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  • 1
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    Baroclinic flow around planetary islands in a double gyre : a mechanism for cross-gyre flow (2010)
    American Meteorological Society
    Details
    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 Physical Oceanography 40 (2010): 1075-1086, doi:10.1175/2009JPO4375.1.
    Description: A quasigeostrophic, two-layer model is used to study the baroclinic circulation around a thin, meridionally elongated island. The flow is driven by either buoyancy forcing or wind stress, each of whose structure would produce an antisymmetric double-gyre flow. The ocean bottom is flat. When the island partially straddles the intergyre boundary, fluid from one gyre is forced to flow into the other. The amount of the intergyre flow depends on the island constant, that is, the value of the geostrophic streamfunction on the island in each layer. That constant is calculated in a manner similar to earlier studies and is determined by the average, along the meridional length of the island, of the interior Sverdrup solution just to the east of the island. Explicit solutions are given for both buoyancy and wind-driven flows. The presence of an island of nonzero width requires the determination of the baroclinic streamfunction on the basin’s eastern boundary. The value of the boundary term is proportional to the island’s area. This adds a generally small additional baroclinic intergyre flow. In all cases, the intergyre flow produced by the island is not related to topographic steering of the flow but rather the pressure anomaly on the island as manifested by the barotropic and baroclinic island constants. The vertical structure of the flow around the island is a function of the parameterization of the vertical mixing in the problem and, in particular, the degree to which long baroclinic Rossby waves can traverse the basin before becoming thermally damped.
    Description: This research was supported in part by NSF Grant OCE 0451086.
    Keywords: Gyres ; Baroclinic flows ; Topographic effects ; Streamfunction ; Orographic effects
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Lateral coupling in baroclinically unstable flows (2010)
    American Meteorological Society
    Details
    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): 1267-1277, doi:10.1175/2007JPO3906.1.
    Description: A two-layer quasigeostrophic model in a channel is used to study the influence of lateral displacements of regions of different sign mean potential vorticity gradient (Πy) on the growth rate and structure of linearly unstable waves. The mean state is very idealized, with a region of positive Πy in the upper layer and a region of negative Πy in the lower layer; elsewhere Πy is zero. The growth rate and structure of the model’s unstable waves are quite sensitive to the amount of overlap between the two regions. For large amounts of overlap (more than several internal deformation radii), the channel modes described by Phillips’ model are recovered. The growth rate decreases abruptly as the amount of overlap decreases below the internal deformation radius. However, unstable modes are also found for cases in which the two nonzero Πy regions are separated far apart. In these cases, the wavenumber of the unstable waves decreases such that the aspect ratio of the wave remains O(1). The waves are characterized by a large-scale barotropic component that has maximum amplitude near one boundary but extends all the way across the channel to the opposite boundary. Near the boundaries, the wave is of mixed barotropic–baroclinic structure with cross-front scales on the order of the internal deformation radius. The perturbation heat flux is concentrated near the nonzero Πy regions, but the perturbation momentum flux extends all the way across the channel. The perturbation fluxes act to reduce the isopycnal slopes near the channel boundaries and to transmit zonal momentum from the region of Πy 〉 0 to the region on the opposite side of the channel where Πy 〈 0. These nonzero perturbation momentum fluxes are found even for a mean state that has no lateral shear in the velocity field.
    Description: This work was supported by NSF Grants OPP-0421904, OCE-0423975 (MAS), and OCE- 85108600 (JP).
    Keywords: Baroclinic flows ; Barotropic flows
    Repository Name: Woods Hole Open Access Server
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  • 3
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    The role of whitecapping in thickening the ocean surface boundary layer (2015)
    American Meteorological Society
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    Publication Date: 2022-05-25
    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): 2006–2024, doi:10.1175/JPO-D-14-0234.1.
    Description: The effects of wind-driven whitecapping on the evolution of the ocean surface boundary layer are examined using an idealized one-dimensional Reynolds-averaged Navier–Stokes numerical model. Whitecapping is parameterized as a flux of turbulent kinetic energy through the sea surface and through an adjustment of the turbulent length scale. Simulations begin with a two-layer configuration and use a wind that ramps to a steady stress. This study finds that the boundary layer begins to thicken sooner in simulations with whitecapping than without because whitecapping introduces energy to the base of the boundary layer sooner than shear production does. Even in the presence of whitecapping, shear production becomes important for several hours, but then inertial oscillations cause shear production and whitecapping to alternate as the dominant energy sources for mixing. Details of these results are sensitive to initial and forcing conditions, particularly to the turbulent length scale imposed by breaking waves and the transfer velocity of energy from waves to turbulence. After 1–2 days of steady wind, the boundary layer in whitecapping simulations has thickened more than the boundary layer in simulations without whitecapping by about 10%–50%, depending on the forcing and initial conditions.
    Description: We thank Skidmore College for financial and infrastructure support, and Skidmore and the National Science Foundation for funding travel to meetings where early versions of this work were presented. We also thank the National Science Foundation, Oregon State University, Jonathan Nash, and Joe Jurisa for funding and hosting a workshop on River Plume Mixing in October, 2013, where ideas and context for this paper were developed.
