al, and Dewar and Killworth merging events indicates a substantial agreement in the kinematics of the DYOCRSS. Parameter sensitivity experiments show that the decrease of the baroclinicity parameter of the system, Γ^2, [defined as Γ^2 = (D^2 fo^2)/ (No^2 H^2)], increases the speed of merging while its increase slows down the merging. However, the halting elfect of baroclinicity (large Γ^2 or small Rossby radii of deformation) reaches a saturation level where the merging becomes insensitive to larger F2 values. Furthermore, we show that a regime of small Γ^2 exists at which the merged baroclinic vortex is unstable (metastable) and breaks again into two new vortices, Thus, in the baroelinic case the range of Γ^2 detemines the stability of the merged vortex. We analyze these results by local energy and vorticity balances, showing that the horizontal divergence of pressure work term [∇ *(pv)] and the relative-vorticity advection term (v * ∇ (∇ ^2 φ) trigger the merging during the first phase. Due to this horizontal redistribution process, a net kinetic to gravitational energy conversion occurs via buoyancy work in the region external to the cores of the vortices. The second phase of merging is dominated by a direct baroclinic conversion of available gravitational energy into kinetic energy, which in tum triggers a horizontal energy redistribution producing the final fusion of the vortex centers. This energy and vorticity analysis supports the hypothesis that merging is an internal mixing process triggered by a horizontal redistribution of kinetic energy.

Description: The work has been financed by a grant from the Progetto Finalizzato "Calcolo Parallelo"

Description: Published

Description: 1618/1637

Description: 4A. Clima e Oceani

Description: JCR Journal

Description: restricted

Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C06014, doi:10.1029/2006JC003947.

Description: In aerial surveys conducted during the Tropical Ocean–Global Atmosphere Coupled Ocean-Atmosphere Response Experiment and the low-wind component of the Coupled Boundary Layer Air-Sea Transfer (CBLAST-Low) oceanographic field programs, sea surface temperature (SST) variability at relatively short spatial scales (O(50 m) to O(1 km)) was observed to increase with decreasing wind speed. A unique set of coincident surface and subsurface oceanic temperature measurements from CBLAST-Low is used to investigate the subsurface expression of this spatially organized SST variability, and the SST variability is linked to internal waves. The data are used to test two previously hypothesized mechanisms for SST signatures of oceanic internal waves: a modulation of the cool-skin effect and a modulation of vertical mixing within the diurnal warm layer. Under conditions of weak winds and strong insolation (which favor formation of a diurnal warm layer), the data reveal a link between the spatially periodic SST fluctuations and subsurface temperature and velocity fluctuations associated with oceanic internal waves, suggesting that some mechanism involving the diurnal warm layer is responsible for the observed signal. Internal-wave signals in skin temperature very closely resemble temperature signals measured at a depth of about 20 cm, indicating that the observed internal-wave SST signal is not a result of modulation of the cool-skin effect. Numerical experiments using a one-dimensional upper ocean model support the notion that internal-wave heaving of the warm-layer base can produce alternating bands of relatively warm and cool SST through the combined effects of surface heating and modulation of wind-driven vertical shear.

Description: We gratefully acknowledge funding for this research from the Office of Naval Research through the CBLAST Departmental Research Initiative (grants N00014-01-1-0029, N00014-05-10090, N00014-01-1-0081, N00014-04-1-0110, N00014-05-1-0036, N00014-01-1-0080) and the Secretary of the Navy/Chief of Naval Operations Chair (grant N00014-99-1-0090).

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 2003

Description: Inertial terms dominate the single-gyre ocean model and prevent western-intensification when the viscosity is small. This occurs long before the oceanically-appropriate parameter range. It is demonstrated here that the circulation is controlled if a mechanism for ultimate removal of vorticity exists, even if it is active only in a narrow region near the boundary. Vorticity removal is modeled here as a viscosity enhanced very near the solid boundaries to roughly parameterize missing boundary physics like topographic interaction and three dimensional turbulence over the shelf. This boundary-enhanced viscosity allows western-intensified mean flows even when the inertial boundary width, is much wider than the frictional region because eddies flux vorticity from within the interior streamlines to the frictional region for removal. Using boundary-enhanced viscosity, western-intensified calculations are possible with lower interior viscosity than in previous studies. Interesting behaviors result: a boundary-layer balance novel to the model, calculations with promise for eddy parameterization, eddy-driven gyres rotating opposite the wind, and temporal complexity including basin resonances. I also demonstrate that multiple-gyre calculations have weaker mean circulation than single-gyres with the same viscosity and subtropical forcing. Despite traditional understanding, almost no inter-gyre flux occurs if no-slip boundary conditions are used. The inter-gyre eddy flux is in control only with exactly symmetric gyres and free slip boundaries. Even without the inter-gyre flux, the multiple-gyre circulation is weak because of sinuous instabilities on the jet which are not present in the single-gyre model. These modes efficiently flux vorticity to the boundary and reduce the circulation without an inter-gyre flux, postponing inertial domination to much smaller viscosities. Then sinuous modes in combination with boundary-enhanced viscosity can control the circulation.

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 Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 2001

Description: A comparison of monthly biogeochemical measurements made from 1993 to 1995, combined with hydrography and satellite altimetry, was used to observe the impacts of nine eddy events on primary productivity and particle flux in the Sargasso Sea. Measurements of primary production, thorium-234 flux, nitrate+nitrite, and photosynthetic pigments made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site were used. During the three years of this study, four out of six high thorium- 234 flux events over 1000 dpm/m2/d occurred during the passage of an eddy. Primary production nearly as high as the spring bloom maximum was observed in two modewater eddies (May 1993 and July 1995). The 1994 spring bloom at BATS was suppressed by the passage of an anticyclone. Distinct phytoplankton community shifts were observed in mode-water eddies, which had an increased percentage diatoms and dinoflagelletes, and in cyclones, which had an increased percentage cyanobacteria (excluding Prochlorococcus). The difference in the observations of mode-water eddies and cyclones may result from the age of the eddy, which was very important to the biological response. In general, eddies that were one to two months old elicited a large biological response; eddies that were three months old may show a biological response and were accompanied by high thorium flux measurements; eddies that were four months old or older did not show a biological response or high thorium flux. Our conceptual model depicting the importance of temporal changes during eddy upwelling and decay fit the observations well in all 7 upwelling eddies. Additional information is needed to determine the importance of deeper mixed layers and winter mixing to the magnitude of the eddy impacts. Also, sampling generally captured only the beginning, end, and lor edge of an eddy due to the monthly to semi-monthly frequency of the measurements made at BATS. Lagrangian studies, higher resolution time-series, and/or more spatial coverage is needed to provide additional information for improved C and N budgets in the Sargasso Sea and to complete our understanding of the temporal changes that occur in an eddy.

Description: Funding for this work was provided by NASA and NSF through the JGOFS Synthesis and Modeling Program.

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: This thesis addresses several aspects of the problem of determining the effect of the low-frequency eddy variability on the mean circulation of the Western North Atlantic. A framework for this study is first established by scale analysis of the eddy and mean terms in the mean momentum, vorticity, and heat balances in three regions of the Western North Atlantic -- the northern recirculation, the southern recirculation, and the mid-ocean. The data from the last decade of field experiments suggest somewhat different conclusions from the earlier analysis of Harrison (1980). In the momentum balance we confirm that the eddy terms are negligible compared to the lowest order mean geostrophic balance. The eddy term may be an 0(1) term in the vorticity balance only in the northern recirculation region where the mean flow is anisotropic. In the mean heat balance, if the mean temperature advection is scaled using the thermal wind relation, then the eddy heat flux is negligible in the mid-ocean, but it may be important in the recirculation areas. For all the balances the eddy terms are comparable to or an order of magnitude larger than the mean advective terms. We conclude from the scale analysis that the eddy field is most likely to be important in the Gulf Stream recirculation region. These balances are subsequently examined in more detail using data from the Local Dynamics Experiment (LDE). Several inconsistencies are first shown in McWilliams' (1983) model for the mean dynamical balances in the LDE. The sampling uncertainties do not allow us to draw conclusions about the long-term dynamical balances. However, it is shown that if we assume that the linear vorticity balance holds between the surface and the thermocline for a finite record, then the vertical velocity induced by the eddy heat flux divergence is non-zero. The local effect of the mesoscale eddy field on the mean potential vorticity distribution of the Gulf Stream recirculation region is determined from the quasigeostrophic eddy potential vorticity flux. This flux is calculated by finite difference of current and temperature time series from the Local Dynamics Experiment. This long-term array of moorings is the only experimental data from which the complete eddy flux can be calculated. The total eddy flux is dominated by the term due to the time variation in the thickness of isopycnal layers. This thickness flux is an order of magnitude larger than the relative vorticity flux. The total flux is statistically significant and directed 217° T to the southwest with a magnitude of 1.57 x 10 -5 cm/2s. The direction of the eddy flux with respect to the mean large scale potential vorticity gradient from hydrographic data indicates that eddies in this region tend to reduce the mean potential vorticity gradient. The results are qualitatively consistent with numerical model results and with other data from the Gulf Stream recirculation region. We find that the strength of the eddy transfer in the enstrophy cascade is comparable to the source terms in the mean enstrophy balance. The Austauch coefficient for potential vorticity mixing is estimated to be 0(107cm2/sec). An order of magnitude estimate of the enstrophy dissipation due only to the internal wave field shows that other processes must be important in enstrophy dissipation. The measured eddy potential vorticity fluxes are compared to the linear stability model of Gill, Green, and Simmons (1974). An earlier study (Hogg, 1984) has shown agreement between the empirical orthogonal modes of the data and the predicted wavenumbers, growth rates, and phase speeds of the most unstable waves. However, we show substantial disagreement in a comparison of the higher moments the eddy heat and potential vorticity fluxes. Because the critical layer of the model is located near the surface, the model predicts that most of the eddy potential vorticity and eddy heat flux should occur within about 300 meters of the surface. The data show much greater deep eddy heat flux than predicted by the model. It is suggested that the unstable modes in the ocean have a longer vertical scale because of the reduction in the buoyancy frequency near the surface. The evidence for in situ instability is also examined in the decay region of the Gulf Stream from an array of current and temperature recorders. Although there is vertical phase propagation in the empirical orthogonal modes for some of the variables at some of the moorings, there is not much evidence for a strong ongoing process of wave generation.

Description: This research has been conducted under NSF contract numbers OCE 77-19403, ATM 79-21431, and OCE 82-00154.

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, 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: Author Posting. © American Meteorological Society, 2011. 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 41 (2011): 889–910, doi:10.1175/2010JPO4496.1.

Description: This paper examines interaction between a barotropic point vortex and a steplike topography with a bay-shaped shelf. The interaction is governed by two mechanisms: propagation of topographic Rossby waves and advection by the forcing vortex. Topographic waves are supported by the potential vorticity (PV) jump across the topography and propagate along the step only in one direction, having higher PV on the right. Near one side boundary of the bay, which is in the wave propagation direction and has a narrow shelf, waves are blocked by the boundary, inducing strong out-of-bay transport in the form of detached crests. The wave–boundary interaction as well as out-of-bay transport is strengthened as the minimum shelf width is decreased. The two control mechanisms are related differently in anticyclone- and cyclone-induced interactions. In anticyclone-induced interactions, the PV front deformations are moved in opposite directions by the point vortex and topographic waves; a topographic cyclone forms out of the balance between the two opposing mechanisms and is advected by the forcing vortex into the deep ocean. In cyclone-induced interactions, the PV front deformations are moved in the same direction by the two mechanisms; a topographic cyclone forms out of the wave–boundary interaction but is confined to the coast. Therefore, anticyclonic vortices are more capable of driving water off the topography. The anticyclone-induced transport is enhanced for smaller vortex–step distance or smaller topography when the vortex advection is relatively strong compared to the wave propagation mechanism.

Description: Y. Zhang acknowledges the support of theMIT-WHOI Joint Programin Physical Oceanography, NSF OCE-9901654 and OCE-0451086. J. Pedlosky acknowledges the support of NSF OCE- 9901654 and OCE-0451086.

Description: Author Posting. © American Meteorological Society, 2011. 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 Climate 24 (2011): 4844–4858, doi:10.1175/2011JCLI4130.1.

Description: The factors that determine the heat transport and overturning circulation in marginal seas subject to wind forcing and heat loss to the atmosphere are explored using a combination of a high-resolution ocean circulation model and a simple conceptual model. The study is motivated by the exchange between the subpolar North Atlantic Ocean and the Nordic Seas, a region that is of central importance to the oceanic thermohaline circulation. It is shown that mesoscale eddies formed in the marginal sea play a major role in determining the mean meridional heat transport and meridional overturning circulation across the sill. The balance between the oceanic eddy heat flux and atmospheric cooling, as characterized by a nondimensional number, is shown to be the primary factor in determining the properties of the exchange. Results from a series of eddy-resolving primitive equation model calculations for the meridional heat transport, overturning circulation, density of convective waters, and density of exported waters compare well with predictions from the conceptual model over a wide range of parameter space. Scaling and model results indicate that wind effects are small and the mean exchange is primarily buoyancy forced. These results imply that one must accurately resolve or parameterize eddy fluxes in order to properly represent the mean exchange between the North Atlantic and the Nordic Seas, and thus between the Nordic Seas and the atmosphere, in climate models.

Description: This study was supported by the National Science Foundation under Grants OCE-0726339 and OCE-0850416.

Description: Author Posting. © American Meteorological Society, 2012. 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 42 (2012): 1524–1547, doi:10.1175/JPO-D-11-0117.1.

Description: Evidence is presented for the transfer of energy from low-frequency inertial–diurnal internal waves to high-frequency waves in the band between 6 cpd and the buoyancy frequency. This transfer links the most energetic waves in the spectrum, those receiving energy directly from the winds, barotropic tides, and parametric subharmonic instability, with those most directly involved in the breaking process. Transfer estimates are based on month-long records of ocean velocity and temperature obtained continuously over 80–800 m from the research platform (R/P) Floating Instrument Platform (FLIP) in the Hawaii Ocean Mixing Experiment (HOME) Nearfield (2002) and Farfield (2001) experiments, in Hawaiian waters. Triple correlations between low-frequency vertical shears and high-frequency Reynolds stresses, uiw∂Ui/∂z, are used to estimate energy transfers. These are supported by bispectral analysis, which show significant energy transfers to pairs of waves with nearly identical frequency. Wavenumber bispectra indicate that the vertical scales of the high-frequency waves are unequal, with one wave of comparable scale to that of the low-frequency parent and the other of much longer scale. The scales of the high-frequency waves contrast with the classical pictures of induced diffusion and elastic scattering interactions and violates the scale-separation assumption of eikonal models of interaction. The possibility that the observed waves are Doppler shifted from intrinsic frequencies near f or N is explored. Peak transfer rates in the Nearfield, an energetic tidal conversion site, are on the order of 2 × 10−7 W kg−1 and are of similar magnitude to estimates of turbulent dissipation that were made near the ridge during HOME. Transfer rates in the Farfield are found to be about half the Nearfield values.

Description: This work was supported by the National Science Foundation and the Office of Naval Research.

Description: 2013-03-01

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 February 2013

Description: 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 flow and banded structures in the low-frequency component of the eddy field. A combination of a realistic eddy-permitting ocean state estimate and simplified theoretical models is used to address the following specific questions. (1) What are the major spatial characteristics of eddy-mean flow interaction from an energy perspective? Is eddy-mean flow interaction a local process in most ocean regions? (2) The banded structures in the low-frequency eddy field 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 flow, influence the origin and characteristics of striations? How much do striations contribute to the energy budget and tracer mixing?

Description: This research was supported by the National Aeronautics and Space Administration contracts NNX09AI87G and NNX08AR33G.

Description: Author Posting. © American Meteorological Society, 2013. 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 43 (2013): 17–28, doi:10.1175/JPO-D-11-0108.1.

Description: Observational evidence is presented for transfer of energy from the internal tide to near-inertial motions near 29°N in the Pacific Ocean. The transfer is accomplished via parametric subharmonic instability (PSI), which involves interaction between a primary wave (the internal tide in this case) and two smaller-scale waves of nearly half the frequency. The internal tide at this location is a complex superposition of a low-mode waves propagating north from Hawaii and higher-mode waves generated at local seamounts, making application of PSI theory challenging. Nevertheless, a statistically significant phase locking is documented between the internal tide and upward- and downward-propagating near-inertial waves. The phase between those three waves is consistent with that expected from PSI theory. Calculated energy transfer rates from the tide to near-inertial motions are modest, consistent with local dissipation rate estimates. The conclusion is that while PSI does befall the tide near a critical latitude of 29°N, it does not do so catastrophically.

Description: This work was sponsored by NSF OCE 04-25283.

Description: 2013-07-01

Description: Author Posting. © American Meteorological Society, 2013. 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 43 (2013): 283–300, doi:10.1175/JPO-D-11-0240.1.

