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  • 1
    Publication Date: 2017-01-04
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA1024, doi:10.1029/2003PA000903.
    Description: We apply a shock-capturing numerical model based on the single-layer shallow water equations to an idealized geometry of the Black Sea and the Sea of Marmara in order to test the implications of a suggested sudden Black Sea infill 8400 years ago. The model resolves the two-dimensional flow upstream and downstream of the hydraulic jump provoked by the cascade of water from the Sea of Marmara into the Black Sea, which would occur during a sudden Black Sea infill. The modeled flow downstream of the hydraulic jump in the Black Sea would consist of a jet that is in part constrained by bathymetric contours. Guided by the Bosporus Canyon, the modeled jet reaches depths of up to 2000 m and could explain the origin of the sediment waves observed at this depth. At a late stage of the infill the modeled jet is attached to the coast and might account for the course of a submerged channel at the mouth of the Bosporus. The preservation of continuous barrier-washover-lagoonal fill systems occurring on the Black Sea shelf is, however, not easily reconcilable with the large flows over the southwest Black Sea shelf predicted by the model. Intensified flow in the upstream basin (Sea of Marmara) is restricted to the immediate vicinity of the Bosporus, suggesting that a sudden reconnection need not have disturbed sediments in the wider Sea of Marmara.
    Description: L. Pratt and K. Helfrich were supported under O.N.R. grant N00014-010100167 and N.S.F. grant OCE-0132903. L. Giosan was supported by a postdoctoral scholarship grant from CICOR (a Joint Institute of Woods Hole Oceanographic Institution and NOAA).
    Keywords: Black Sea ; Flood hypothesis ; Dam break
    Repository Name: Woods Hole Open Access Server
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  • 2
    Publication Date: 2017-01-05
    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): 2779-2799, doi:10.1175/2009JPO4075.1.
    Description: The hydraulic state of the exchange circulation through the Strait of Gibraltar is defined using a recently developed critical condition that accounts for cross-channel variations in layer thickness and velocity, applied to the output of a high-resolution three-dimensional numerical model simulating the tidal exchange. The numerical model uses a coastal-following curvilinear orthogonal grid, which includes, in addition to the Strait of Gibraltar, the Gulf of Cadiz and the Alboran Sea. The model is forced at the open boundaries through the specification of the surface tidal elevation that is characterized by the two principal semidiurnal and two diurnal harmonics: M2, S2, O1, and K1. The simulation covers an entire tropical month. The hydraulic analysis is carried out approximating the continuous vertical stratification first as a two-layer system and then as a three-layer system. In the latter, the transition zone, generated by entrainment and mixing between the Atlantic and Mediterranean flows, is considered as an active layer in the hydraulic model. As result of these vertical approximations, two different hydraulic states have been found; however, the simulated behavior of the flow only supports the hydraulic state predicted by the three-layer case. Thus, analyzing the results obtained by means of the three-layer hydraulic model, the authors have found that the flow in the strait reaches maximal exchange about 76% of the tropical monthlong period.
    Keywords: Channel flows ; Seas/gulfs/bays ; Mediterranean Sea ; Tides ; Orographic effects
    Repository Name: Woods Hole Open Access Server
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  • 3
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    Copernicus Publications on behalf of the European Geosciences Union and the American Geophysical Union
    Publication Date: 2016-09-23
    Description: © The Author(s), 2011. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Nonlinear Processes in Geophysics 18 (2011): 977-987, doi:10.5194/npg-18-977-2011.
    Description: It is argued that the complexity of fluid particle trajectories provides the basis for a new method, referred to as the Complexity Method (CM), for estimation of Lagrangian coherent structures in aperiodic flows that are measured over finite time intervals. The basic principles of the CM are explained and the CM is tested in a variety of examples, both idealized and realistic, and in different reference frames. Two measures of complexity are explored in detail: the correlation dimension of trajectory, and a new measure – the ergodicity defect. Both measures yield structures that strongly resemble Lagrangian coherent structures in all of the examples considered. Since the CM uses properties of individual trajectories, and not separation rates between closely spaced trajectories, it may have advantages for the analysis of ocean float and drifter data sets in which trajectories are typically widely and non-uniformly spaced.
    Description: Work supported by grants NSF-CMG- 82469600, NSF-CMG-0825547 and ONR-N00014-11-1-0087.
    Repository Name: Woods Hole Open Access Server
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  • 4
    Publication Date: 2017-01-04
    Description: Author Posting. © Cambridge University Press, 2013. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 738 (2014): 143-183, doi:10.1017/jfm.2013.583.
