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  • Articles  (108)
  • Internal waves  (38)
  • Diapycnal mixing  (33)
  • Ocean models  (33)
  • Baroclinic flows
  • American Meteorological Society  (108)
  • MDPI Publishing
  • 101
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    Marginal sea overflows and the upper ocean interaction (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    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): 387-403, doi:10.1175/2008JPO3934.1.
    Description: Marginal sea overflows and the overlying upper ocean are coupled in the vertical by two distinct mechanisms—by an interfacial mass flux from the upper ocean to the overflow layer that accompanies entrainment and by a divergent eddy flux associated with baroclinic instability. Because both mechanisms tend to be localized in space, the resulting upper ocean circulation can be characterized as a β plume for which the relevant background potential vorticity is set by the slope of the topography, that is, a topographic β plume. The entrainment-driven topographic β plume consists of a single gyre that is aligned along isobaths. The circulation is cyclonic within the upper ocean (water columns are stretched). The transport within one branch of the topographic β plume may exceed the entrainment flux by a factor of 2 or more. Overflows are likely to be baroclinically unstable, especially near the strait. This creates eddy variability in both the upper ocean and overflow layers and a flux of momentum and energy in the vertical. In the time mean, the eddies accompanying baroclinic instability set up a double-gyre circulation in the upper ocean, an eddy-driven topographic β plume. In regions where baroclinic instability is growing, the momentum flux from the overflow into the upper ocean acts as a drag on the overflow and causes the overflow to descend the slope at a steeper angle than what would arise from bottom friction alone. Numerical model experiments suggest that the Faroe Bank Channel overflow should be the most prominent example of an eddy-driven topographic β plume and that the resulting upper-layer transport should be comparable to that of the overflow. The overflow-layer eddies that accompany baroclinic instability are analogous to those observed in moored array data. In contrast, the upper layer of the Mediterranean overflow is likely to be dominated more by an entrainment-driven topographic β plume. The difference arises because entrainment occurs at a much shallower location for the Mediterranean case and the background potential vorticity gradient of the upper ocean is much larger.
    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. JY has also been supported by NSF Grant OCE-0351055.
    Keywords: Baroclinic flows ; Mass fluxes/transport ; Entrainment ; Topographic effects ; Potential vorticity
    Repository Name: Woods Hole Open Access Server
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  • 102
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    Seasonal kinetic energy variability of near-inertial motions (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    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).
    Keywords: Kinetic energy ; Internal waves ; Intraseasonal variability ; North Atlantic Ocean ; In situ observations
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  • 103
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    Circulation and exchange in choked marginal seas (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    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): 2639-2661, doi:10.1175/2008JPO3946.1.
    Description: A theory for the exchange between a rotating, buoyancy-forced marginal sea and an ocean is developed and tested numerically. Cooling over the marginal sea leads to sinking and sets up a two-layer exchange flow, with a warm surface layer entering from the ocean and a cool layer exiting at depth. The connecting strait is sufficiently narrow and shallow to cause the exchange flow to be hydraulically controlled. The incoming surface layer forms a baroclinically unstable boundary current that circles the marginal sea in a cyclonic sense and feeds heat to the interior by way of eddies. Consistent with the overall heat and volume balances for the marginal sea, there is a continuous family of hydraulically controlled states with critical flow at the most constricted section of the strait. Included in this family is a limiting “maximal-exchange” solution with two sections of hydraulic control in the strait and with fixed layer depths at the most constricted section. The state of exchange for a given forcing is predicted using a theory that assumes energy conservation over a certain path connecting the strait to the marginal sea or, in some cases, the ocean. Depending on the configuration of the exchange, long-wave information may be blocked from entering the strait from the marginal sea, from the open ocean, or both. The scenario that holds determines what is predicted and what needs to be input. Numerical tests of the prediction for the temperature difference and the state of exchange are carried out for straits with a pure contraction in width and for a constant width strait with a topographic sill. The comparison is reasonable in most cases, though the numerical model is not able to reproduce cases of multiple states predicted by the theory for certain forcing values. The analytical model is an alternative to the Price and Yang and Siddall et al. models of a marginal sea outflow.