Description: Motivated by the recent interest in ocean energetics, the widespread use of horizontal eddy viscosity in models, and the promise of high horizontal resolution data from the planned wide-swath satellite altimeter, this paper explores the impacts of horizontal eddy viscosity and horizontal grid resolution on geostrophic turbulence, with a particular focus on spectral kinetic energy fluxes Π(K) computed in the isotropic wavenumber (K) domain. The paper utilizes idealized two-layer quasigeostrophic (QG) models, realistic high-resolution ocean general circulation models, and present-generation gridded satellite altimeter data. Adding horizontal eddy viscosity to the QG model results in a forward cascade at smaller scales, in apparent agreement with results from present-generation altimetry. Eddy viscosity is taken to roughly represent coupling of mesoscale eddies to internal waves or to submesoscale eddies. Filtering the output of either the QG or realistic models before computing Π(K) also greatly increases the forward cascade. Such filtering mimics the smoothing inherent in the construction of present-generation gridded altimeter data. It is therefore difficult to say whether the forward cascades seen in present-generation altimeter data are due to real physics (represented here by eddy viscosity) or to insufficient horizontal resolution. The inverse cascade at larger scales remains in the models even after filtering, suggesting that its existence in the models and in altimeter data is robust. However, the magnitude of the inverse cascade is affected by filtering, suggesting that the wide-swath altimeter will allow a more accurate determination of the inverse cascade at larger scales as well as providing important constraints on smaller-scale dynamics.

Description: BKA received support from Office of Naval Research Grant N00014-11-1-0487, National Science Foundation (NSF) Grants OCE-0924481 and OCE- 09607820, and University of Michigan startup funds. KLP acknowledges support from Woods Hole Oceanographic Institution bridge support funds. RBS acknowledges support from NSF grants OCE-0960834 and OCE-0851457, a contract with the National Oceanography Centre, Southampton, and a NASA subcontract to Boston University. JFS and JGR were supported by the projects ‘‘Global and remote littoral forcing in global ocean models’’ and ‘‘Agesotrophic vorticity dynamics of the ocean,’’ respectively, both sponsored by the Office of Naval Research under program element 601153N.

Description: 2013-08-01

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 Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2006

Description: The continuous supply of heat and fresh water from the boundaries to the interior of the Labrador Sea plays an important role for the dynamics of the region and in particular, for the Labrador Sea Water formation. Thus, it is necessary to understand the factors governing the exchange of properties between the boundary and interior. A significant fraction of heat and fresh water, needed to balance the annual heat loss and to contribute to the seasonal freshening of the Labrador Sea, is thought to be provided by coherent long-lived anticyclonic eddies shed by the Irminger Current. The population, some properties, rates and direction of propagation of these anomalies are known but the evolution and the mechanism of their decay are still far from obvious. In this work I investigated their water mass properties and evolution under the strong wintertime forcing using hydrographic data from 1990-2004 and a 1-dimensional mixed layer model. There were 50 eddies found in the hydrographic data record, 48 of which were identified as anticyclones. Vertical structure of the eddies was investigated, leading to the categorization of all the anticyclones into three classes: 12 - with a fresh surface layer and no mixed layer, 18 - without a fresh layer and at least one mixed layer, and 18 with ambiguous vertical structure. Four eddies of the second group appeared to have cores extending to as deep as 1500 m vertically and an isopycnal displacement of 400-600 m. A number of eddies without a fresh water cap contained Labrador Sea Water from the previous year at mid-depths.

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: 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): 880–895, doi:10.1175/2007JPO3750.1.

Description: The oceanic response to overflows is explored using a two-layer isopycnal model. Overflows enter the open ocean as dense gravity currents that flow along and down the continental slope. While descending the slope, overflows typically double their volume transport by entraining upper oceanic water. The upper oceanic layer must balance this loss of mass, and the resulting convergent flow produces significant vortex stretching. Overflows thus represent an intense and localized mass and vorticity forcing for the upper ocean. In this study, simulations show that the upper ocean responds to the overflow-induced forcing by establishing topographic β plumes that are aligned more or less along isobaths and that have a transport that is typically a few times larger than that of the overflows. For the topographic β plume driven by the Mediterranean overflow, the occurrence of eddies near Cape St. Vincent, Portugal, allows the topographic β plume to flow across isobaths. The modeled topographic β-plume circulation forms two transatlantic zonal jets that are analogous to the Azores Current and the Azores Countercurrent. In other cases (e.g., the Denmark Strait overflow), the same kind of circulation remains trapped along the western boundary and hence would not be readily detected.

Description: SK’s support during the time of his Ph.D. research in the MIT/WHOI Joint Program was provided by the National Science Foundation through Grant OCE04-24741. JP and JY have also received support from the Climate Process Team on Gravity Current Entrainment, NSF Grant OCE-0611530.

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): 380-399, doi:10.1175/2007JPO3728.1.

Description: Barotropic to baroclinic conversion and attendant phenomena were recently examined at the Kaena Ridge as an aspect of the Hawaii Ocean Mixing Experiment. Two distinct mixing processes appear to be at work in the waters above the 1100-m-deep ridge crest. At middepths, above 400 m, mixing events resemble their open-ocean counterparts. There is no apparent modulation of mixing rates with the fortnightly cycle, and they are well modeled by standard open-ocean parameterizations. Nearer to the topography, there is quasi-deterministic breaking associated with each baroclinic crest passage. Large-amplitude, small-scale internal waves are triggered by tidal forcing, consistent with lee-wave formation at the ridge break. These waves have vertical wavelengths on the order of 400 m. During spring tides, the waves are nonlinear and exhibit convective instabilities on their leading edge. Dissipation rates exceed those predicted by the open-ocean parameterizations by up to a factor of 100, with the disparity increasing as the seafloor is approached. These observations are based on a set of repeated CTD and microconductivity profiles obtained from the research platform (R/P) Floating Instrument Platform (FLIP), which was trimoored over the southern edge of the ridge crest. Ocean velocity and shear were resolved to a 4-m vertical scale by a suspended Doppler sonar. Dissipation was estimated both by measuring overturn displacements and from microconductivity wavenumber spectra. The methods agreed in water deeper than 200 m, where sensor resolution limitations do not limit the turbulence estimates. At intense mixing sites new phenomena await discovery, and existing parameterizations cannot be expected to apply.

Description: This work was funded by the National Science Foundation as a component of the Hawaii Ocean Mixing Program. Added support for FLIP was provided by the Office of Naval Research.

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): 133–145, doi:10.1175/2007JPO3782.1.

Description: Five ice-tethered profilers (ITPs), deployed between 2004 and 2006, have provided detailed potential temperature θ and salinity S profiles from 21 anticyclonic eddy encounters in the central Canada Basin of the Arctic Ocean. The 12–35-m-thick eddies have center depths between 42 and 69 m in the Arctic halocline, and are shallower and less dense than the majority of eddies observed previously in the central Canada Basin. They are characterized by anomalously cold θ and low stratification, and have horizontal scales on the order of, or less than, the Rossby radius of deformation (about 10 km). Maximum azimuthal speeds estimated from dynamic heights (assuming cyclogeostrophic balance) are between 9 and 26 cm s−1, an order of magnitude larger than typical ambient flow speeds in the central basin. Eddy θ–S and potential vorticity properties, as well as horizontal and vertical scales, are consistent with their formation by instability of a surface front at about 80°N that appears in historical CTD and expendable CTD (XCTD) measurements. This would suggest eddy lifetimes longer than 6 months. While the baroclinic instability of boundary currents cannot be ruled out as a generation mechanism, it is less likely since deeper eddies that would originate from the deeper-reaching boundary flows are not observed in the survey region.

Description: The engineering design work for the ITP was initiated by the Cecil H. and Ida M. Green Technology Innovation Program (an internal program at the Woods Hole Oceanographic Institution). Prototype development and construction were funded jointly by the U.S. National Science Foundation (NSF) Oceanographic Technology and Interdisciplinary Coordination Program and Office of Polar Programs (OPP) under Award OCE-0324233. Continued support has been provided by the OPP Arctic Sciences Section under Award ARC-0519899 and internal WHOI funding.

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): 686-701, doi:10.1175/2007JPO3826.1.

Description: The disintegration of a first-mode internal tide into shorter solitary-like waves is considered. Since observations frequently show both tides and waves with amplitudes beyond the restrictions of weakly nonlinear theory, the evolution is studied using a fully nonlinear, weakly nonhydrostatic two-layer theory that includes rotation. In the hydrostatic limit, the governing equations have periodic, nonlinear inertia–gravity solutions that are explored as models of the nonlinear internal tide. These long waves are shown to be robust to weak nonhydrostatic effects. Numerical solutions show that the disintegration of an initial sinusoidal linear internal tide is closely linked to the presence of these nonlinear waves. The initial tide steepens due to nonlinearity and sheds energy into short solitary waves. The disintegration is halted as the longwave part of the solution settles onto a state close to one of the nonlinear hydrostatic solutions, with the short solitary waves superimposed. The degree of disintegration is a function of initial amplitude of the tide and the properties of the underlying nonlinear hydrostatic solutions, which, depending on stratification and tidal frequency, exist only for a finite range of amplitudes (or energies). There is a lower threshold below which no short solitary waves are produced. However, for initial amplitudes above another threshold, given approximately by the energy of the limiting nonlinear hydrostatic inertia–gravity wave, most of the initial tidal energy goes into solitary waves. Recent observations in the South China Sea are briefly discussed.

Description: KRH was supported by a Woods Hole Oceanographic Institution Mellon Independent Study Award and ONR Grant N000140610798.

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): 1644-1668, doi:10.1175/2007JPO3829.1.

Description: The mean structure and time-dependent behavior of the shelfbreak jet along the southern Beaufort Sea, and its ability to transport properties into the basin interior via eddies are explored using high-resolution mooring data and an idealized numerical model. The analysis focuses on springtime, when weakly stratified winter-transformed Pacific water is being advected out of the Chukchi Sea. When winds are weak, the observed jet is bottom trapped with a low potential vorticity core and has maximum mean velocities of O(25 cm s−1) and an eastward transport of 0.42 Sv (1 Sv ≡ 106 m3 s−1). Despite the absence of winds, the current is highly time dependent, with relative vorticity and twisting vorticity often important components of the Ertel potential vorticity. An idealized primitive equation model forced by dense, weakly stratified waters flowing off a shelf produces a mean middepth boundary current similar in structure to that observed at the mooring site. The model boundary current is also highly variable, and produces numerous strong, small anticyclonic eddies that transport the shelf water into the basin interior. Analysis of the energy conversion terms in both the mooring data and the numerical model indicates that the eddies are formed via baroclinic instability of the boundary current. The structure of the eddies in the basin interior compares well with observations from drifting ice platforms. The results suggest that eddies shed from the shelfbreak jet contribute significantly to the offshore flux of heat, salt, and other properties, and are likely important for the ventilation of the halocline in the western Arctic Ocean. Interaction with an anticyclonic basin-scale circulation, meant to represent the Beaufort gyre, enhances the offshore transport of shelf water and results in a loss of mass transport from the shelfbreak jet.

Description: This study was supported by the National Science Foundation Office of Polar Programs under Grants 0421904 and 035268 (MS), and by the Office of Naval Research Grant N00014-02-1-0317 (RP and PF). Analysis by AJP was supported by the Office of Naval Research under Grant N00014-97-1-0135 and by the National Science Foundation under Grant OPP-9815303.

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): 1992-2002, doi:10.1175/2008JPO3669.1.

Description: This paper extends A. Bracco and J. Pedlosky’s investigation of the eddy-formation mechanism in the eastern Labrador Sea by including a more realistic depiction of the boundary current. The quasigeostrophic model consists of a meridional, coastally trapped current with three vertical layers. The current configuration and topographic domain are chosen to match, as closely as possible, the observations of the boundary current and the varying topographic slope along the West Greenland coast. The role played by the bottom-intensified component of the boundary current on the formation of the Labrador Sea Irminger Rings is explored. Consistent with the earlier study, a short, localized bottom-trapped wave is responsible for most of the perturbation energy growth. However, for the instability to occur in the three-layer model, the deepest component of the boundary current must be sufficiently strong, highlighting the importance of the near-bottom flow. The model is able to reproduce important features of the observed vortices in the eastern Labrador Sea, including the polarity, radius, rate of formation, and vertical structure. At the time of formation, the eddies have a surface signature as well as a strong circulation at depth, possibly allowing for the transport of both surface and near-bottom water from the boundary current into the interior basin. This work also supports the idea that changes in the current structure could be responsible for the observed interannual variability in the number of Irminger Rings formed.

Description: AB is supported by WHOI unrestricted funds, JP by the National Science Foundation OCE 85108600, and RP by 0450658.

Description: Author Posting. © American Meteorological Society, 2007. 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 37 (2007): 1103-1121, doi:10.1175/jpo3041.1.

Description: The role of mesoscale oceanic eddies is analyzed in a quasigeostrophic coupled ocean–atmosphere model operating at a large Reynolds number. The model dynamics are characterized by decadal variability that involves nonlinear adjustment of the ocean to coherent north–south shifts of the atmosphere. The oceanic eddy effects are diagnosed by the dynamical decomposition method adapted for nonstationary external forcing. The main effects of the eddies are an enhancement of the oceanic eastward jet separating the subpolar and subtropical gyres and a weakening of the gyres. The flow-enhancing effect is due to nonlinear rectification driven by fluctuations of the eddy forcing. This is a nonlocal process involving generation of the eddies by the flow instabilities in the western boundary current and the upstream part of the eastward jet. The eddies are advected by the mean current to the east, where they backscatter into the rectified enhancement of the eastward jet. The gyre-weakening effect, which is due to the time-mean buoyancy component of the eddy forcing, is a result of the baroclinic instability of the westward return currents. The diagnosed eddy forcing is parameterized in a non-eddy-resolving ocean model, as a nonstationary random process, in which the corresponding parameters are derived from the control coupled simulation. The key parameter of the random process—its variance—is related to the large-scale flow baroclinicity index. It is shown that the coupled model with the non-eddy-resolving ocean component and the parameterized eddies correctly simulates climatology and low-frequency variability of the control eddy-resolving coupled solution.

Description: Funding for this work came from NSF Grants OCE 02-221066 and OCE 03-44094. Additional funding for PB was provided by the U.K. Royal Society Fellowship and by WHOI Grants 27100056 and 52990035.

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 1999

Description: This thesis addresses the parameterization of the heat and momentum transporting properties of eddy motions for use in three-dimensional, primitive equation, z-coordinate atmosphere and ocean models. Determining the transport characteristics of these eddies is fundamental to understanding their effect on the large-scale ocean circulation and global climate. The approach is to transform the primitive equations to yield the altered 'transformed Eulerian mean' (TEM) equations. The assumption is made that the eddy motions obey quasigeostrophic dynamics while the mean flow obeys the primitive equations. With this assumption, the TEM framework leads to the eddies appearing as one term, which acts as a body force in the momentum equations. This force manifests itself as a flux of potential vorticity (PV) - a quantity that incorporates both eddy momentum and heat transporting properties. Moreover, the dynamic velocities are those of the residual mean circulation, a much more relevant velocity for understanding heat and tracer transport. Closure for the eddy PV flux is achieved through a flux-gradient relationship, which directs the flux down the large scale PV gradient. For zonal flows, care is taken to ensure that the resulting force does not generate any net momentum, acting only to redistribute it. Neglect of relative vorticity fluxes in the PV flux yields the parameterization scheme of Gent and McWilliams. The approach is investigated by comparing a zonally-averaged parameterized model with a three dimensional eddy-resolving calculation of flow in a stress-driven channel. The stress at the upper surface is communicated down the water column to the bottom by eddy form drag. Moreover, lateral eddy momentum fluxes act to strengthen and sharpen the mean flow, transporting eastward momentum up its large scale gradient. Both the vertical momentum transfer and lateral, upgradient momentum transfer by eddies, are captured in the parameterized model. The advantages of this approach are demonstrated in two further zonal cases: 1) the spin-down of a baroclinic zone, and 2) the atmospheric jet stream. The time mean TEM approach and the eddy PV flux closure are explored in the context of an eddy-resolving closed basin flow which breaks the zonal symmetry. Decomposition of eddy PV fluxes into components associated with advective and dissipative effects suggest that the component associated with eddy flux divergence, and therefore forcing of the mean flow, is mainly directed down the large scale gradient and can be parameterized as before. Thus, the approach can be used to capture eddy transport properties for both zonal mean and time mean flows. The PV flux embodies both the eddy heat and momentum fluxes and so presents a more unified picture of their transferring properties. It therefore provides a powerful conceptual and practical framework for representing eddies in numerical models of the atmsophere and ocean.

Description: The work in this thesis was funded by grants from NSF, (OCE-9634331, OCE- 9503895), ONR (NOOOI4-95-1-0967), and by a fellowship from the Joint Program on the Science and Policy of Global Change at MIT.

Description: Author Posting. © American Meteorological Society, 2011. 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 41 (2011): 2168–2186, doi:10.1175/JPO-D-11-08.1.