    Description: We investigate and quantify stirring due to chaotic advection within a steady, three-dimensional, Ekman-driven, rotating cylinder flow. The flow field has vertical overturning and horizontal swirling motion, and is an idealization of motion observed in some ocean eddies. The flow is characterized by strong background rotation, and we explore variations in Ekman and Rossby numbers, E and Ro, over ranges appropriate for the ocean mesoscale and submesoscale. A high-resolution spectral element model is used in conjunction with linear analytical theory, weakly nonlinear resonance analysis and a kinematic model in order to map out the barriers, manifolds, resonance layers and other objects that provide a template for chaotic stirring. As expected, chaos arises when a radially symmetric background state is perturbed by a symmetry-breaking disturbance. In the background state, each trajectory lives on a torus and some of the latter survive the perturbation and act as barriers to chaotic transport, a result consistent with an extension of the KAM theorem for three-dimensional, volume-preserving flow. For shallow eddies, where E is O(1), the flow is dominated by thin resonant layers sandwiched between KAM-type barriers, and the stirring rate is weak. On the other hand, eddies with moderately small E experience thicker resonant layers, wider-spread chaos and much more rapid stirring. This trend reverses for sufficiently small E, corresponding to deep eddies, where the vertical rigidity imposed by strong rotation limits the stirring. The bulk stirring rate, estimated from a passive tracer release, confirms the non-monotonic variation in stirring rate with E. This result is shown to be consistent with linear Ekman layer theory in conjunction with a resonant width calculation and the Taylor–Proudman theorem. The theory is able to roughly predict the value of E at which stirring is maximum. For large disturbances, the stirring rate becomes monotonic over the range of Ekman numbers explored. We also explore variation in the eddy aspect ratio.
    Description: L.J.P., I.I.R., T.M.O. and P.W. have been supported on DOD (MURI) grant N000141110087, administered by the Office of Naval Research. I.I.R. and L.J.P. received additional support from Grant NSF-OCE-0725796 from the National Science Foundation.
    Description: 2014-12-05
    Keywords: Chaotic advection ; Geophysical and geological flows ; Ocean processes
    Repository Name: Woods Hole Open Access Server
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  • 5
    Publication Date: 2017-01-04
    Description: Author Posting. © American Geophysical Union, 2014. 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: Oceans 119 (2014): 2238–2262, doi:10.1002/2013JC009004.
    Description: The overturning circulation in the Red Sea exhibits a distinct seasonally reversing pattern and is studied using high-resolution MIT general circulation model simulations. In the first part of this study, the vertical and horizontal structure of the summer overturning circulation and its dynamical mechanisms are presented from the model results. The seasonal water exchange in the Strait of Bab el Mandeb is successfully simulated, and the structures of the intruding subsurface Gulf of Aden intermediate water are in good agreement with summer observations in 2011. The model results suggest that the summer overturning circulation is driven by the combined effect of the shoaling of the thermocline in the Gulf of Aden resulting from remote winds in the Arabian Sea and an upward surface slope from the Red Sea to the Gulf of Aden set up by local surface winds in the Red Sea. In addition, during late summer two processes associated, respectively, with latitudinally differential heating and increased salinity in the southern Red Sea act together to cause the reversal of the contrast of the vertical density structure and the cessation of the summer overturning circulation. Dynamically, the subsurface northward pressure gradient force is mainly balanced by vertical viscosity resulting from the vertical shear and boundary friction in the Strait of Bab el Mandeb. Unlike some previous studies, the three-layer summer exchange flows in the Strait of Bab el Mandeb do not appear to be hydraulically controlled.
    Description: Partial support for this effort was provided by the Saudi Aramco Marine Environmental Research Center at KAUST.
    Description: 2014-10-14
    Keywords: Red Sea ; MITgcm ; Overturning
    Repository Name: Woods Hole Open Access Server
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  • 6
    Publication Date: 2017-01-04
    Description: Author Posting. ©0American Geophysical Union, 2014. 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: Oceans 119 (2014): 2263–2289, doi:10.1002/2013JC009331.
    Description: The shallow winter overturning circulation in the Red Sea is studied using a 50 year high-resolution MITgcm (MIT general circulation model) simulation with realistic atmospheric forcing. The overturning circulation for a typical year, represented by 1980, and the climatological mean are analyzed using model output to delineate the three-dimensional structure and to investigate the underlying dynamical mechanisms. The horizontal model circulation in the winter of 1980 is dominated by energetic eddies. The climatological model mean results suggest that the surface inflow intensifies in a western boundary current in the southern Red Sea that switches to an eastern boundary current north of 24°N. The overturning is accomplished through a cyclonic recirculation and a cross-basin overturning circulation in the northern Red Sea, with major sinking occurring along a narrow band of width about 20 km along the eastern boundary and weaker upwelling along the western boundary. The northward pressure gradient force, strong vertical mixing, and horizontal mixing near the boundary are the essential dynamical components in the model's winter overturning circulation. The simulated water exchange is not hydraulically controlled in the Strait of Bab el Mandeb; instead, the exchange is limited by bottom and lateral boundary friction and, to a lesser extent, by interfacial friction due to the vertical viscosity at the interface between the inflow and the outflow.