    Description: This work was supported by the National Science Foundation under Grants OCE-0525729 and OCE-0423975.
    Keywords: Ocean circulation ; Coastlines ; Ocean models
    Repository Name: Woods Hole Open Access Server
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  • 104
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    Milankovitch forcing and meridional moisture flux in the atmosphere : insight from a zonally averaged ocean–atmosphere model (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    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 Climate 23 (2010): 4841–4855, doi:10.1175/2010JCLI3273.1.
    Description: A 1-Myr-long time-dependent solution of a zonally averaged ocean–atmosphere model subject to Milankovitch forcing is examined to gain insight into long-term changes in the planetary-scale meridional moisture flux in the atmosphere. The model components are a one-dimensional (latitudinal) atmospheric energy balance model with an active hydrological cycle and an ocean circulation model representing four basins (Atlantic, Indian, Pacific, and Southern Oceans). This study finds that the inclusion of an active hydrological cycle does not significantly modify the responses of annual-mean air and ocean temperatures to Milankovitch forcing found in previous integrations with a fixed hydrological cycle. Likewise, the meridional overturning circulation of the North Atlantic Ocean is not significantly affected by hydrological changes. Rather, it mainly responds to precessionally driven variations of ocean temperature in subsurface layers (between 70- and 500-m depth) of this basin. On the other hand, annual and zonal means of evaporation rate and meridional flux of moisture in the atmosphere respond notably to obliquity-driven changes in the meridional gradient of annual-mean insolation. Thus, when obliquity is decreased (increased), the meridional moisture flux in the atmosphere is intensified (weakened). This hydrological response is consistent with deuterium excess records from polar ice cores, which are characterized by dominant obliquity cycles.
    Description: A. A. thanks the Global Environmental and Climate Change Centre of McGill University for a Network Grant that made possible an enriching twoweek stay at WHOI during June 2007. O. M. acknowledges support from theU.S.National Science Foundation. Support from a Canadian NSERC Discovery Grant awarded to L.A.M. is gratefully acknowledged.
    Keywords: Forcing ; Moisture ; Fluxes ; Ocean models ; Coupled models ; Southern Ocean ; Pacific Ocean ; Atlantic Ocean ; Indian Ocean
    Repository Name: Woods Hole Open Access Server
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  • 105
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    The baroclinic adjustment of time-dependent shear flows (2010)
    American Meteorological Society
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    Publication Date: 2022-05-26
    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): 1851-1865, doi:10.1175/2010JPO4217.1.
    Description: Motivated by the fact that time-dependent currents are ubiquitous in the ocean, this work studies the two-layer Phillips model on the beta plane with baroclinic shear flows that are steady, periodic, or aperiodic in time to understand their nonlinear evolution better. When a linearly unstable basic state is slightly perturbed, the primary wave grows exponentially until nonlinear advection adjusts the growth. Even though for long time scales these nearly two-dimensional motions predominantly cascade energy to large scales, for relatively short times the wave–mean flow and wave–wave interactions cascade energy to smaller horizontal length scales. The authors demonstrate that the manner through which these mechanisms excite the harmonics depends significantly on the characteristics of the basic state. Time-dependent basic states can excite harmonics very rapidly in comparison to steady basic states. Moreover, in all the simulations of aperiodic baroclinic shear flows, the barotropic component of the primary wave continues to grow after the adjustment by the nonlinearities. Furthermore, the authors find that the correction to the zonal mean flow can be much larger when the basic state is aperiodic compared to the periodic or steady limits. Finally, even though time-dependent baroclinic shear on an f plane is linearly stable, the authors show that perturbations can grow algebraically in the linear regime because of the erratic variations in the aperiodic flow. Subsequently, baroclinicity adjusts the growing wave and creates a final state that is more energetic than the nonlinear adjustment of any of the unstable steady baroclinic shears that are considered.
    Description: FJP was supported by NSERC and JP was supported by NSF OCE 0925061 during the research and writing of this manuscript.