Description: This paper studies the interaction of an Antarctic Circumpolar Current (ACC)–like wind-driven channel flow with a continental slope and a flat-bottomed bay-shaped shelf near the channel’s southern boundary. Interaction between the model ACC and the topography in the second layer induces local changes of the potential vorticity (PV) flux, which further causes the formation of a first-layer PV front near the base of the topography. Located between the ACC and the first-layer slope, the newly formed PV front is constantly perturbed by the ACC and in turn forces the first-layer slope with its own variability in an intermittent but persistent way. The volume transport of the slope water across the first-layer slope edge is mostly directly driven by eddies and meanders of the new front, and its magnitude is similar to the maximum Ekman transport in the channel. Near the bay’s opening, the effect of the topographic waves, excited by offshore variability, dominates the cross-isobath exchange and induces a mean clockwise shelf circulation. The waves’ propagation is only toward the west and tends to be blocked by the bay’s western boundary in the narrow-shelf region. The ensuing wave–coast interaction amplifies the wave amplitude and the cross-shelf transport. Because the interaction only occurs near the western boundary, the shelf water in the west of the bay is more readily carried offshore than that in the east and the mean shelf circulation is also intensified along the bay’s western boundary.

Description: Y. Zhang acknowledges the support of the MIT-WHOI Joint Program in Physical Oceanography and NSF OCE-9901654 and OCE- 0451086. J. Pedlosky acknowledges the support of NSF OCE-9901654 and OCE-0451086.

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 1992

Description: Oceanic profiles of temperature, salinity, horizontal velocity, rate of dissipation of turbulent kinetic energy (ε) and rate of dissipation of thermal variance (χ) are used to examine the parameterization of turbulent mixing in the ocean due to internal waves. Turbulent mixing is quantified through eddy diffusivity parameterizations of the mass (Kρ; Osborn, 1980) and heat fluxes (Kτ; Osborn and Cox, 1972) in turbulent production/dissipation balances. Turbulence in the ocean is generally held to result from the occurrence of shear instability in regions where the Richardson number is locally supercritical (i.e. Ri ≤ 1/4), permitting the growth of small-scale waves which break and result in turbulent mixing. The occurrence of shear instability results from the local intensification of the shear in the internal wave field. The energy dissipated in such events is provided by the energy flux to higher wavenumber due to nonlinear wave/wave interactions on scales of 10's to 100's of meters. In turn, the strength of the wave/wave interactions depends generally on the energy content of the internal wave field, which can vary considerably over even larger scales due to the presence of topography or background flows. The magnitude of turbulent mixing is linked to internal wave dynamics by equating the turbulent dissipation with the energy flux through the vertical wavenumber spectrum under the priviso that the model spectrum which forms the basis for the analysis is statistically stationary with respect to the nonlinear interactions. Dynamical models (McComas and Muller, 1981; Henyey et al., 1986) indicate that the Garrett and Munk (GM; Munk, 1981) spectrum is stationary. Observations from the far field of a seamount in a region of negligible large-scale flow were examined to address the issue of the buoyancy scaling of ε. These data exhibited large variations in background stratification with depth, but the internal wave characteristics were not substantially differentiable from the GM prescription. The magnitude of ε and its functional dependence upon internal wave energy levels (E) and buoyancy frequency (N) was best described by the dynamical model ofHenyey et al. (1986) (ε ~ E2N2). The Richardson number scaling model of Kunze et al. (1990) produced consistent estimates. A second dynamical model, McComas and Muller (1981), predicted an appropriate (E,N) scaling, but overestimated the observed dissipation rates by a factor of five. Two kinematical dissipation parameterizations (Garmett and Holloway (1984) and Munk (1981)) predicted buoyancy scalings of N3/2 which were inconsistent with the observed scaling. Data from an upper-ocean front, a warm core ring and a region of steep topography were analyzed in order to examine the parameter dependence of E in internal wave fields which exhibited potentially nonstationary characteristics. Evidence was provided which implied the internal wave field in an upper ocean front was interacting with and modified by the background flow. Inhomogeneity and anisotropy of the internal wave field were noted in that data set. The model of Gregg (1989), which in turn was based upon the model of Henyey et al., effectively collapsed the observed diffusivity estimates from the front. The warm core ring profiles were noted to be anisotropic, dominated by near-inertial frequencies and to have a peaked vertical wavenumber shear spectrum. The data from a region of steep topography were noted to have a peaked vertical wavenumber spectrum and were characterized by higher than GM frequency motions. For the latter two data sets, application of a frequency based correction to the Henyey et al. model (Henyey, 1991) reduced more than an order of magnitude scatter in the parameterized estimates of E to less than a factor of four. Of the possible non-equilibrium conditions in the internal wave field, the (E,N) scaled dissipation rates were most sensitive to deviations in wave field frequency content. On the basis of a number of theoretical Richardson number probability distributions (Ri = N2/S2, where S2 is the sum of the squared vertical derivatives of horizontal velocity), the nominal dissipation scaling of the Kunze et al. model was determined to be E2N3. This scaling is altered to the observed ε ~ E2N2 scaling by a statistical dependence between N2 and S2 which reduces the occurrence of supercritical Ri values. This statistical dependence is hypothesized to be an effect of the turbulent momentum and buoyancy fluxes on the internal wave shear and strain profiles caused by shear instability. The statistical dependence between N2 and S2 exhibited a buoyancy scaling which was interpreted as resulting from the decreasing ratio between the time scale of the shear instability mechanism [T- 2π/N] and the adiabatic time scale [T - 2π/(Nf)1/2] of the internal wave field (f is the Coriolis parameter). This phenomenology is interpreted in light of saturated spectral theories which suggest that the magnitude and shape of the vertical wavenumber spectrum is controlled by instability mechanisms at large wavenumber ( ≥ .1 cpm). We argue that saturated spectral theories are valid only in the limit where a separation exists between the two time scales, i.e. for large N, low internal wave frequency content, and small f. These results have immediate implications for oceanic mixing driven by internal wave motions. First, background diffusivities are small: at GM energy levels, Kρ - .03x10-4 m2/s (Kρ = .25ε/N2). Secondly, since Kρ is independent of N at constant E, some process or collection of processes must be responsible for heightened E values in the abyss if internal waves cause the 0(1-10x10-4 m2/s) diffusivities generally inferred from deep ocean hydrographic data. We view internal wave reflection and/or internal wave generation associated with topographic features to be likely candidates.

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 August 1989

Description: Given well known environmental conditions, matched field processing has been shown to be a promising signal processing technique for the localization of acoustic sources. However, when environmental data are incomplete or inaccurate, a 'mismatch' occurs between the measured field and model field which can lead to a severe degradation of the localization estimator. We investigate the possible mismatch effects of surface and internal waves on matched field processing in a shallow water waveguide. We utilize a modified ray theory, based on the work of Tindle, to calculate the acoustic pressure field. This allows us to simply incorporate range dependent environmental conditions as well as to generalize our work to deeper waveguides. In general, the conventional (Bartlett) matched field beamformer does not provide sufficient resolution to unambiguously locate a source, even in a perfectly matched environment. The maximum likelihood method (MLM) matched field beamformer has much better resolution but is extremely susceptible to mismatch. The mismatch due to surface roughness can result in a large reduction of the estimator peak. Part, but not all, of the peak can be regained by 1)using a model which includes incomplete reflection at the surface based on actual sea surface statistics and 2) short time averaging of the measured signal, with times on the order of the period of the surface waves. Mismatch due to internal waves can also result in a large degradation of the estimator. Averaging over the same time period as surface waves provides little improvement and leads one to surmise that internal waves may be a limiting constraint on matched field processing. Finally, we combine the surface and internal wave fields with a slowly moving source. This example highlights the necessity for the development of a beamformer which has a broader mainlobe while maintaining adequate sidelobe suppression, and we address this issue by looking at two such beamformers.

Description: Author Posting. © American Meteorological Society, 2012. 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 42 (2012): 1981–2000, doi:10.1175/JPO-D-12-028.1.

Description: Packets of nonlinear internal waves (NLIWs) in a small area of the Mid-Atlantic Bight were 10 times more energetic during a local neap tide than during the preceding spring tide. This counterintuitive result cannot be explained if the waves are generated near the shelf break by the local barotropic tide since changes in shelfbreak stratification explain only a small fraction of the variability in barotropic to baroclinic conversion. Instead, this study suggests that the occurrence of strong NLIWs was caused by the shoaling of distantly generated internal tides with amplitudes that are uncorrelated with the local spring-neap cycle. An extensive set of moored observations show that NLIWs are correlated with the internal tide but uncorrelated with barotropic tide. Using harmonic analysis of a 40-day record, this study associates steady-phase motions at the shelf break with waves generated by the local barotropic tide and variable-phase motions with the shoaling of distantly generated internal tides. The dual sources of internal tide energy (local or remote) mean that shelf internal tides and NLIWs will be predictable with a local model only if the locally generated internal tides are significantly stronger than shoaling internal tides. Since the depth-integrated internal tide energy in the open ocean can greatly exceed that on the shelf, it is likely that shoaling internal tides control the energetics on shelves that are directly exposed to the open ocean.

Description: This research was supported by ONR Grants N00014-05-1-0271, N00014-08-1-0991, N00014-04- 1-0146, and N00014-11-1-0194.

Description: 2013-05-01

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 August 1996

Description: A field experiment was undertaken during July and August of 1995 aimed at understanding the interaction of acoustic signals with the internal wave field off the coast of New Jersey. As part of SWARM (Shallow Water Acoustics in a Random Medium), physical data were collected in 75 m of water near 39°15.34'N, 72°56.59'W with three thermistor strings, a bottom-mounted ADCP, and yo-yo CTDs. These data spanned a two-week period of the month-long study. With the exception of a time following a storm event, during which the generation mechanism near the shelf break was effectively switched off, large-amplitude (up to 20 meters), rank-ordered groups of internal solitons were observed traveling through the region approximately every 12.4 hours. These groups of solitons progressed across the shelf with phase speeds of 61.8 ± 14.9 cm/s with a heading of 280 ± 31° T. Two-layer finite-depth theory was tested on this data and shown to consistently overpredict the phase speed of the internal solitons within each group. Predictions of horizontal scale, particle velocities, and displacements were in qualitative agreement with two-layer finite-depth dynamics.

Description: Support for this work was provided by a National Science Foundation grant OCE-9313670.

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 1997

Description: As part of the Shallow Water Acoustics in a Random Medium (SWARM) experiment [1], a sixteen element WHOI vertical line array (WVLA) was moored in 70 meters of water off the New Jersey coast. This array was sampled at 1395 Hz or higher for the seven days it was deployed. Tomography sources with carrier frequencies of 224 and 400 Hz were moored about 32 km shoreward, such that the acoustic path was anti-parallel to the primary propagation direction for shelf generated internal wave solitons. Two models for the propagation of normal modes through a 2-D waveguide with solitary internal wave (soliton) scattering included are developed to help in understanding the very complicated mode arrivals seen at the WVLA. The simplest model uses the Preisig and Duda [2] sharp interface approximation for solitons, allowing for rapid analysis of the effects of various numbers of solitons on mode arrival statistics. The second model, using SWARM thermistor string data to simulate the actual SWARM waveguides, is more realistic, but much slower. The analysis of the actual WVLA data yields spread, bias, wander, and intensity fluctuation signals that are modulated at tidal frequencies. The signals are consistent with predicted relationships to the internal wave distributions in the waveguides.

Description: The funds for my education were provided by the Office of Naval Research through an ONR Fellowship (MIT award 002734-001); the funds for SWARM were also provided by the Office of Naval Research through ONR Grant N00014-95-0051.

Description: Author Posting. © American Meteorological Society, 2012. 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 42 (2012): 2143–2152, doi:10.1175/JPO-D-12-027.1.

Description: Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) is constricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10−8 W kg−1 at abyssal and middepths. The mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 × 10−4 m2 s−1. This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smoother bathymetry. Outside of the frontal regions, dissipation rates are O(10−10) W kg−1, comparable to the background levels of turbulence found throughout most mid- and low-latitude regions of the global ocean.

Description: This work was supported by the U.S. National Science Foundation and by the Natural Environment Research Council of the United Kingdom.

Description: 2013-06-01

Description: Author Posting. © American Meteorological Society, 2013. 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 43 (2013): 698–705, doi:10.1175/JPO-D-12-0119.1.

Description: Owing to the larger thermal expansion coefficient at higher temperatures, more buoyancy is put into the ocean by heating than is removed by cooling at low temperatures. The authors show that, even with globally balanced thermal and haline surface forcing at the ocean surface, there is a negative density flux and hence a positive buoyancy flux. As shown by McDougall and Garrett, this must be compensated by interior densification on mixing due to the nonlinearity of the equation of state (cabbeling). Three issues that arise from this are addressed: the estimation of the annual input of density forcing, the effects of the seasonal cycle, and the total cabbeling potential of the ocean upon complete mixing. The annual expansion through surface density forcing in a steady-state ocean driven by balanced evaporation–precipitation–runoff (E–P–R) and net radiative budget at the surface Qnet is estimated as 74 000 m3 s−1 (0.07 Sv; 1 Sv ≡ 106 m3 s−1), which would be equivalent to a sea level rise of 6.3 mm yr−1. This is equivalent to approximately 3 times the estimated rate of sea level rise or 450% of the average Mississippi River discharge. When seasonal variations are included, this density forcing increases by 35% relative to the time-mean case to 101 000 m3 s−1 (0.1 Sv). Likely bounds are established on these numbers by using different Qnet and E–P–R datasets and the estimates are found to be robust to a factor of ~2. These values compare well with the cabbeling-induced contraction inferred from independent thermal dissipation rate estimates. The potential sea level decrease upon complete vertical mixing of the ocean is estimated as 230 mm. When horizontal mixing is included, the sea level drop is estimated as 300 mm.

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

Description: 2013-10-01

Description: Author Posting. © American Meteorological Society, 2012. 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 42 (2012): 2206–2228, doi:10.1175/JPO-D-11-0191.1.

Description: This study investigates the anisotropic properties of the eddy-induced material transport in the near-surface North Atlantic from two independent datasets, one simulated from the sea surface height altimetry and one derived from real-ocean surface drifters, and systematically examines the interactions between the mean- and eddy-induced material transport in the region. The Lagrangian particle dispersion, which is widely used to characterize the eddy-induced tracer fluxes, is quantified by constructing the “spreading ellipses.” The analysis consistently demonstrates that this dispersion is spatially inhomogeneous and strongly anisotropic. The spreading is larger and more anisotropic in the subtropical than in the subpolar gyre, and the largest ellipses occur in the Gulf Stream vicinity. Even at times longer than half a year, the spreading exhibits significant nondiffusive behavior in some parts of the domain. The eddies in this study are defined as deviations from the long-term time-mean. The contributions from the climatological annual cycle, interannual, and subannual (shorter than one year) variability are investigated, and the latter is shown to have the strongest effect on the anisotropy of particle spreading. The influence of the mean advection on the eddy-induced particle spreading is investigated using the “eddy-following-full-trajectories” technique and is found to be significant. The role of the Ekman advection is, however, secondary. The pronounced anisotropy of particle dispersion is expected to have important implications for distributing oceanic tracers, and for parameterizing eddy-induced tracer transfer in non-eddy-resolving models.

Description: IR was supported by Grant NSF-OCE-0725796. IK would like to acknowledge support by the National Science foundation Grant OCE-0842834.

Description: 2013-06-01

Description: Author Posting. © American Meteorological Society, 2013. 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 43 (2013): 905–919, doi:10.1175/JPO-D-12-0150.1.

Description: Interactions between vortices and a shelfbreak current are investigated, with particular attention to the exchange of waters between the continental shelf and slope. The nonlinear, three-dimensional interaction between an anticyclonic vortex and the shelfbreak current is studied in the laboratory while varying the ratio ε of the maximum azimuthal velocity in the vortex to the maximum alongshelf velocity in the shelfbreak current. Strong interactions between the shelfbreak current and the vortex are observed when ε 〉 1; weak interactions are found when ε 〈 1. When the anticyclonic vortex comes in contact with the shelfbreak front during a strong interaction, a streamer of shelf water is drawn offshore and wraps anticyclonically around the vortex. Measurements of the offshore transport and identification of the particle trajectories in the shelfbreak current drawn offshore from the vortex allow quantification of the fraction of the shelfbreak current that is deflected onto the slope; this fraction increases for increasing values of ε. Experimental results in the laboratory are strikingly similar to results obtained from observations in the Middle Atlantic Bight (MAB); after proper scaling, measurements of offshore transport and offshore displacement of shelf water for vortices in the MAB that span a range of values of ε agree well with laboratory predictions.

Description: Laboratory work was supported by the National Science Foundation through Grant OCE- 0081756. Glider observations in March–April 2006 were supported by the National Science Foundation through Grant OCE-0220769. Glider observations in July– October 2007 were supported by a grant from Raytheon. RET was supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Cooperative Institute for the North Atlantic Region. The REMUS observations were funded by the Office of Naval Research. GGG was supported by the National Science Foundation through Grant OCE-1129125 for analysis and writing.