    Description: Partial support for this effort was provided by the Saudi Aramco Marine Environmental Research Center at KAUST.
    Description: 2014-10-14
    Keywords: Red Sea ; MITgcm ; Overturning
    Repository Name: Woods Hole Open Access Server
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  • 7
    Publication Date: 2017-01-04
    Description: Author Posting. © American Geophysical Union, 2014. 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: Oceans 119 (2014): 1445–1464, doi:10.1002/2013JC009351.
    Description: Surface drifters released in the central Red Sea during April 2010 detected a well-defined anticyclonic eddy around 23°N. This eddy was ∼45–60 km in radius, with a swirl speed up to ∼0.5 m/s. The eddy feature was also evident in monthly averaged sea surface height fields and in current profiles measured on a cross-isobath, shipboard CTD/ADCP survey around that region. The unstructured-grid, Finite-Volume Community Ocean Model (FVCOM) was configured for the Red Sea and process studies were conducted to establish the conditions necessary for the eddy to form and to establish its robustness. The model was capable of reproducing the observed anticyclonic eddy with the same location and size. Diagnosis of model results suggests that the eddy can be formed in a Red Sea that is subject to seasonally varying buoyancy forcing, with no wind, but that its location and structure are significantly altered by wind forcing, initial distribution of water stratification and southward coastal flow from the upstream area. Momentum analysis indicates that the flow field of the eddy was in geostrophic balance, with the baroclinic pressure gradient forcing about the same order of magnitude as the surface pressure gradient forcing.
    Description: This project was supported by the King Abdullah University of Science and Technology (KAUST). The development of Global-FVCOM was supported by NSF grants ARC0712903, ARC0732084, ARC0804029 and OCE-1203393. C. Chen’s contributions were also supported by the International Center for Marine Studies at Shanghai Ocean University through the ‘‘Shanghai Universities First-class Disciplines Project.’’ L. Pratt was also supported by National Science Foundation Grant OCE0927017.
    Description: 2014-08-25
    Keywords: Eddies in the Red Sea
    Repository Name: Woods Hole Open Access Server
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  • 8
    Publication Date: 2017-01-05
    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): 1410–1425, doi:10.1175/JPO-D-14-0192.1.
    Description: The west-to-east crossover of boundary currents has been seen in mean circulation schemes from several past models of the Red Sea. This study investigates the mechanisms that produce and control the crossover in an idealized, eddy-resolving numerical model of the Red Sea. The authors also review the observational evidence and derive an analytical estimate for the crossover latitude. The surface buoyancy loss increases northward in the idealized model, and the resultant mean circulation consists of an anticyclonic gyre in the south and a cyclonic gyre in the north. In the midbasin, the northward surface flow crosses from the western boundary to the eastern boundary. Numerical experiments with different parameters indicate that the crossover latitude of the boundary currents changes with f0, β, and the meridional gradient of surface buoyancy forcing. In the analytical estimate, which is based on quasigeostrophic, β-plane dynamics, the crossover is predicted to lie at the latitude where the net potential vorticity advection (including an eddy component) is zero. Various terms in the potential vorticity budget can be estimated using a buoyancy budget, a thermal wind balance, and a parameterization of baroclinic instability.
    Description: This work is supported by Award USA 00002, KSA 00011, and KSA 00011/02 made by King Abdullah University of Science and Technology (KAUST), by National Science Foundation Grants OCE0927017, OCE1154641, and OCE85464100, and by the Woods Hole Oceanographic Institution Academic Program Office.
    Description: 2015-11-01
    Keywords: Circulation/ Dynamics ; Boundary currents ; Buoyancy ; Ocean circulation ; Ocean dynamics
    Repository Name: Woods Hole Open Access Server
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  • 9
    Publication Date: 2017-01-05
    Description: Author Posting. © American Geophysical Union, 2015. 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: Oceans 120 (2015): 6542–6563, doi:10.1002/2015JC010751.