    Keywords: Baroclinic flows ; Shear structure/flows
    Repository Name: Woods Hole Open Access Server
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  • 106
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    Rapid mixed layer depening by the combination of Langmuir and shear instabilities : a case study (2010)
    American Meteorological Society
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    Publication Date: 2022-05-26
    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.
    Keywords: Mixed layer ; Shear structure/flows ; Wind effects ; Turbulence ; Thermocline ; Internal waves ; Advection
    Repository Name: Woods Hole Open Access Server
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  • 107
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    Acoustical measurement of nonlinear internal waves using the inverted echo sounder (2010)
    American Meteorological Society
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    Publication Date: 2022-05-26
    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 Atmospheric and Oceanic Technology 26 (2009): 2228-2242, doi:10.1175/2009JTECHO652.1.
    Description: The performance of pressure sensor–equipped inverted echo sounders for monitoring nonlinear internal waves is examined. The inverted echo sounder measures the round-trip acoustic travel time from the sea floor to the sea surface and thus acquires vertically integrated information on the thermal structure, from which the first baroclinic mode of thermocline motion may be inferred. This application of the technology differs from previous uses in that the wave period (30 min) is short, requiring a more rapid transmission rate and a different approach to the analysis. Sources of error affecting instrument performance include tidal effects, barotropic adjustment to internal waves, ambient acoustic noise, and sea surface roughness. The latter two effects are explored with a simulation that includes surface wave reconstruction, acoustic scattering based on the Kirchhoff approximation, wind-generated noise, sound propagation, and the instrument’s signal processing circuitry. Bias is introduced as a function of wind speed, but the simulation provides a basis for bias correction. The assumption that the waves do not significantly affect the mean stratification allows for a focus on the dynamic response. Model calculations are compared with observations in the South China Sea by using nearby temperature measurements to provide a test of instrument performance. After applying corrections for ambient noise and surface roughness effects, the inverted echo sounder exhibits an RMS variability of approximately 4 m in the estimated depth of the eigenfunction maximum in the wind speed range 0 ≤ U10 ≤ 10 m s−1. This uncertainty may be compared with isopycnal excursions for nonlinear internal waves of 100 m, showing that the observational approach is effective for measurements of nonlinear internal waves in this environment.
    Description: This project was supported by the ONR Nonlinear Wave Program under Contract N0014-05-1-0286.
    Keywords: Acoustic measurements/effects ; Internal waves ; Instrumentation/sensors ; Temperature
    Repository Name: Woods Hole Open Access Server
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  • 108
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    The impact of abyssal mixing parameterizations in an ocean general circulation model (2010)
    American Meteorological Society
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    Publication Date: 2022-05-26
    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): 1756-1775, doi:10.1175/2009JPO4085.1.
    Description: A parameterization of vertical diffusivity in ocean general circulation models has been implemented in the ocean model component of the Community Climate System Model (CCSM). The parameterization represents the dynamics of the mixing in the abyssal ocean arising from the breaking of internal waves generated by the tides forcing stratified flow over rough topography. This parameterization is explored over a range of parameters and compared to the more traditional ad hoc specification of the vertical diffusivity. Diapycnal mixing in the ocean is thought to be one of the primary controls on the meridional overturning circulation and the poleward heat transport by the ocean. When compared to the traditional approach with uniform mixing, the new mixing parameterization has a noticeable impact on the meridional overturning circulation; while the upper limb of the meridional overturning circulation appears to be only weakly impacted by the transition to the new parameterization, the deep meridional overturning circulation is significantly strengthened by the change. The poleward ocean heat transport does not appear to be strongly affected by the mixing in the abyssal ocean for reasonable parameter ranges. The transport of the Antarctic Circumpolar Current through the Drake Passage is related to the amount of mixing in the deep ocean. The new parameterization is found to be energetically consistent with the known constraints on the ocean energy budget.
    Description: This work was supported by the National Science Foundation under Grants ATM 02-00929 and OCE 02-41061.
    Keywords: General circulation models ; Parameterization ; Abyssal circulation ; Ocean models
    Repository Name: Woods Hole Open Access Server
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