Description: 2013-11-01

Description: Author Posting. © American Meteorological Society, 2013. 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 Climate 26 (2013): 9839–9859, doi:10.1175/JCLI-D-12-00647.1.

Description: Spatial and temporal covariability between the atmospheric transient eddy heat fluxes (i.e., υ′T′ and υ′q′) in the Northern Hemisphere winter (January–March) and the paths of the Gulf Stream (GS), Kuroshio Extension (KE), and Oyashio Extension (OE) are examined based on an atmospheric reanalyses and ocean observations for 1979–2009. For the climatological winter mean, the northward heat fluxes by the synoptic (2–8 days) transient eddies exhibit canonical storm tracks with their maxima collocated with the GS and KE/OE. The intraseasonal (8 days–3 months) counterpart, while having overall similar amplitude, shows a spatial pattern with more localized maxima near the major orography and blocking regions. Lateral heat flux divergence by transient eddies as the sum of the two frequency bands exhibits very close coupling with the exact locations of the ocean fronts. Linear regression is used to examine the lead–lag relationship between interannual changes in the northward heat fluxes by the transient eddies and the meridional changes in the paths of the GS, KE, and OE, respectively. One to three years prior to the northward shifts of each ocean front, the atmospheric storm tracks shift northward and intensify, which is consistent with wind-driven changes of the ocean. Following the northward shifts of the ocean fronts, the synoptic storm tracks weaken in all three cases. The zonally integrated northward heat transport by the synoptic transient eddies increases by ~5% of its maximum mean value prior to the northward shift of each ocean front and decreases to a similar amplitude afterward.

Description: Support from the National Aeronautics and Space Administration (NASA) Physical Oceanography Program (NNX09AF35G to TJ and Y-OK) and the Department of Energy (DOE) Climate and Environmental Sciences Division (DE-SC0007052 to Y-OK) is gratefully acknowledged.

Description: 2014-06-15

Description: Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 606–612, doi:10.1175/JPO-D-14-0221.1.

Description: Mesoscale intrathermocline lenses are observed throughout the World Ocean and are commonly attributed to water mass anomalies advected from a distant origin. An alternative mechanism of local generation is offered herein, in which eddy–wind interaction can create lens-shaped disturbances in the thermocline. Numerical simulations illustrate how eddy–wind-driven upwelling in anticyclones can yield a convex lens reminiscent of a mode water eddy, whereas eddy–wind-driven downwelling in cyclones produces a concave lens that thins the mode water layer (a cyclonic “thinny”). Such transformations should be observable with long-term time series in the interiors of mesoscale eddies.

Description: Support of this research by the National Science Foundation and National Aeronautics and Space Administration is gratefully acknowledged.

Description: 2015-08-01

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 February 2015

Description: The scattering of low-mode internal tides by ocean-floor topography is extensively studied through analytical models and field observations at the Line Islands Ridge (LIR). An existing Green function method is utilized to examine the generation of internal tides by idealized topographic shapes as well as realistic transects of the LIR. The method is also applied to examine the scattering of a mode-1 internal tide at these topographies to determine the relative high mode energy flux due to generated and scattered internal tides at the realistic transects. A method of determining the modal content of an internal wave field is advanced to account for arbitrary stratification and rotation. It is then adjusted to allow for image loss as is common to oceanographic studies. Its performance is compared to the existing regression method widely used by oceanographers to determine the modal content of internal tides. The results from this comparison are used to inform the analysis of the field observations. This thesis concludes by examining the modal content of the LIR as determined from measurements taken during the 150-day EXperiment on Internal Tide Scattering (EXITS) NSF field study. Motivated by satellite altimetry data and three-dimensional numerical model studies, the EXITS cruise sought to observe the internal tide scattering process in the ocean for the first time. The data from three moorings equipped with moored profilers, spanning total depths of 3000-5000 m is analyzed to determine the modal content of the southward propagating internal tide before and after it encounters the ridge for evidence of topographic scattering.

Description: Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 966–987, doi:10.1175/JPO-D-14-0110.1.

Description: A key remaining challenge in oceanography is the understanding and parameterization of small-scale mixing. Evidence suggests that topographic features play a significant role in enhancing mixing in the Southern Ocean. This study uses 914 high-resolution hydrographic profiles from novel EM-APEX profiling floats to investigate turbulent mixing north of the Kerguelen Plateau, a major topographic feature in the Southern Ocean. A shear–strain finescale parameterization is applied to estimate diapycnal diffusivity in the upper 1600 m of the ocean. The indirect estimates of mixing match direct microstructure profiler observations made simultaneously. It is found that mixing intensities have strong spatial and temporal variability, ranging from O(10−6) to O(10−3) m2 s−1. This study identifies topographic roughness, current speed, and wind speed as the main factors controlling mixing intensity. Additionally, the authors find strong regional variability in mixing dynamics and enhanced mixing in the Antarctic Circumpolar Current frontal region. This enhanced mixing is attributed to dissipating internal waves generated by the interaction of the Antarctic Circumpolar Current and the topography of the Kerguelen Plateau. Extending the mixing observations from the Kerguelen region to the entire Southern Ocean, this study infers a large water mass transformation rate of 17 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) across the boundary of Antarctic Intermediate Water and Upper Circumpolar Deep Water in the Antarctic Circumpolar Current. This work suggests that the contribution of mixing to the Southern Ocean overturning circulation budget is particularly significant in fronts.

Description: AM was supported by the joint CSIRO–University of Tasmania Quantitative Marine Science (QMS) program and the 2009 CSIRO Wealth from Ocean Flagship Collaborative Fund. BMS was supported by the Australian Climate Change Science Program, jointly funded by the Department of the Environment and CSIRO. KLPs salary support was provided by Woods Hole Oceanographic Institution bridge support funds.

Description: 2015-10-01

Description: Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 2820–2835, doi:10.1175/JPO-D-15-0101.1.

Description: The response of a convective ocean basin to variations in atmospheric temperature is explored using numerical models and theory. The results indicate that the general behavior depends strongly on the frequency at which the atmosphere changes relative to the local response time to air–sea heat flux. For high-frequency forcing, the convective region in the basin interior is essentially one-dimensional and responds to the integrated local surface heat flux anomalies. For low-frequency forcing, eddy fluxes from the boundary current into the basin interior become important and act to suppress variability forced by the atmosphere. A theory is developed to quantify this time-dependent response and its influence on various oceanic quantities. The amplitude and phase of the temperature and salinity of the convective water mass, the meridional overturning circulation, the meridional heat flux, and the air–sea heat flux predicted by the theory compare well with that diagnosed from a series of numerical model calculations in both strongly eddying and weakly eddying regimes. Linearized analytic solutions provide direct estimates of each of these quantities and demonstrate their dependence on the nondimensional numbers that characterize the domain and atmospheric forcing. These results highlight the importance of mesoscale eddies in modulating the mean and time-dependent ocean response to atmospheric variability and provide a dynamical framework with which to connect ocean observations with changes in the atmosphere and surface heat flux.

Description: This study was supported by the National Science Foundation under Grant OCE-1232389.

Description: 2016-05-01

Description: Author Posting. © American Meteorological Society, 2016. 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 46 (2016): 2645-2662, doi:10.1175/JPO-D-15-0191.1.

Description: The occurrence, drivers, and implications of small-scale O(2–5) km diameter coherent vortices, referred to as submesoscale eddies, over the inner shelf south of Martha’s Vineyard, Massachusetts, are examined using high-frequency (HF), radar-based, high-resolution (400 m) observations of surface currents. Within the 300 km2 study area, eddies occurred at rates of 1 and 4 day−1 in winter and summer, respectively. Most were less than 5 h in duration, smaller than 4 km in diameter, and rotated less than once over their lifespan; 60% of the eddies formed along the eastern edge of study area, adjacent to Wasque Shoal, and moved westward into the interior, often with relative vorticity greater than f. Eddy generation was linked to vortex stretching on the ebb and flood tide as well as the interaction of the spatially variable tide and the wind-driven currents; however, these features had complex patterns of surface divergence and stretching. Eddies located away from Wasque Shoal were related to the movement of wind-driven surface currents, as wind direction controlled where eddies formed as well as density effects. Using an analysis of particles advected within the radar-based surface currents, the observed eddies were found to be generally leaky, losing 60%–80% of particles over their lifespan, but still more retentive than the background flow. As a result, the combined translation and rotational effects of the observed eddies were an important source of lateral exchange for surface waters over the inner shelf.

Description: The HF radar data utilized here were obtained using internal funding from the Woods Hole Oceanographic Institution. The analysis was supported by NSF OCE Grant 1332646.

Description: 2017-02-19

Description: Author Posting. © American Meteorological Society, 2017. 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 47 (2017): 1291-1305, doi:10.1175/JPO-D-16-0160.1.

Description: Along-stream variations in the dynamics of the Antarctic Circumpolar Current (ACC) impact heat and tracer transport, regulate interbasin exchange, and influence closure of the overturning circulation. Topography is primarily responsible for generating deviations from zonal-mean properties, mainly through standing meanders associated with regions of high eddy kinetic energy. Here, an idealized channel model is used to explore the spatial distribution of energy exchange and its relationship to eddy geometry, as characterized by both eddy momentum and eddy buoyancy fluxes. Variations in energy exchange properties occur not only between standing meander and quasi-zonal jet regions, but throughout the meander itself. Both barotropic and baroclinic stability properties, as well as the magnitude of energy exchange terms, undergo abrupt changes along the path of the ACC. These transitions are captured by diagnosing eddy fluxes of energy and by adopting the eddy geometry framework. The latter, typically applied to barotropic stability properties, is applied here in the depth–along-stream plane to include information about both barotropic and baroclinic stability properties of the flow. These simulations reveal that eddy momentum fluxes, and thus barotropic instability, play a leading role in the energy budget within a standing meander. This result suggests that baroclinic instability alone cannot capture the dynamics of ACC standing meanders, a challenge for models where eddy fluxes are parameterized.

Description: The authors all acknowledge support from NSF OCE-1235488. MKY also acknowledges support from the AMS Graduate Student Fellowship.

Description: 2017-10-12

Description: Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 739-748, doi:10.1175/JPO-D-17-0089.1.

Description: McDougall and Ferrari have estimated the global deep upward diapycnal flow in the boundary layer overlying continental slopes that must balance both downward diapycnal flow in the deep interior and the formation of bottom water around Antarctica. The decrease of perimeter of isopycnal surfaces with depth and the observed decay with height above bottom of turbulent dissipation in the deep ocean play a key role in their estimate. They argue that because the perimeter of seamounts increases with depth, the net effect of mixing around seamounts is to produce net downward diapycnal flow. While this is true along much of a seamount, it is shown here that diapycnal flow of the densest water around the seamount is upward, with buoyancy being transferred from water just above. The same is true for midocean ridges, whose perimeter is constant with depth. It is argued that mixing around seamounts and especially midocean ridges contributes positively to the global deep overturning circulation, reducing the amount of turbulence demanded over the continental slopes to balance the buoyancy budget for the bottom and deep water.

Description: This work was supported by National Science Foundation Grant OCE- 1232962.

Description: 2018-09-29

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, 1980

Description: The structure of the inertial peak in deep ocean kinetic energy spectra is studied here. Records were obtained from Polymode arrays deployed in the Western North Atlantic Ocean (40°W to 70°W, 15°N to 42°N). The results are interpreted both in terms of local sources and of turning point effects on internal waves generated at lower latitudes. In most of the data, there is a prominent inertial peak slightly above f; however, the peak height above the background continuum varies with depth and geographical environment. Three classes of environment and their corresponding spectra emerge from peak height variations: class 1 is the 1500 m level near the Mid-Atlantic Ridge, with the greatest peak height of 18 db; class 2 includes (a) the upper ocean (depth less than 2000 m), (b) the deep ocean (depth greater than 2000 m) over rough topography, and (c) the deep ocean underneath the Gulf Stream, with intermediate peak height of 11.5 db; class 3 is the deep ocean over smooth topography, with the lowest peak height of 7.5 db. Near f, the horizontal coherence scale is 0(60 km) at depths from 200 m to 600 m, and the vertical coherence scale is O(200 m) just below the main thermocline. A one turning point model is developed to describe inertial waves at mid-latitudes, based on the assumption that inertial waves are randomly generated at lower latitudes (global generation) where their frequency-wavenumber spectrum is given by the model of Garrett and Munk (1972 a, 1975). Using the globally valid wave functions obtained by Munk and Phillips (1968), various frequency spectra near f are calculated numerically. The model yields a prominent inertial peak of 7 db in the horizontal velocity spectrum but no peaks in the temperature spectrum. The model is latitudinally dependent: the frequency shift and bandwidth of the inertial peak decrease with latitude; energy level near f is minimum at about 30° and higher at low and high latitudes. The observations of class 3 can be well-described by the model; a low zonal wavenumber cutoff is required to produce the observed frequency shift of the inertial peak. The differences between the global generation model and the observations of class 1 and class 2 are interpreted as the effects of local sources. A locally forced model is developed based on the latitudinal modal decomposition of a localized source function. Asymptotic eigensolutions of the Laplace's tidal equation are therefore derived and used as a set of expansion functions. The forcing is through a vertical velocity field specified at the top or bottom boundaries of the ocean. For white noise forcing, the horizontal velocity spectrum of the response has an inertial peak which diminishes in the far-field. With the forcing located at either the surface or the bottom, several properties of the class 2 observations can be described qualitatively by a combination of the global and local models. The reflection of inertial waves from a turbulent benthic boundary layer is studied by a slab model of given depth. Frictional effects are confined to the boundary layer and modelled by a quadratic drag law. For given incident waves, reflection coefficients are found to be greater than 0.9 for the long waves which contain most of the energy. This result suggests that energy-containing inertial waves can propagate over great distance as is required by the validity of the model of global generation.

Description: This work was supported by the National Science Foundation through grant OCE 76-80210 and its continuation OEE 78-19833.

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 1982

Description: The development of nonlinear surface and internal wave groups is investigated. Surface wave evolution was observed in an unusually long wave channel as a function of steepness and group length. Dissipation and frequency downshifting were important characteristics of the long-time evolution. The amplitude and phase modulations were obtained using the Hilbert transform and specified as an initial condition to the cubic nonlinear Schrodinger equation, which was solved numerically. This equation is known to govern the slowly varying complex modulation envelope of gravity waves on deep water. When dissipation was included, the model compared quite well with the observations. Phase modulation was used to interpret the long-time behavior, using the phase evolution of exact asymptotic solutions as a guide. The wave groups exhibited a long-time coherence but not the recurrence predicted by the inviscid theory. An oceanic field study of the generation of groups of large amplitude internal waves by stratified tidal flow over a submarine ridge indicates that the large amplitude and asymmetry of the topography are critical in determining the type of flow response. The calculated Froude numbers response length scale and duration differ markedly between the two phases of the tide due to the asymmetry.

Description: Research assistantship provided by the Office of Naval Research contract no. N00014-80-C-0273

Description: Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): C03020, doi:10.1029/2004JC002377.

Description: During July–August 2001, oceanographic variability on the New England inner continental shelf was investigated with an emphasis on temporal scales shorter than tidal periods. Mooring and ship survey data showed that subtidal variation of inner shelf stratification was in response to regional Ekman upwelling and downwelling wind driven dynamics. High-frequency variability in the vertical structure of the water column at an offshore mooring site was linked to the baroclinic internal tide and the onshore propagation of nonlinear solitary waves of depression. Temperature, salinity, and velocity data measured at an inshore mooring detected a bottom bore that formed on the flood phase of the tide. During the ebb tide, a second bottom discontinuity and series of nonlinear internal waves of elevation (IWOE) formed when the water column became for a time under hydraulic control. A surface manifestation of these internal wave crests was also observed in aircraft remote sensing imagery. The coupling of IWOE formation to the offshore solitary waves packets was investigated through internal wave breaking criterion derived in earlier laboratory studies. Results suggested that the offshore solitons shoaled on the sloping shelf, and transformed from waves of depression to waves of elevation. The coupling of inshore bore formation to the offshore solitary waves and the possible impact of these periodic features on mixing on the inner shelf region are discussed.

Description: This work was funded by the Office of Naval Research under grant N00014-99-1-0029 and Secretary of the Navy/CNO Chair grant N00014-99-1-0090 to R. A. W.

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): 1551-1573, doi:10.1175/2008JPO4152.1.

Description: A conceptually simple model is presented for predicting the amplitude and periodicity of eddies generated by a steady poleward outflow in a 1½-layer β-plane formulation. The prediction model is rooted in linear quasigeostrophic dynamics but is capable of predicting the amplitude of the β plume generated by outflows in the nonlinear range. Oscillations in the plume amplitude are seen to represent a near-zero group velocity response to an adjustment process that can be traced back to linear dynamics. When the plume-amplitude oscillations become large enough so that the coherent β plume is replaced by a robust eddy field, the eddy amplitude is still constrained by the plume-amplitude prediction model. The eddy periodicity remains close to that of the predictable, near-zero group-velocity linear oscillations. Striking similarities between the patterns of variability in the model and observations south of Indonesia’s Lombok Strait suggest that the processes investigated in this study may play an important role in the generation of the observed eddy field of the Indo-Australian Basin.