    Description: Hydrographic data, chlorofluorocarbon-12 (CFC-12) and sulfur hexafluoride (SF6) measurements collected in March 2010 and September–October 2011 in the Red Sea, as well as an idealized numerical experiment are used to study the formation and spreading of Red Sea Outflow Water (RSOW) in the Red Sea. Analysis of inert tracers, potential vorticity distributions, and model results confirm that RSOW is formed through mixed-layer deepening caused by sea surface buoyancy loss in winter in the northern Red Sea and reveal more details on RSOW spreading rates, pathways, and vertical structure. The southward spreading of RSOW after its formation is identified as a layer with minimum potential vorticity and maximum CFC-12 and SF6. Ventilation ages of seawater within the RSOW layer, calculated from the partial pressure of SF6 (pSF6), range from 2 years in the northern Red Sea to 15 years at 17°N. The distribution of the tracer ages is in agreement with the model circulation field which shows a rapid transport of RSOW from its formation region to the southern Red Sea where there are longer circulation pathways and hence longer residence time due to basin wide eddies. The mean residence time of RSOW within the Red Sea estimated from the pSF6 age is 4.7 years. This time scale is very close to the mean transit time (4.8 years) for particles from the RSOW formation region to reach the exit at the Strait of Bab el Mandeb in the numerical experiment.
    Description: King Abdullah University of Science and Technology (KAUST) Grant Numbers: USA 00002, KSA 00011, KSA 00011/02; National Science Foundation; WHOI Academic Program Office Grant Number: OCE0927017
    Description: 2016-03-29
    Keywords: Anthropogenic tracers ; Red Sea Outflow Water ; Transit time ; Formation ; Spreading ; Residence time
    Repository Name: Woods Hole Open Access Server
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  • 10
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    Massachusetts Institute of Technology and Woods Hole Oceanographic Institution
    Publication Date: 2017-01-04
    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 April 1982
    Description: The dynamics of steady and unsteady channel flow over large obstacles is studied analytically and numerically in an attempt to determine the applicability of classical hydraulic concepts to such flows. The study is motivated by a need to understand the influence of deep ocean straits and sills on the abyssal circulation. Three types of channel flow are considered: nonrotating one dimensional (Chapter 2); semigeostrophic, constant potential vorticity (Chapter 3); and dispersive, zero potential vorticity (Chapter 4). In each case the discussion centers around the time-dependent adjustment that occurs as a result of sudden obtrusion of an obstacle into a uniform initial flow or the oscillatory upstream forcing of a steady flow over topography. For nondispersive (nonrotating or semigeostrophic) flow, nonlinear adjustment to obstacle obtrusion is examined using a characteristic formulation and numerical results obtained from a Lax-Wendroff scheme. The adjustment process and asymptotic state are found to depend upon the height of the obstacle bO in relation to a critical height bc and a blocking height bb. For bO 〈 bc 〈 bb, isolated packets of nondispersive (long gravity or Kelvin) waves are generated which propagate away from the obstacle, leaving the far field unaffected. For bc 〈 bO 〈 bb, a bore is generated which moves upstream and partially blocks the flow. In the semigeostrophic case, the potential vorticity of the flow is changed by the bore at a rate proportional to the differential rate of energy dissipation along the line of breakage. For bb 〈 bO the flow is completely blocked. Dispersive results in the parameter range bO 〈 bc are obtained from a linear model of the adjustment that results from obstacle obtrusion into a uniform, rotating-channel flow. The results depend on the initial Froude number Fd (based on the Kelvin wave speed). The dispersive modes set up a decaying response about the obstacle if Fd 〈 1 and (possibly resonant) lee waves if Fd 〉 1. However, the far-field upstream response is found to depend on the behavior of the nondispersive Kelvin modes and is therefore nil. Nonlinear steady solutions to nondispersive flow are obtained through direct integration of the equations of motion. The characteristic formulation is used to evaluate the stability of various steady solutions with respect to small disturbances. Of the four types of steady solution, the one in which hydraulic control occurs is found to be the most stable. This is verified by numerical experiments in which the steady, controlled flow is perturbed by disturbances generated upstream. If the topography is complicated (contains more than sill, say), then controlled flows may become destabilized and oscillations may be excited near the topography. The transmission across the obstacle of energy associated with upstream-forced oscillations is studied using a reflection theory for small amplitude waves. The theory assumes quasi-steady flow over the obstacle and is accurate for waves long compared to the obstacle. For nonrotating flow, the reflection coefficients are bounded below by a value of 1/3. For semigeostrophic flow, however, the reflection coefficient can be arbitrarily small for large values of potential vorticity. This is explained as a result of the boundary-layer character of the semigeostrophic flow.
    Keywords: Hydraulics ; Channels ; Fluid dynamics ; Rotating masses of fluid
    Repository Name: Woods Hole Open Access Server
    Type: Thesis
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