Description: This work was completed at the Woods Hole Oceanographic Institution while TS Durland was supported by the Ocean and Climate Change Institute. MA Spall was supported by NSF Grant OCE-0423975 and J Pedlosky by NSF Grant OCE-0451086. TS Durland acknowledges additional report preparation support from NASA Grant NNG05GN98G.

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): 1035-1049, doi:10.1175/2008JPO3920.1.

Description: Seasonal variability of near-inertial horizontal kinetic energy is examined using observations from a series of McLane Moored Profiler moorings located at 39°N, 69°W in the western North Atlantic Ocean in combination with a one-dimensional, depth-integrated kinetic energy model. The time-mean kinetic energy and shear vertical wavenumber spectra of the high-frequency motions at the mooring site are in reasonable agreement with the Garrett–Munk internal wave description. Time series of depth-dependent and depth-integrated near-inertial kinetic energy are calculated from available mooring data after filtering to isolate near-inertial-frequency motions. These data document a pronounced seasonal cycle featuring a wintertime maximum in the depth-integrated near-inertial kinetic energy deriving chiefly from the variability in the upper 500 m of the water column. The seasonal signal in the near-inertial kinetic energy is most prominent for motions with vertical wavelengths greater than 100 m but observable wintertime enhancement is seen down to wavelengths of the order of 10 m. Rotary vertical wavenumber spectra exhibit a dominance of clockwise-with-depth energy, indicative of downward energy propagation and implying a surface energy source. A simple depth-integrated near-inertial kinetic energy model consisting of a wind forcing term and a dissipation term captures the order of magnitude of the observed near-inertial kinetic energy as well as its seasonal cycle.

Description: Funding to initiate the McLane Moored Profiler observations at Line W were provided by grants from the G. Unger Vetlesen Foundation and the Comer Charitable Fund to the Woods Hole Oceanographic Institution’s Ocean and Climate Change Institute. Ongoing moored observations at Line W are supported by the National Science Foundation (NSF Grant OCE-0241354).

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 2000

Description: The plausibility of local baroclinic instability as a generation mechanism for midocean mesoscale eddies is examined with a two-layer, quasi-geostrophic (QG) model forced by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated through bottom Ekman friction, Explanations are sought for two observed features of mid-ocean eddies: 1) substantial energy is retained in the baroclinic mode and in the associated deformation radius (Rd) scale, and 2) the ratio of eddy to mean kinetic energy is much larger than one, The tendency of QG to cascade energy into the barotropic mode and into scales larger than Rd can be counteracted when stratification is surface-trapped, for then the baroclinic mode is weakly damped, and hence enhanced, Numerical experiments are performed with both surface-trapped and uniform stratification to quantify this, Experiments with equal Ekman frictions in the two layers are also performed for purposes of contrast, Interpretation is aided with an inequality derived from the energy and enstrophy equations, The inequality forbids the simultaneous retention of substantial energy in the baroclinic mode and in scales near Rd when Ekman friction is symmetric, but points towards surface-trapped stratification and bottomtrapped friction as an environment in which both of these can be achieved, The dissertation also contains a systematic study of geostrophic turbulence forced by nonzonal flows, Narrow zonal jets emerge when shear-induced mean potential vorticity (PV) gradients are small compared to the planetary gradient (β), and energy is a strong function of the angle shear presents to the east-west direction, When shear-induced PV gradients are comparable to β, and the mean shear has a westward component, fields of monopolar vortices form and persist, Energy is asymmetric between fields of cyclones and anticyclones, Such asymmetry was commonly thought not to occur in QG, but is shown here to be introduced by the nonzonal basic state, In both jet and vortex regimes, eddy energy can be much larger than mean kinetic energy, contrary to the expectation that β stabilizes weak shear flows,

Description: My first three years here were funded by an Office of Naval Research/National Defense Science and Engineering Graduate Fellowship administered by Jeff Jarocz at the American Society for Engineering Education. During the last three years, most of my support has come the National Science Foundation via grant OCE-9617848, with some additional support coming from the Office of Naval Research via grant N00014-95-1-0824.

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: The evolution of a coastal ocean undergoing uniform surface heat loss is examined. The dynamics of this ocean are initially modulated by the intense vertical mixing driven by surface cooling. The strong vertical mixing prevents the formation of geostrophic flows and inhibits the cross-shelf flux of heat. The vertical mixing is eventually suppressed by the advective transport of cold, dense water offshore. Once this happens, alongshore geostrophic flows form, and become baroclinically unstable. The surface heat flux is then balanced by a cross-shelf eddy heat flux. Scales are found for the cross-shelf density gradient which results from this balance. Solutions for linear internal waves are found for a wedge-shaped bathymetry with bottom friction. Bottom friction is capable of entirely dissipating the waves before they reach the coast, and waves traveling obliquely offshore are reflected back to the coast from a caustic. The internal wave climate near two moorings of the Coastal Ocean Dynamics Experiment observation program is analyzed. The high frequency internal wave energy levels were elevated above the Garrett and Munk spectrum, and the spectrum becomes less red as one moves to the shore. The wave field is dominated by vertical-mode one waves, and internal wave energy propagates shoreward.

Description: This work was funded by an Office of Naval Research fellowship and and Office of Naval Research AASERT fellowship, N00014-95:-1-0746.

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 2011

Description: This thesis describes the physics of fully three-dimensional low frequency acoustic interaction with internal waves, bottom sediment waves and surface swell waves that are often observed in shallow waters and on continental slopes. A simple idealized model of the ocean waveguide is used to analytically study the properties of acoustic normal modes and their perturbations due to waves of each type. The combined approach of a semi-quantitative study based on the geometrical acoustics approximation and on fully three-dimensional coupled mode numerical modeling is used to examine the azimuthal dependence of sound wave horizontal reflection from, transmission through and ducting between straight parallel waves of each type. The impact of the natural crossings of nonlinear internal waves on horizontally ducted sound energy is studied theoretically and modeled numerically using a three-dimensional parabolic equation acoustic propagation code. A realistic sea surface elevation is synthesized from the directional spectrum of long swells and used for three-dimensional numerical modeling of acoustic propagation. As a result, considerable normal mode amplitude scintillations were observed and shown to be strongly dependent on horizontal azimuth, range and mode number. Full field numerical modeling of low frequency sound propagation through large sand waves located on a sloped bottom was performed using the high resolution bathymetry of the mouth of San Francisco Bay. Very strong acoustic ducting is shown to steer acoustic energy beams along the sand wave’s curved crests.

Description: Office of Naval Research for the financial support of this work.

Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Reviews of Geophysics 49 (2011): RG4003, doi:10.1029/2010RG000329.

Description: Many major oceanographic internal wave observational programs of the last 4 decades are reanalyzed in order to characterize variability of the deep ocean internal wavefield. The observations are discussed in the context of the universal spectral model proposed by C. J. R. Garrett and W. H. Munk. The Garrett and Munk model is a good description of wintertime conditions at Site D on the continental rise north of the Gulf Stream. Elsewhere and at other times, significant deviations in terms of amplitude, separability of the 2-D vertical wavenumber-frequency spectrum, and departure from the model's functional form are reported. Specifically, the Garrett and Munk model overestimates annual average frequency domain spectral levels both at Site D and in general. The bias at Site D is associated with the Garrett and Munk model being a fit to wintertime data from Site D and the presence of an annual cycle in high-frequency energy in the western subtropical North Atlantic having a maximum in winter. The wave spectrum is generally nonseparable, with near-inertial waves typically having greater bandwidth (occupying smaller vertical scales) than continuum frequency waves. Separability is a better approximation for more energetic states, such as wintertime conditions at Site D. Subtle geographic differences from the high-frequency and high vertical wavenumber power laws of the Garrett and Munk spectrum are apparent. Such deviations tend to covary: whiter frequency spectra are partnered with redder vertical wavenumber spectra. We review a general theoretical framework of statistical radiative balance equations and interpret the observed variability in terms of the interplay between generation, propagation, and nonlinearity. First, nonlinearity is a fundamental organizing principle in this work. The observed power laws lie close to the induced diffusion stationary states of the resonant kinetic equation describing the lowest-order nonlinear transfers. Second, eddy variability and by implication wave mean interactions are also an organizing principle. Observations from regions of low eddy variability tend to be outliers in terms of their parametric spectral representation; other data tend to cluster in two regions of parameter space. More tentatively, the seasonal cycle of high-frequency energy is in phase with the near-inertial seasonal cycle in regions of significant eddy variability. In regions of low eddy variability, the seasonal cycle in high-frequency energy lags that of near-inertial energy. The induced diffusion stationary states are approximate analytic solutions to the resonant kinetic equation, and the Garrett and Munk spectrum represents one such analytic solution. We present numerical solutions of the resonant kinetic equation, however, that are inconsistent with the Garrett and Munk model representing a stationary state, either alone or in combination with other physical mechanisms. We believe this to be the case for other regional characterizations as well. We argue that nonstationarity of the numerical solutions is related to local transfers in horizontal wavenumber, whereas the analytic induced diffusion stationary states consider only nonlocal transfers in vertical wavenumber. Consequences for understanding the pathways by which energy is transferred from sources to sinks are considered. Further progress likely requires self-consistent solutions to a broadened kinetic equation.

Description: We gratefully acknowledge funding provided by a Collaborations in Mathematical Geosciences (CMG) grant from the National Science Foundation. Y.L. additionally acknowledges NSF DMS grant 0807871 and ONR award N00014‐09‐1‐0515.

Description: 2012-05-10

Description: Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1993

Description: Travel time perturbations of adiabatic normal modes due to an internal tide and internal mode field in the Barents Sea are examined. A formalism for the travel time perturbation due to a change in sound speed is presented. Internal tide and internal wave amplitude spectra are calculated from Brancker temperature loggers which were deployed on moorings in the Barents Sea during the August 1992 Barents Sea Polar Front Experiment. In particular, the first three internal wave mode amplitudes are estimated from the four Brancker temperature loggers on the southwest mooring of the array. Modal perturbations in acoustic pulse travel time and the travel time covariance are calculated and compared for consistency to a simple ray model. These perturbations are small for the modal arrivals that the vertical acoustic array which was deployed is expected to resolve. The third internal wave mode has the largest impact on the acoustic arrivals, per unit amplitude, but the first internal wave mode dominates the scattering due to having a much larger amplitude overall.

Description: Author Posting. © American Meteorological Society, 2013. 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 43 (2013): 259–282, doi:10.1175/JPO-D-11-0194.1.

Description: This study reports on observations of turbulent dissipation and internal wave-scale flow properties in a standing meander of the Antarctic Circumpolar Current (ACC) north of the Kerguelen Plateau. The authors characterize the intensity and spatial distribution of the observed turbulent dissipation and the derived turbulent mixing, and consider underpinning mechanisms in the context of the internal wave field and the processes governing the waves’ generation and evolution. The turbulent dissipation rate and the derived diapycnal diffusivity are highly variable with systematic depth dependence. The dissipation rate is generally enhanced in the upper 1000–1500 m of the water column, and both the dissipation rate and diapycnal diffusivity are enhanced in some places near the seafloor, commonly in regions of rough topography and in the vicinity of strong bottom flows associated with the ACC jets. Turbulent dissipation is high in regions where internal wave energy is high, consistent with the idea that interior dissipation is related to a breaking internal wave field. Elevated turbulence occurs in association with downward-propagating near-inertial waves within 1–2 km of the surface, as well as with upward-propagating, relatively high-frequency waves within 1–2 km of the seafloor. While an interpretation of these near-bottom waves as lee waves generated by ACC jets flowing over small-scale topographic roughness is supported by the qualitative match between the spatial patterns in predicted lee wave radiation and observed near-bottom dissipation, the observed dissipation is found to be only a small percentage of the energy flux predicted by theory. The mismatch suggests an alternative fate to local dissipation for a significant fraction of the radiated energy.

Description: SW acknowledges the support of the Grantham Institute for Climate Change, Imperial College London. ACNG acknowledges the support of a NERC Advanced Research Fellowship (Grant NE/C517633/1). KLP acknowledges support from Woods Hole Oceanographic Institution bridge support funds.

Description: 2013-08-01

Description: Author Posting. © American Meteorological Society, 2013. 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 43 (2013): 766–789, doi:10.1175/JPO-D-12-0141.1.

Description: Nonlinear energy transfers from the semidiurnal internal tide to high-mode, near-diurnal motions are documented near Kaena Ridge, Hawaii, an energetic generation site for the baroclinic tide. Data were collected aboard the Research Floating Instrument Platform (FLIP) over a 35-day period during the fall of 2002, as part of the Hawaii Ocean Mixing Experiment (HOME) Nearfield program. Energy transfer terms for a PSI resonant interaction at midlatitude are identified and compared to those for near-inertial PSI close to the M2 critical latitude. Bispectral techniques are used to demonstrate significant energy transfers in the Nearfield, between the low-mode M2 internal tide and subharmonic waves with frequencies near M2/2 and vertical wavelengths of O(120 m). A novel prefilter is used to test the PSI wavenumber resonance condition, which requires the subharmonic waves to propagate in opposite vertical directions. Depth–time maps of the interactions, formed by directly estimating the energy transfer terms, show that energy is transferred predominantly from the tide to subharmonic waves, but numerous reverse energy transfers are also found. A net forward energy transfer rate of 2 × 10−9 W kg−1 is found below 400 m. The suggestion is that the HOME observations of energy transfer from the tide to subharmonic waves represent a first step in the open-ocean energy cascade. Observed PSI transfer rates could account for a small but significant fraction of the turbulent dissipation of the tide within 60 km of Kaena Ridge. Further extrapolation suggests that integrated PSI energy transfers equatorward of the M2 critical latitude may be comparable to PSI energy transfers previously observed near 28.8°N.

Description: This work was supported by the National Science Foundation and the Office of Naval Research.

Description: 2013-10-01

Description: Author Posting. © American Meteorological Society, 2014. 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 44 (2014): 229–245, doi:10.1175/JPO-D-12-0218.1.

Description: Data from a mooring deployed at the edge of the East Greenland shelf south of Denmark Strait from September 2007 to October 2008 are analyzed to investigate the processes by which dense water is transferred off the shelf. It is found that water denser than 27.7 kg m−3—as dense as water previously attributed to the adjacent East Greenland Spill Jet—resides near the bottom of the shelf for most of the year with no discernible seasonality. The mean velocity in the central part of the water column is directed along the isobaths, while the deep flow is bottom intensified and veers offshore. Two mechanisms for driving dense spilling events are investigated, one due to offshore forcing and the other associated with wind forcing. Denmark Strait cyclones propagating southward along the continental slope are shown to drive off-shelf flow at their leading edges and are responsible for much of the triggering of individual spilling events. Northerly barrier winds also force spilling. Local winds generate an Ekman downwelling cell. Nonlocal winds also excite spilling, which is hypothesized to be the result of southward-propagating coastally trapped waves, although definitive confirmation is still required. The combined effect of the eddies and barrier winds results in the strongest spilling events, while in the absence of winds a train of eddies causes enhanced spilling.

Description: The authors wish to thank Paula Fratantoni, Frank Bahr, and Dan Torres for processing the mooring data. The mooring array was capably deployed by the crew of the R/V Arni Fridriksson and recovered by the crew of the R/V Knorr. We thank Hedinn Valdimarsson for his assistance in the field work. Ken Brink provided valuable insights regarding the dynamics of shelf waves. Funding for the study was provided by National Science Foundation Grant OCE-0722694, the Arctic Research Initiative of the Woods Hole Oceanographic Institution. We also wish to thank the Natural Environment Research Council for Ph.D. studentship funding, and the University of East Anglia’s Roberts Fund and Royal Meteorological Society for supporting travel for collaboration.

Description: 2014-07-01

Description: Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 1822–1842, doi:10.1175/JPO-D-14-0147.1.

Description: Influences of time-dependent precipitation on water mass transformation and heat budgets in an idealized marginal sea are examined using theoretical and numerical models. The equations proposed by Spall in 2012 are extended to cases with time-dependent precipitation whose form is either a step function or a sinusoidal function. The theory predicts the differences in temperature and salinity between the convective water and the boundary current as well as the magnitudes of heat fluxes into the marginal sea and across the sea surface. Moreover, the theory reveals that there are three inherent time scales: relaxation time scales for temperature and salinity and a precipitation time scale. The relaxation time scales are determined by a steady solution of the theoretical model with steady precipitation. The relaxation time scale for temperature is always smaller than that for salinity as a result of not only the difference in the form of fluxes at the surface but also the variation in the eddy transport from the boundary current. These three time scales and the precipitation amplitude determine the strength of the ocean response to changes in precipitation and the phase relation between precipitation, changes in salinity and temperature, and changes in heat fluxes. It is demonstrated that the theoretical predictions agree qualitatively well with results from the eddy-resolving numerical model. This demonstrates the fundamental role of mesoscale eddies in the ocean response to time-dependent forcing and provides a framework with which to assess the extent to which observed variability in marginal sea convection and water mass transformation are consistent with an external forcing by variations in precipitation.

Description: This work was initiated at the 2013 WHOI Geophysical Fluid Dynamics Summer Program, which was supported by the National Science Foundation and the Office of Naval Research. This work was also supported by Grant-in-Aid for Research Fellow (25·8466) of the Ministry of Education, Culture, Sports and Technology (MEXT), Japan, the Program for Leading Graduate Schools, MEXT, Japan (YY), and by the National Science Foundation Grant OCE-1232389 (MAS).

Description: 2016-01-01

Description: Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 2773–2789, doi:10.1175/JPO-D-15-0031.1.

Description: Tidal oscillatory salt transport, induced by the correlation between tidal variations in salinity and velocity, is an important term for the subtidal salt balance under the commonly used Eulerian method of salt transport decomposition. In this paper, its mechanisms in a partially stratified estuary are investigated with a numerical model of the Hudson estuary. During neap tides, when the estuary is strongly stratified, the tidal oscillatory salt transport is mainly due to the hydraulic response of the halocline to the longitudinal variation of topography. This mechanism does not involve vertical mixing, so it should not be regarded as oscillatory shear dispersion, but instead it should be regarded as advective transport of salt, which results from the vertical distortion of exchange flow obtained in the Eulerian decomposition by vertical fluctuations of the halocline. During spring tides, the estuary is weakly stratified, and vertical mixing plays a significant role in the tidal variation of salinity. In the spring tide regime, the tidal oscillatory salt transport is mainly due to oscillatory shear dispersion. In addition, the transient lateral circulation near large channel curvature causes the transverse tilt of the halocline. This mechanism has little effect on the cross-sectionally integrated tidal oscillatory salt transport, but it results in an apparent left–right cross-channel asymmetry of tidal oscillatory salt transport. With the isohaline framework, tidal oscillatory salt transport can be regarded as a part of the net estuarine salt transport, and the Lagrangian advective mechanism and dispersive mechanism can be distinguished.

Description: Tao Wang was supported by the Open Research Fund of State Key Laboratory of Estuarine and Coastal Research (Grant SKLEC-KF201509) and Chinese Scholarship Council. Geyer was supported by by NSF Grant OCE 0926427. Wensheng Jiang was supported by NSFC-Shandong Joint Fund for Marine Science Research Centers (Grant U1406401).

Description: 2016-05-01

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 2016

Description: Eddies in the ocean move westwards. Those shed by western boundary currents must then interact with continental shelf-slope topography at the western boundary. The presence of other eddies and mean lows complicates this simple picture, yet satellite images show that mesoscale eddies translating near the shelfbreak routinely affect the continental shelves of the Mid-Atlantic Bight, the Gulf of Mexico etc. The consequent cross-shelfbreak transports are currently of unknown importance to shelf budgets of heat, salt and volume. Thus motivated, this thesis uses idealized continuously stratified numerical experiments to explore eddy-slope interactions under four questions: 1. Can the continental slope prevent an eddy from reaching the shelfbreak? 2. What is the structure of the eddy-driven offshore low? 3. How is the continental shelf affected by an eddy at the shelfbreak? 4. Given surface observations, can one estimate the volume of water transported across the shelfbreak? The experiments show that the efficiency of Rossby wave radiation from the eddy controls whether it can cross isobaths: by radiating energy the eddy becomes shallow enough to move into shallower water. For wide continental slopes, relative to an eddy diameter, a slope can prevent an anticyclone from reaching the shelfbreak by shutting down such radiation. For narrow continental slopes, the interaction repeatedly produces dipoles, whose cyclonic halves contain shelf-slope water stacked over eddy water. The formation of such cyclones is explained. Then, the structure of shelf lows forced by the eddy are studied: their vertical structures are rationalized and scalings derived for their cross-isobath scales; for example, the extent to which the eddy influences the shelf. A recipe for estimating cross-isobath transports based on eddy surface properties is put forward. Finally, the findings are tested against observations in the Middle Atlantic Bight of the northeastern United States.

Description: I acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation. The research presented here was funded by National Science Foundation grants OCE-1059632 and OCE-1433953. Funding support from the Academic Programs Oice, WHOI is also gratefully acknowledged.

Description: Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 773-794, doi:10.1175/JPO-D-17-0205.1.

Description: Fourteen autonomous profiling floats, equipped with CTDs, were deployed in the deep eastern and western basins of the Gulf of Mexico over a four-year interval (July 2011–August 2015), producing a total of 706 casts. This is the first time since the early 1970s that there has been a comprehensive survey of water masses in the deep basins of the Gulf, with better vertical resolution than available from older ship-based surveys. Seven floats had 14-day cycles with parking depths of 1500 m, and the other half from the U.S. Argo program had varying cycle times. Maps of characteristic water masses, including Subtropical Underwater, Antarctic Intermediate Water (AAIW), and North Atlantic Deep Water, showed gradients from east to west, consistent with their sources being within the Loop Current (LC) and the Yucatan Channel waters. Altimeter SSH was used to characterize profiles being in LC or LC eddy water or in cold eddies. The two-layer nature of the deep Gulf shows isotherms being deeper in the warm anticyclonic LC and LC eddies and shallower in the cold cyclones. Mixed layer depths have an average seasonal signal that shows maximum depths (~60 m) in January and a minimum in June–July (~20 m). Basin-mean steric heights from 0–50-m dynamic heights and altimeter SSH show a seasonal range of ~12 cm, with significant interannual variability. The translation of LC eddies across the western basin produces a region of low homogeneous potential vorticity centered over the deepest part of the western basin.

Description: The authors were supported by the Department of the Interior, Bureau of Ocean Energy Management (BOEM), Contract M08PC20043 to Leidos, Inc., Raleigh, North Carolina.

Description: 2018-10-04

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, 1976

Description: Measurements of horizontal and vertical current by propeller cluster current meters and temperature by thermistors mounted on a rigid array 8 m high and 20 m long moored in the oceanic main thermocline near Bermuda are interpreted in terms of thermocline-trapped internal wave modes in the presence of temperature and density fine-structure. Two turning-point uniformly valid asymptotic solutions to the internal wave equation are developed to describe the wave functions. Mode decay beyond the turning point in depth or frequency produces a sharp cutoff in vertical current spectra above the local buoyancy frequency N(z). An internal wave wavenumber-frequency spectral model Ε(α,ω) = E(ω/No)-2 (α./α0)-2 describes vertical current spectra and potential energy to horizontal kinetic energy ratios. The red wavenumer shape suppresses peaks in both these quantities at frequencies near N(z). The data are consistent with time-averaged horizontal isotropy of the wave field. A dip in the vertical current spectra at 0.5 cph not predicted by the model appears related to the bottom slope. Temperature fine-structure is modeled as a passive vertical field advected by internal waves. Quasi-permanent fine-scale features of the stratification and vertically small-scale internal waves are indistinguishable in this study. The model of McKean (1974) is generalized to include fine-structure fields specified by their vertical wavenumber spectra as well as different Poisson-distributed layer models. Together with the trapped internal wave model, moored temperature spectra, temperature vertical difference spectra, and coherence over vertical separations are described using a fine-structure vertical wavenumber spectrum PT(k) =ATk-5/2 which agrees with other spectra made using vertical profiling instruments in the range 0.1 to 1.0 cpm. Horizontal current fine-structure is also modeled as a passive field advected vertically by long internal waves. The model describes moored horizontal current spectra (least successfully at frequencies near N(z)) and finite-difference vertical shear spectra. Contours of temperature in depth versus time indicate possible mixing events. These events appear concurrently with high shear and Richardson numbers O. 25≤ R ≤ 1.0. Over 7 m a cutoff in Ri at 0.25 is observed, indicating saturation of the internal wave spectrum. Spectra of finite-difference approximations to shear and buoyancy frequency are dominated by fine-structure contributions over nearly the whole internal wave range, suggesting that breaking is enhanced by fine-structure. Breaking appears equally likely at all frequencies in the internal wave range.

Description: This research was supported by Office of Naval Research contract N00014-67-0204-0047 and continuation contract NOOOl4-75-C-0291.

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 2003

Description: The importance of eddies and nonlinearities in ß-plume dynamics in the deep ocean was investigated using reduced gravity models of the deep ocean forced by a small region of cross isopycnal transport in the interior. The effect of topography on ß-plumes was also examined by placing a Gaussian bump in the forcing region. Despite the fact that the mean flow is weak in the deep ocean interior, it was found that the nonlinearity and instabilities are still important for realistic parameter and forcing values. The flow was dominated by eddies and was remarkably different from what would be expected from a linear solution.

Description: This study was supported by Woods Hole Oceanographic Institution Academic Programs Office Fellowship and National Science Foundation Grant 89542700.

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 May 1982

Description: Velocity and temperature time series from Hudson Submarine Canyon and hydrographic surveys of seven canyons of the Middle Atlantic Bight indicate that the effects of storms, tides, and incoming internal waves are intensified in submarine canyons. Storms with strong eastward and westward wind stress were found to cause strong upwelling and downwelling through the upper layers of Hudson Canyon. Storm-forced upwelling also caused strong down-canyon flows at the canyon floor. Internal waves were found to be concentrated in the canyon head and near the floor, in agreement with theoretical predictions. Slope water apparently circulates slowly through the outer part of the canyon and is mixed in near-floor layers which could be caused by breaking internal waves. Internal tides are generated at the floor in the central part of the canyon. Oscillations at tidal frequencies dominate the near-floor velocity field below the thermocline, and are accompanied by high-frequency spikes that may be nonlinear interface waves propagating on the top of the bottom mixed layer. A numerical model was used to calculate mixing in the canyon's bottom boundary layer caused by an unstable density gradient during flood tide. Energetic internal wave activity is apparently responsible for sediment sorting in the canyon head; the internal waves become more energetic as the sediment grain size increases. Below the thermocline, the tidal oscillations vary in amplitude with the phases of the moon; the observed deposition of mud can easily occur during weeks of low velocity.

Description: Foundation graduate fellowship and by the Office of Naval Research under Contracts N00014-75-C-029l and N00014-80-C-0273.

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: Nonlinear quasigeostrophic flows in two layers over a topographic slope are considered. The evolution depends on the size of two parameters which indicate the degree of nonlinearity at depth. The first measures the importance of relative vorticity advection and the second of stretching vorticity. Two types of isolated vortex are used to examine the parameter dependence. An initially barotropic vortex remains barotropic only when the first parameter is large, otherwise topographic waves dominate at depth. An Initially surface-trapped vortex larger than deformation scale is baroclinically unstable when the second is large, but is stabilized by the slope otherwise. Both parameters are also relevant to cascading geostrophic turbulence. If the stretching parameter is large, a "barotropic cascade" occurs at the deformation radius (Rhines, 1977) and the cascade "arrests" when the relative vorticity parameter is order unity. If small, layer coupling is hindered and the cascade is arrested at the deformation scale, with the flow dominated by isotropic surface vortices. In both cases, the distinction between vortices and waves is transparent when viewing potential vorticity. It is more difficult to identify waves and vortices from the streamfunction fields, because the waves are present in both layers.

Description: Funding for this research was provided by Office of Naval Research Coastal Science Code, grants N00014-92-J-1643 and N00014-92-J-1528.

Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C03005, doi:10.1029/2008JC004762.

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 1987

Description: Garrett and Munk use linear dynamics to synthesize frequency-wavenumber energy spectra for internal waves (GM72, GM75, GM79). The GM internal wave models are horizontally isotropic, vertically symmetric, purely propagating, and universal in both time and space. This set of properties effectively eliminates all the interesting physics, since such models do not allow localized sources and sinks of energy. Thus an important step in understanding internal wave dynamics is to make measurements of deviations from the simple GM models. This thesis continues the search for deviations from the GM models. It has three advantages over earlier work: extensive data from an equatorial region, long time series (2 years), and relatively sophisticated linear internal wave models. Since the GM models are based on mid-latitude data, having data from an equatorial region which has a strong mean current system offers an opportunity to examine a region with a distinctly different basic state. The longer time series mean there is a larger statistical ensemble of realizations, making it possible to detect smaller internal wave signals. The internal wave models include several important extensions to the GM models: horizontal anisotropy and vertical asymmetry, resolution between standing modes and propagating waves, general vertical structure, and kinematic effects of mean shear flow. Also investigated are the effects of scattering on internal waves, effects that are especially strong on the equator because the buoyancy frequency variability is a factor of ten higher than at mid-latitudes. In the high frequency internal wave field considered (frequencies between .125 cph and .458 cph), several features are found that are not included in the GM models. Both the kinematic effects of a mean shear flow and the phase-locking that distinguishes standing modes from propagating waves are observed. There is a seasonal dependence in energy level of roughly 10% of the mean level. At times the wave field is zonally and vertically asymmetric, with resulting energy fluxes that are a small (4% to 10%) fraction of the maximum energy flux the internal wave field could support. The fluxes are, however, as big as many of the postulated sources of energy for the internal wave field.

Description: This work has been supported under grants from the National Science Foundation and the Office of Naval Research, grants numbered NSF-89076, ONR-88914, NSF-9l002, NSF-94971, and NSF-93661.

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): 2341–2347, doi:10.1175/2010JPO4465.1.

Description: The mean downwelling in an eddy-resolving model of a convective basin is concentrated near the boundary where eddies are shed from the cyclonic boundary current into the interior. It is suggested that the buoyancy-forced downwelling in the Labrador Sea and the Lofoten Basin is similarly concentrated in analogous eddy formation regions along their eastern boundaries. Use of a transformed Eulerian mean depiction of the density transport reveals the central role eddy fluxes play in maintaining the adiabatic nature of the flow in a nonperiodic region where heat is lost from the boundary current. The vorticity balance in the downwelling region is primarily between stretching of planetary vorticity and eddy flux divergence of relative vorticity, although a narrow viscous boundary layer is ultimately important in closing the regional vorticity budget. This overall balance is similar in some ways to the diffusive–viscous balance represented in previous boundary layer theories, and suggests that the downwelling in convective basins may be properly represented in low-resolution climate models if eddy flux parameterizations are adiabatic, identify localized regions of eddy formations, and allow density to be transported far from the region of eddy formations.

Description: This study was supported by the National Science Foundation under Grants OCE-0726339 and OCE-0850416.

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): 3162-3175, doi:10.1175/2009JPO4239.1.

Description: This study analyzes anisotropic properties of the material transport by eddies and eddy-driven zonal jets in a general circulation model of the North Atlantic through the analysis of Lagrangian particle trajectories. Spreading rates—defined here as half the rate of change in the particle dispersion—in the zonal direction systematically exceed the meridional rates by an order of magnitude. Area-averaged values for the upper-ocean zonal and meridional spreading rates are approximately 8100 and 1400 m2 s−1, respectively, and in the deep ocean they are 2400 and 200 m2 s−1. The results demonstrate that this anisotropy is mainly due to the action of the transient eddies and not to the shear dispersion associated with the time-mean jets. This property is consistent with the fact that eddies in this study have zonally elongated shapes. With the exception of the upper-ocean subpolar gyre, eddies also cause the superdiffusive zonal spreading, significant variations in the spreading rate in the vertical and meridional directions, and the difference between the westward and eastward spreading.

Description: Funding for IK was provided by NSF Grants OCE 0346178, 0749722, and 0842834. Funding for PB was provided by NSF Grants OCE 0344094 and OCE 0725796 and by the research grant from the Newton Trust of the University of Cambridge. For JP the acknowledgement is to NSF OCE-0451086.

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): 1361-1379, doi:10.1175/2008JPO4096.1.

Description: Multiple zonal jets are observed in satellite data–based estimates of oceanic velocities, float measurements, and high-resolution numerical simulations of the ocean circulation. This study makes a step toward understanding the dynamics of these jets in the real ocean by analyzing the vertical structure and dynamical balances within multiple zonal jets simulated in an eddy-resolving primitive equation model of the North Atlantic. In particular, the authors focus on the role of eddy flux convergences (“eddy forcing”) in supporting the buoyancy and relative/potential vorticity (PV) anomalies associated with the jets. The results suggest a central role of baroclinic eddies in the barotropic and baroclinic dynamics of the jets, and significant differences in the effects of eddy forcing between the subtropical and subpolar gyres. Additionally, diabatic potential vorticity sources and sinks, associated with vertical diffusion, are shown to play an important role in supporting the potential vorticity anomalies. The resulting potential vorticity profile does not resemble a “PV staircase”—a distinct meridional structure observed in some idealized studies of geostrophic turbulence.

Description: Funding for IK was provided by NSF Grants OCE 0346178 and 0749722. Funding for PB was provided by NSF Grants OCE 0344094 and OCE 0725796 and by the research grant from the Newton Trust of the University of Cambridge. For JP the acknowledgement is to NSF OCE-0451086.

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 August 2000

Description: With Lagrangian and hydrographic data taken in the deep Brazil Basin, we identify several submesoscale coherent vortices (SCVs). These features contrast with SCV paradigms in that float data indicate approximately equal populations of cyclonic and anticyclonic vortices, and hydrographic data suggest that roughly half exhibit the convex lens shape generally associated with SCVs, while half are instead shaped like a concave lens, with isopycnal surfaces pinched together. There is some evidence that the vortex cores may be enriched in warm, salty North Atlantic Deep Water, suggesting formation in the north or northwest regions of the basin. Data is available from 153 floats which were ballasted for 2500 and 4000 db pressures. They tracked 34 eddies, which are believed to be roughly 30 km in diameter, and rotate with apparent periods of about 30 days. Many floats experienced formation or entrainment events, and destruction or detrainment events, near seamounts.

Description: This work was supported by the National Science Foundation, grants OCE 99-11148 and OCE 94-15509.

Description: Author Posting. © American Meteorological Society, 2011. 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 41 (2011): 1182–1208, doi:10.1175/2010JPO4564.1.

Description: The authors use data collected by a line of tall current meter moorings deployed across the axis of the Kuroshio Extension (KE) jet at the location of maximum time-mean eddy kinetic energy to characterize the mean jet structure, the eddy variability, and the nature of eddy–mean flow interactions observed during the Kuroshio Extension System Study (KESS). A picture of the 2-yr record mean jet structure is presented in both geographical and stream coordinates, revealing important contrasts in jet strength, width, vertical structure, and flanking recirculation structure. Eddy variability observed is discussed in the context of some of its various sources: jet meandering, rings, waves, and jet instability. Finally, various scenarios for eddy–mean flow interaction consistent with the observations are explored. It is shown that the observed cross-jet distributions of Reynolds stresses at the KESS location are consistent with wave radiation away from the jet, with the sense of the eddy feedback effect on the mean consistent with eddy driving of the observed recirculations. The authors consider these results in the context of a broader description of eddy–mean flow interactions in the larger KE region using KESS data in combination with in situ measurements from past programs in the region and satellite altimetry. This demonstrates important consistencies in the along-stream development of time-mean and eddy properties in the KE with features of an idealized model of a western boundary current (WBC) jet used to understand the nature and importance of eddy–mean flow interactions in WBC jet systems.

Description: This work was supported by National Science Foundation funding for the KESS program under Grants OCE-0220161 (SW, NGH, and SRJ), OCE- 0825550 (SW), OCE-0850744 (NGH), and OCE-0849808 (SRJ). SW was also supported by the MIT Presidential Fellowship. The financial assistance of the Houghton Fund, the MIT Student Assistance Fund, and WHOI Academic Programs is also gratefully acknowledged.

Description: Author Posting. © American Meteorological Society, 2011. 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 41 (2011): 2223–2241, doi:10.1175/2011JPO4344.1.

Description: Results are presented from an observational study of stratified, turbulent flow in the bottom boundary layer on the outer southeast Florida shelf. Measurements of momentum and heat fluxes were made using an array of acoustic Doppler velocimeters and fast-response temperature sensors in the bottom 3 m over a rough reef slope. Direct estimates of flux Richardson number Rf confirm previous laboratory, numerical, and observational work, which find mixing efficiency not to be a constant but rather to vary with Frt, Reb, and Rig. These results depart from previous observations in that the highest levels of mixing efficiency occur for Frt 〈 1, suggesting that efficient mixing can also happen in regions of buoyancy-controlled turbulence. Generally, the authors find that turbulence in the reef bottom boundary layer is highly variable in time and modified by near-bed flow, shear, and stratification driven by shoaling internal waves.

Description: Funding was provided by grants from the National Oceanic and Atmospheric Administration’s National Undersea Research Program, National Science Foundation Grants OCE-0622967 and OCE- 0824972 to SGM, and the Singapore Stanford Program. Kristen Davis was supported by a National Defense Science and Engineering Graduate Fellowship and an ARCS Foundation Fellowship.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution February 1994

Description: In this work we study motion of a baroclinic upper-ocean eddy over a large-scale topography which simulates a continental slope. We use a quasigeostrophic f-plane approximation with continuous stratification. To study this problem we develop a new numerical technique which we call "semi-lagrangian contour dynamics". This technique resembles the traditional 2-D contour dynamics method but differs significantly from it in the numerical algorithm. In addition to "Lagrangian" moving contours it includes an underlying "Eulerian" regular grid to which vorticity or density fields are interpolated. To study topographic interactions in a continuously stratified model we use density contours at the bottom in a similar manner as vorticity contours are used in the standard contour dynamics. For the case of a localized upper-ocean vortex moving over a sloping bottom the problem becomes computationally 2-dimensional (we need to follow only bottom density contours and the position of the vortex itself) although the physical domain is still 3-dimensional. Results of the numerical model lndicate importance of baroclinic effects in the vortex-topography interaction. After the initial surge of topographic Rossby waves a vortex moves almost steadily due to the interaction with a bottom density anomaly which is created and supported by a vortex itself. This anomaly is equivalent to a region of opposite-signed vorticity with a total circulation exactly compensating that of a vortex. This results in a vertically aligned dipolar structure with the total barotropic component equal to zero. Analytical considerations explaining this effect are presented and formulated in a more general statement which resembles but does not coincide with the "zero angular momentum theorem" of Flierl, Stern and Whitehead, 1983. In such steady translation the centroid of a bottom density anomaly is displaced horizon tally from the center of an upper-ocean vortex so the whole system moves due to this misalignment, which is known as a "he tonic mechanism". Cyclonic vortices go generally upslope, and anticyclones - in a downslope direction. The along-slope component of their motion depends upon the strength of a vortex, curvature of the bottom slope and background flows. When surrounded by a bowl-shaped topography anticyclonic vortices tend to stay near the deepest center of a basin, even resisting ambient flows which advect them outward. Application of this results to various oceanic examples (particularly to the "Shikmona eddy" in the Eastern Meditenanian) is discussed. Our results show that the behavior of a vortex over a sloping bottom differs significantly from its motion on the planetary beta-plane (but with a flat bottom). To explain this difference we introduce the concept of a "wave-breaking regime" relevant for the case of a planetary beta-effect, and a "wave-gliding regime" which characterizes the interaction of an eddy with a topographic slope.

Description: This work was supported by the NSF grant #OCE 90-12821.

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 August 1995

Description: This thesis investigates the amplitude fluctuation effects in acoustic scattering due to shallow water internal waves. Theoretically, it uses the adiabatic approximation and perturbation methods to statistically evaluate acoustic transmission fluctuations caused by internal waves in the ocean; it also investigates acoustic mode coupling effect due to internal waves. Numerically, this thesis simulates the shallow water internal wave (IW) field using the Garrett-Munk internal wave spectrum model and then evaluates acoustic transmission in the simulated internal wave field with the Kraken normal mode program. Theoretical calculations are also performed using the theory developed in this thesis. Comparisons are made between theory and numerical calculations. The results presented and discussed in this thesis are related to the following issues: coherent and incoherent intensity fluctuations for adiabatic approximation, acoustic mode coupling due to IW's, transmission loss difference between adiabatic and coupled mode methods, and their dependence on range, IW amplitude and frequency.

Description: Author Posting. © American Meteorological Society, 2013. 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 43 (2013): 1611–1626, doi:10.1175/JPO-D-12-0204.1.

Description: A new method is proposed for extrapolating subsurface velocity and density fields from sea surface density and sea surface height (SSH). In this, the surface density is linked to the subsurface fields via the surface quasigeostrophic (SQG) formalism, as proposed in several recent papers. The subsurface field is augmented by the addition of the barotropic and first baroclinic modes, whose amplitudes are determined by matching to the sea surface height (pressure), after subtracting the SQG contribution. An additional constraint is that the bottom pressure anomaly vanishes. The method is tested for three regions in the North Atlantic using data from a high-resolution numerical simulation. The decomposition yields strikingly realistic subsurface fields. It is particularly successful in energetic regions like the Gulf Stream extension and at high latitudes where the mixed layer is deep, but it also works in less energetic eastern subtropics. The demonstration highlights the possibility of reconstructing three-dimensional oceanic flows using a combination of satellite fields, for example, sea surface temperature (SST) and SSH, and sparse (or climatological) estimates of the regional depth-resolved density. The method could be further elaborated to integrate additional subsurface information, such as mooring measurements.

Description: JW and AM were supported by NASA (NNX12AD47G) and NSF (OCE 0928617). JLM was supported by the Office of Naval Research and the Office of Science (BER), U.S. Department of Energy under DE-GF0205ER64119. GRF is supported by OCE-0752346 and JHL by NORSEE (Nordic Seas Eddy Exchanges) funded by the Norwegian Research Council.

Description: 2014-02-01

Description: Author Posting. © American Meteorological Society, 2014. 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 44 (2014): 2938–2950, doi:10.1175/JPO-D-13-0201.1.

Description: Direct observations in the Southern Ocean report enhanced internal wave activity and turbulence in a kilometer-thick layer above rough bottom topography collocated with the deep-reaching fronts of the Antarctic Circumpolar Current. Linear theory, corrected for finite-amplitude topography based on idealized, two-dimensional numerical simulations, has been recently used to estimate the global distribution of internal wave generation by oceanic currents and eddies. The global estimate shows that the topographic wave generation is a significant sink of energy for geostrophic flows and a source of energy for turbulent mixing in the deep ocean. However, comparison with recent observations from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean shows that the linear theory predictions and idealized two-dimensional simulations grossly overestimate the observed levels of turbulent energy dissipation. This study presents two- and three-dimensional, realistic topography simulations of internal lee-wave generation from a steady flow interacting with topography with parameters typical of Drake Passage. The results demonstrate that internal wave generation at three-dimensional, finite bottom topography is reduced compared to the two-dimensional case. The reduction is primarily associated with finite-amplitude bottom topography effects that suppress vertical motions and thus reduce the amplitude of the internal waves radiated from topography. The implication of these results for the global lee-wave generation is discussed.

Description: This research was supported by the National Science Foundation under Award CMG-1024198.

Description: 2015-05-01

Description: Author Posting. © American Meteorological Society, 2014. 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 44 (2014): 413–426, doi:10.1175/JPO-D-13-0117.1.

Description: Salinity and temperature profiles from drifting ice-tethered profilers in the Beaufort gyre region of the Canada Basin are used to characterize and quantify the regional near-inertial internal wave field over one year. Vertical displacements of potential density surfaces from the surface to 750-m depth are tracked from fall 2006 to fall 2007. Because of the time resolution and irregular sampling of the ice-tethered profilers, near-inertial frequency signals are marginally resolved. Complex demodulation is used to determine variations with a time scale of several days in the amplitude and phase of waves at a specified near-inertial frequency. Characteristics and variability of the wave field over the course of the year are investigated quantitatively and related to changes in surface wind forcing and sea ice cover.

Description: The ITP program and J. Toole’s contributions were supported by the National Science Foundation Office of Polar Programs Arctic Observing Network. We acknowledge the support of the Office of Naval Research (Grant N00014-11-1-0454) for this study. Support for H. Dosser was also provided by the Natural Sciences and Engineering Research Council of Canada.

Description: 2014-08-01

Description: Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 1735–1756, doi:10.1175/JPO-D-14-0238.1.

Description: The Lofoten basin of the Nordic Seas is recognized as a crucial component of the meridional overturning circulation in the North Atlantic because of the large horizontal extent of Atlantic Water and winter surface buoyancy loss. In this study, hydrographic and current measurements collected from a mooring deployed in the Lofoten basin from July 2010 to September 2012 are used to describe water mass transformation and the mesoscale eddy field. Winter mixed layer depths (MLDs) are observed to reach approximately 400 m, with larger MLDs and denser properties resulting from the colder 2010 winter. A heat budget of the upper water column requires lateral input, which balances the net annual heat loss of ~80 W m−2. The lateral flux is a result of mesoscale eddies, which dominate the velocity variability. Eddy velocities are enhanced in the upper 1000 m, with a barotropic component that reaches the bottom. Detailed examination of two eddies, from April and August 2012, highlights the variability of the eddy field and eddy properties. Temperature and salinity properties of the April eddy suggest that it originated from the slope current but was ventilated by surface fluxes. The properties within the eddy were similar to those of the mode water, indicating that convection within the eddies may make an important contribution to water mass transformation. A rough estimate of eddy flux per unit boundary current length suggests that fluxes in the Lofoten basin are larger than in the Labrador Sea because of the enhanced boundary current–interior density difference.

Description: The work was supported by NSF OCE 0850416.

Description: 2015-12-01

Description: Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 2381–2406, doi:10.1175/JPO-D-14-0086.1.

Description: While near-inertial waves are known to be generated by atmospheric storms, recent observations in the Kuroshio Front find intense near-inertial internal-wave shear along sloping isopycnals, even during calm weather. Recent literature suggests that spontaneous generation of near-inertial waves by frontal instabilities could represent a major sink for the subinertial quasigeostrophic circulation. An unforced three-dimensional 1-km-resolution model, initialized with the observed cross-Kuroshio structure, is used to explore this mechanism. After several weeks, the model exhibits growth of 10–100-km-scale frontal meanders, accompanied by O(10) mW m−2 spontaneous generation of near-inertial waves associated with readjustment of submesoscale fronts forced out of balance by mesoscale confluent flows. These waves have properties resembling those in the observations. However, they are reabsorbed into the model Kuroshio Front with no more than 15% dissipating or radiating away. Thus, spontaneous generation of near-inertial waves represents a redistribution of quasigeostrophic energy rather than a significant sink.

Description: “The Study of Kuroshio Ecosystem Dynamics for Sustainable Fisheries (SKED)” supported by MEXT, MIT-Hayashi Seed Fund, ONR (Awards N000140910196 and N000141210101), NSF (Award OCE 0928617, 0928138) for support.

Description: 2016-03-01

Description: Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 2497–2521, doi:10.1175/JPO-D-14-0128.1.

Description: Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale LT and the Ozmidov length scale LO. Historic evidence supporting LT ~ LO has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of LT and LO, using vertical profiles of temperature and microstructure shear collected in the Luzon Strait—a site characterized by topographically influenced overturns up to O(100) m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes. A key result is that LT/LO increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin–Helmholtz (K–H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting LT ~ LO are revisited, and conceptual regimes explaining the relationship between LT/LO and a nondimensional overturn size are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.

Description: B.D.M. and S.K.V. gratefully acknowledge the support of the Office of Naval Research under Grants N00014-12-1-0279, N00014-12-1-0282, and N00014-12-1-0938 (Program Manager: Dr. Terri Paluszkiewicz). S.K.V. also acknowledges support of the National Science Foundation under Grant OCE-1151838. L.S.L. acknowledges support for BBTRE by the National Science Foundation by Contract OCE94-15589 and NATRE and IWISE by the Office of Naval Research by Contracts N00014-92-1323 and N00014-10-10315. J.N.M. was supported through Grant 1256620 from the National Science Foundation and the Office of Naval Research (IWISE Project).

Description: 2016-04-01

Description: Author Posting. © American Meteorological Society, 2016. 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 46 (2016): 3263-3278, doi:10.1175/JPO-D-16-0091.1.

Description: The halocline of the Beaufort Gyre varies significantly on interannual to decadal time scales, affecting the freshwater content (FWC) of the Arctic Ocean. This study explores the role of eddies in the Ekman-driven gyre variability. Following the transformed Eulerian-mean paradigm, the authors develop a theory that links the FWC variability to the stability of the large-scale gyre, defined as the inverse of its equilibration time. The theory, verified with eddy-resolving numerical simulations, demonstrates that the gyre stability is explicitly controlled by the mesoscale eddy diffusivity. An accurate representation of the halocline dynamics requires the eddy diffusivity of 300 ± 200 m2 s−1, which is lower than what is used in most low-resolution climate models. In particular, on interannual and longer time scales the eddy fluxes and the Ekman pumping provide equally important contributions to the FWC variability. However, only large-scale Ekman pumping patterns can significantly alter the FWC, with spatially localized perturbations being an order of magnitude less efficient. Lastly, the authors introduce a novel FWC tendency diagnostic—the Gyre Index—that can be conveniently calculated using observations located only along the gyre boundaries. Its strong predictive capabilities, assessed in the eddy-resolving model forced by stochastic winds, suggest that the Gyre Index would be of use in interpreting FWC evolution in observations as well as in numerical models.

Description: GEMacknowledges the support from theHowland Postdoctoral Program Fund at WHOI and the Stanback Fellowship Fund at Caltech.MAS was supported by NSF Grants PLR-1415489 and OCE-1232389. AFT acknowledges support from NASA Award NNN12AA01C.

Description: 2017-04-20

Description: Author Posting. © American Meteorological Society, 2017. 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 47 (2017): 1789-1797, doi:10.1175/JPO-D-16-0240.1.

Description: Internal solitary waves are commonly observed in the coastal ocean where they are known to contribute to mass transport and turbulent mixing. While these waves are often generated by cross-isobath barotropic tidal currents, novel observations are presented suggesting that internal solitary waves result from along-isobath tidal flows over channel-shoal bathymetry. Mooring and ship-based velocity, temperature, and salinity data were collected over a cross-channel section in a stratified estuary. The data show that Ekman forcing on along-channel tidal currents drives lateral circulation, which interacts with the stratified water over the deep channel to generate a supercritical mode-2 internal lee wave. This lee wave propagates onto the shallow shoal and evolves into a group of internal solitary waves of elevation due to nonlinear steepening. These observations highlight the potential importance of three-dimensionality on the conversion of tidal flow to internal waves in the rotating ocean.

Description: National Science Foundation (OCE-1061609)

Description: 2018-01-03

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: Highly migratory marine fishes support valuable commercial fisheries worldwide. Yet, many target species have proven difficult to study due to long-distance migrations and regular deep diving. Despite the dominance of oceanographic features, such as fronts and eddies, in the open ocean, the biophysical interactions occurring at the oceanic (sub)mesoscale (〈 100 km) remain poorly understood. This leads to a paucity of knowledge on oceanographic associations of pelagic fishes and hinders management efforts. With ever-improving oceanographic datasets and modeling outputs, we can leverage these tools both to derive better estimates of animal movements and to quantify fish-environment interactions. In this thesis, I developed analytical tools to characterize the biophysical interactions influencing animal behavior and species’ ecology in the open ocean. A novel, observation-based likelihood framework was combined with a Bayesian state-space model to improve geolocation estimates for archival-tagged fishes using oceanographic profile data. Using this approach, I constructed track estimates for a large basking shark tag dataset using a high-resolution oceanographic model and discovered a wide range of movement strategies. I also applied this modeling approach to track archival-tagged swordfish, which revealed affinity for thermal front and eddy habitats throughout the North Atlantic that was further corroborated by synthesizing these results with a fisheries-dependent conventional tag dataset. An additive modeling approach applied to longline catch-per-unit effort data further highlighted the biophysical interactions that characterize variability in swordfish catch. In the final chapter, I designed a synergistic analysis of high-resolution, 3D shark movements and satellite observations to quantify the influence of mesoscale oceanography on blue shark movements and behavior. This work demonstrated the importance of eddies in structuring the pelagic ocean by influencing the movements of an apex predator and governing the connectivity between deep scattering layer communities and deep-diving, epipelagic predators. Together, these studies demonstrate the breadth and depth of information that can be garnered through the integration of traditional animal tagging and oceanographic research with cutting-edge analytical approaches and high-resolution oceanographic model and remote sensing datasets, the product of which provides a transformative view of the biophysical interactions occurring in and governing the structure of the pelagic ocean.

Description: Supported by the NASA Earth and Space Science Fellowship, the MIT John S. Hennessy Fellowship, the MIT Martin Family Society of Fellows for Sustainability Fellowship, the WHOI Ocean Venture, Grassle, and James Stratton Fellowships and the WHOI Academic Programs Office. This research and its dissemination was supported by funds from National Geographic, Amazon Web Services, the Explorers Club, Rolex, Sigma Xi, the MIT Center for International Studies, WHOI Access to the Sea and Coastal Ocean Institute Funds, MIT Graduate Student Council, MIT Student Assistance Fund, WHOI Biology Department, American Fisheries Society, WHOI Academic Programs Office

Description: Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 1969-1993, doi:10.1175/JPO-D-18-0031.1.

Description: Upstream mean semidiurnal internal tidal energy flux has been found in the Gulf Stream in hydrodynamical model simulations of the Atlantic Ocean. A major source of the energy in the simulations is the south edge of Georges Bank, where strong and resonant Gulf of Maine tidal currents are found. An explanation of the flux pattern within the Gulf Stream is that internal wave modal rays can be strongly redirected by baroclinic currents and even trapped (ducted) by current jets that feature strong velocities above the thermocline that are directed counter to the modal wavenumber vector (i.e., when the waves travel upstream). This ducting behavior is analyzed and explained here with ray-based wave propagation studies for internal wave modes with anisotropic wavenumbers, as occur in mesoscale background flow fields. Two primary analysis tools are introduced and then used to analyze the strong refraction and ducting: the generalized Jones equation governing modal properties and ray equations that are suitable for studying waves with anisotropic wavenumbers.

Description: The Woods Hole research was supported by National Science Foundation Grant OCE-1060430 and by the Office of Naval Research Grants N00014-11-1-0701 and N00014-17-1-2624. The USM research was supported by ONR Grant N00014-15-1-2288 and National Science Foundation Grant OCE-1537449.

Description: 2019-02-28

Description: Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 1375-1384, doi:10.1175/JPO-D-17-0266.1.

Description: The relationship between net mixing and the estuarine exchange flow may be quantified using a salinity variance budget. Here “mixing” is defined as the rate of destruction of volume-integrated salinity variance, and the exchange flow is quantified using the total exchange flow. These concepts are explored using an idealized 3D model estuary. It is shown that in steady state (e.g., averaging over the spring–neap cycle) the volume-integrated mixing is approximately given by Mixing ≅ SinSoutQr, where Sin and Sout are the representative salinities of in- and outflowing layers at the mouth and Qr is the river volume flux. This relationship provides an extension of the familiar Knudsen relation, in which the exchange flow is diagnosed based on knowledge of these same three quantities, quantitatively linking mixing to the exchange flow.

Description: The work was supported by the National Science Foundation through Grants OCE-1736242 to PM and OCE-1736539 to WRG and by the German Research Foundation through Grants TRR 181 and GRK 2000 to HB.

Description: Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 2703-2719, doi:10.1175/JPO-D-17-0245.1.

Description: A new set of deep float trajectory data collected in the Gulf of Mexico from 2011 to 2015 at 1500- and 2500-m depths is analyzed to describe mesoscale processes, with particular attention paid to the western Gulf. Wavelet analysis is used to identify coherent eddies in the float trajectories, leading to a census of the basinwide coherent eddy population and statistics of the eddies’ kinematic properties. The eddy census reveals a new formation region for anticyclones off the Campeche Escarpment, located northwest of the Yucatan Peninsula. These eddies appear to form locally, with no apparent direct connection to the upper layer. Once formed, the eddies drift westward along the northern edge of the Sigsbee Abyssal Gyre, located in the southwestern Gulf of Mexico over the abyssal plain. The formation mechanism and upstream sources for the Campeche Escarpment eddies are explored: the observational data suggest that eddy formation may be linked to the collision of a Loop Current eddy with the western boundary of the Gulf. Specifically, the disintegration of a deep dipole traveling under the Loop Current eddy Kraken, caused by the interaction with the northwestern continental slope, may lead to the acceleration of the abyssal gyre and the boundary current in the Bay of Campeche region.

Description: The authors were supported by the Department of the Interior, Bureau of Ocean Energy Management (BOEM), Contract M10PC00112 to Leidos, Inc., Raleigh, North Carolina.

Description: 2019-05-07

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): 2381-2400, doi:10.1175/2010JPO4403.1.

Description: Langmuir circulation (LC) is a turbulent upper-ocean process driven by wind and surface waves that contributes significantly to the transport of momentum, heat, and mass in the oceanic surface layer. The authors have previously performed a direct comparison of large-eddy simulations and observations of the upper-ocean response to a wind event with rapid mixed layer deepening. The evolution of simulated crosswind velocity variance and spatial scales, as well as mixed layer deepening, was only consistent with observations if LC effects are included in the model. Based on an analysis of these validated simulations, in this study the fundamental differences in mixing between purely shear-driven turbulence and turbulence with LC are identified. In the former case, turbulent kinetic energy (TKE) production due to shear instabilities is largest near the surface, gradually decreasing to zero near the base of the mixed layer. This stands in contrast to the LC case in which at middepth range TKE production can be dominated by Stokes drift shear. Furthermore, the Eulerian mean vertical shear peaks near the base of the mixed layer so that TKE production by mean shear flow is elevated there. LC transports horizontal momentum efficiently downward leading to an along-wind velocity jet below LC downwelling regions at the base of the mixed layer. Locally enhanced vertical shear instabilities as a result of this jet efficiently erode the thermocline. In turn, enhanced breaking internal waves inject cold deep water into the mixed layer, where LC currents transport temperature perturbation advectively. Thus, LC and locally generated shear instabilities work intimately together to facilitate strongly the mixed layer deepening process.

Description: This research was supported by the Office of Naval Research through Grants N00014-09-M-0112 (TK) and N00014-06-1-0178 (AP, JT). Author TK also received support from a Woods Hole Oceanographic Institution Cooperative Institute for Climate and Ocean Research Postdoctoral Scholarship.

Description: Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): C12049, doi:10.1029/2010JC006331.

Description: The generation, propagation, and dissipation processes of large-amplitude nonlinear internal waves in Massachusetts Bay during the stratified season were examined using the nonhydrostatic Finite-Volume Coastal Ocean Model (FVCOM-NH). The model reproduced well the characteristics of the high-frequency internal waves observed in Massachusetts Bay in August 1998. The model experiments suggested that internal waves over Stellwagen Bank are generated by the interaction of tidal currents with steep bottom topography through a process of forming a large-density front on the western slope of the bank by the release of an initial density perturbation near ebb-flood transition, nonlinear steepening of the density front into a deep density depression, and disintegrating of the density depression into a wave train. Earth's rotation tends to transfer the cross-bank tidal kinetic energy into the along-bank direction and thus reduces the intensity of the density perturbation at ebb-flood transition and density depression in the flood period. The internal wave packet propagates as a leading edge feature of the internal tidal wave, and the faster propagation speed of the high-frequency internal waves in Massachusetts Bay is caused by Earth's rotation. The model experiments suggested that bottom friction can significantly influence the cross-bank scale of the density perturbation and thus the density depression during wave generation and the dissipation during the wave's shoaling. Inclusion of vertical mixing using the Mellor-Yamada level 2.5 turbulence closure model had only a marginal effect on wave evolution. The model results support the internal wave theory proposed by Lee and Beardsley (1974) but are in disagreement with the lee-wave mechanism proposed by Maxworthy (1979).

Description: This research was supported by NOAA g r a n t s DOC/NOAA/NA04NMF4720332 and DOC/NOAA/ NA05NMF4721131, U.S. GLOBEC Northwest Atlantic/Georges Bank Program NSF grants (OCE‐0606928, OCE‐0712903, OCE‐0732084, OCE‐0726851, OCE0814505), and MIT Sea Grant funds (2006‐RC‐103 and 2010‐R/RC‐116), NOAA NERACOOS Program for the UMASSD team and the Smith Chair in Coastal Oceanography, and NOAA grant (NA‐17RJ1223) for R.C. Beardsley. C. Chen’s contribution is also supported by Shanghai Ocean University under grants A‐2302‐10‐0003 and 09320503700 and the State Key Laboratory for Estuarine and Coastal Research, East China Normal University.

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 March 1992

Description: Rotating baroclinic and barotropic boundary currents flowing around a corner in the laboratory were studied in order to discover the circumstances under which eddies were produced at the corner. Such flows are reminiscent of oceanic coastal flows around capes. When the baroclinic currents, which consisted of surface flows bounded by a density front, encountered a sharp corner, immediately downstream of the corner an anticyclone grew in the surface layer for an angle of greater than 40 degrees. Varying the initial condition of the flow or the depth of the lower layer did not noticeably affect the gyre's properties except for its growth speed, which was greater when the lower layer was shallower. The barotropic currents were pumped along a sloping bottom, and also formed anticyclonic gyres which quickly attained an approximately steady state. For a given topography, the size of the gyre was proportional to the inertial radius u/f. Volume flux calculations based on the surface velocity revealed vertical shear which increased with gyre size. Hydraulic models were also applied to flow around gently curving topography to determine the critical separation curvature as a function of upstream parameters.

Description: This work was supported by the National Science Foundation grant OCE 89-15408.

Description: Author Posting. © American Meteorological Society, 2014. 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 44 (2014): 834-849, doi:10.1175/JPO-D-13-0179.1.

Description: A hydrostatic numerical model with alongshore-uniform barotropic M2 tidal boundary forcing and idealized shelfbreak canyon bathymetries is used to study internal-tide generation and onshore propagation. A control simulation with Mid-Atlantic Bight representative bathymetry is supported by other simulations that serve to identify specific processes. The canyons and adjacent slopes are transcritical in steepness with respect to M2 internal wave characteristics. Although the various canyons are symmetrical in structure, barotropic-to-baroclinic energy conversion rates Cυ are typically asymmetrical within them. The resulting onshore-propagating internal waves are the strongest along beams in the horizontal plane, with the stronger beam in the control simulation lying on the side with higher Cυ. Analysis of the simulation results suggests that the cross-canyon asymmetrical Cυ distributions are caused by multiple-scattering effects on one canyon side slope, because the phase variation in the spatially distributed internal-tide sources, governed by variations in the orientation of the bathymetry gradient vector, allows resonant internal-tide generation. A less complex, semianalytical, modal internal wave propagation model with sources placed along the critical-slope locus (where the M2 internal wave characteristic is tangent to the seabed) and variable source phasing is used to diagnose the physics of the horizontal beams of onshore internal wave radiation. Model analysis explains how the cross-canyon phase and amplitude variations in the locally generated internal tides affect parameters of the internal-tide beams. Under the assumption that strong internal tides on continental shelves evolve to include nonlinear wave trains, the asymmetrical internal-tide generation and beam radiation effects may lead to nonlinear internal waves and enhanced mixing occurring preferentially on one side of shelfbreak canyons, in the absence of other influencing factors.

Description: All three authors were supported by Office of Naval Research (ONR) Grant N00014-11-1-0701. WGZ was additionally supported by the National Science Foundation (NSF) Grant OCE-1154575, and TFD was additionally supported by NSF Grant OCE-1060430.

Description: 2014-09-01

Description: Author Posting. © American Meteorological Society, 2014. 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 44 (2014): 1466–1492, doi:10.1175/JPO-D-12-0154.1.

Description: Simultaneous full-depth microstructure measurements of turbulence and finestructure measurements of velocity and density are analyzed to investigate the relationship between turbulence and the internal wave field in the Antarctic Circumpolar Current. These data reveal a systematic near-bottom overprediction of the turbulent kinetic energy dissipation rate by finescale parameterization methods in select locations. Sites of near-bottom overprediction are typically characterized by large near-bottom flow speeds and elevated topographic roughness. Further, lower-than-average shear-to-strain ratios indicative of a less near-inertial wave field, rotary spectra suggesting a predominance of upward internal wave energy propagation, and enhanced narrowband variance at vertical wavelengths on the order of 100 m are found at these locations. Finally, finescale overprediction is typically associated with elevated Froude numbers based on the near-bottom shear of the background flow, and a background flow with a systematic backing tendency. Agreement of microstructure- and finestructure-based estimates within the expected uncertainty of the parameterization away from these special sites, the reproducibility of the overprediction signal across various parameterization implementations, and an absence of indications of atypical instrument noise at sites of parameterization overprediction, all suggest that physics not encapsulated by the parameterization play a role in the fate of bottom-generated waves at these locations. Several plausible underpinning mechanisms based on the limited available evidence are discussed that offer guidance for future studies.

Description: The SOFine project is funded by the United Kingdom’s Natural Environmental Research Council (NERC) (Grant NE/G001510/1). SW acknowledges the support of anARCDiscovery Early CareerResearchAward (Grant DE120102927), as well as the Grantham Institute for Climate Change, Imperial College London, and the ARC Centre of Excellence for Climate System Science (Grant CE110001028). ACNG acknowledges the support of a NERC Advanced Research Fellowship (Grant NE/C517633/1).KLP acknowledges support fromWoods Hole Oceanographic Institution bridge support funds.

Description: 2014-11-01

Description: Author Posting. © American Meteorological Society, 2014. 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 44 (2014): 1854–1872, doi:10.1175/JPO-D-13-0104.1.

Description: The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10−4) m2 s−1 and above 1000-m depth is O(10−5) m2 s−1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.

Description: This research was funded by the Climate Process Team (CPT) on internal wave–driven mixing throughNSF GrantOCE-0968721. GSC acknowledges support from NSF Grants OCE-0825266 (EXITS), OCE-1029483 (SPAM), and OCE-1029722 (MIXET). LDT and CBW acknowledge support from NSF Grant OCE-0927650. SWand ACNG acknowledge support from NERC Grant NE/G001510/1 (SOFine).

Description: 2015-01-01

Description: Author Posting. © American Meteorological Society, 2014. 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 44 (2014): 2593–2616, doi:10.1175/JPO-D-13-0120.1.

Description: The first direct estimate of the rate at which geostrophic turbulence mixes tracers across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is 710 ± 260 m2 s−1 at 1500-m depth. The estimate is based on an extrapolation of the tracer-based diffusivity using output from numerical tracers released in a one-twentieth of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s−1 and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. These vertical variations are not captured by ocean models presently used for climate studies, but they significantly affect the ventilation of different water masses.

Description: NSF support through Awards OCE-1233832, OCE-1232962, and OCE-1048926 is gratefully acknowledged.

Description: 2015-04-01

Description: Author Posting. © American Meteorological Society, 2016. 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 46 (2016): 1309-1321, doi:10.1175/JPO-D-15-0068.1.