    Description: 2016-02-01
    Keywords: Circulation/ Dynamics ; Mixing ; Turbulence ; Wave breaking ; Wind stress ; Atm/Ocean Structure/ Phenomena ; Mixed layer
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Coral reef drag coefficients—surface gravity wave enhancement (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    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): 1555-1566, doi:10.1175/JPO-D-17-0231.1.
    Description: A primary challenge in modeling flow over shallow coral reefs is accurately characterizing the bottom drag. Previous studies over continental shelves and sandy beaches suggest surface gravity waves should enhance the drag on the circulation over coral reefs. The influence of surface gravity waves on drag over four platform reefs in the Red Sea is examined using observations from 6-month deployments of current and pressure sensors burst sampling at 1Hz for 4–5min. Depth-average current fluctuations U0 within each burst are dominated by wave orbital velocities uw that account for 80%–90%of the burst variance and have a magnitude of order 10 cm s21, similar to the lower-frequency depth-average current Uavg. Previous studies have shown that the cross-reef bottom stress balances the pressure gradient over these reefs. A bottom stress estimate that neglects the waves (rCdaUavgjUavgj, where r is water density and Cda is a drag coefficient) balances the observed pressure gradient when uw is smaller than Uavg but underestimates the pressure gradient when uw is larger than Uavg (by a factor of 3–5 when uw 5 2Uavg), indicating the neglected waves enhance the bottom stress. In contrast, a bottom stress estimate that includes the waves [rCda(Uavg 1 U0)jUavg 1 U0j)] balances the observed pressure gradient independent of the relative size of uw and Uavg, indicating that this estimate accounts for the wave enhancement of the bottom stress. A parameterization proposed by Wright and Thompson provides a reasonable estimate of the total bottom stress (including the waves) given the burst-averaged current and the wave orbital velocity.
    Description: The Red Sea field program was supported by Awards USA 00002 and KSA 00011 made by KAUST. S. Lentz was supported for the analysis by NSF Award OCE-1558343.
    Description: 2019-01-13
    Keywords: Coastal flows ; Currents ; Dynamics ; Gravity waves ; Turbulence
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Observations of turbulence in the ocean surface boundary layer : energetics and transport (2010)
    American Meteorological Society
    Details
    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): 1077–1096, doi:10.1175/2008JPO4044.1.
    Description: Observations of turbulent kinetic energy (TKE) dynamics in the ocean surface boundary layer are presented here and compared with results from previous observational, numerical, and analytic studies. As in previous studies, the dissipation rate of TKE is found to be higher in the wavy ocean surface boundary layer than it would be in a flow past a rigid boundary with similar stress and buoyancy forcing. Estimates of the terms in the turbulent kinetic energy equation indicate that, unlike in a flow past a rigid boundary, the dissipation rates cannot be balanced by local production terms, suggesting that the transport of TKE is important in the ocean surface boundary layer. A simple analytic model containing parameterizations of production, dissipation, and transport reproduces key features of the vertical profile of TKE, including enhancement near the surface. The effective turbulent diffusion coefficient for heat is larger than would be expected in a rigid-boundary boundary layer. This diffusion coefficient is predicted reasonably well by a model that contains the effects of shear production, buoyancy forcing, and transport of TKE (thought to be related to wave breaking). Neglect of buoyancy forcing or wave breaking in the parameterization results in poor predictions of turbulent diffusivity. Langmuir turbulence was detected concurrently with a fraction of the turbulence quantities reported here, but these times did not stand out as having significant differences from observations when Langmuir turbulence was not detected.
    Description: The Office of Naval Research funded this work as a part of CBLAST-Low.
    Keywords: Turbulence ; Boundary layer ; Sea/ocean surface ; Air-sea interaction ; Energy transport
    Repository Name: Woods Hole Open Access Server
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  • 6
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    The influence of lateral advection on the residual estuarine circulation : a numerical modeling study of the Hudson River Estuary (2010)
    American Meteorological Society
    Details
    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): 107-124, doi:10.1175/2008JPO3952.1.
    Description: In most estuarine systems it is assumed that the dominant along-channel momentum balance is between the integrated pressure gradient and bed stress. Scaling the amplitude of the estuarine circulation based on this balance has been shown to have predictive skill. However, a number of authors recently highlighted important nonlinear processes that contribute to the subtidal dynamics at leading order. In this study, a previously validated numerical model of the Hudson River estuary is used to examine the forces driving the residual estuarine circulation and to test the predictive skill of two linear scaling relationships. Results demonstrate that the nonlinear advective acceleration terms contribute to the subtidal along-channel momentum balance at leading order. The contribution of these nonlinear terms is driven largely by secondary lateral flows. Under a range of forcing conditions in the model runs, the advective acceleration terms nearly always act in concert with the baroclinic pressure gradient, reinforcing the residual circulation. Despite the strong contribution of the nonlinear advective terms to the subtidal dynamical balance, a linear scaling accurately predicts the strength of the observed residual circulation in the model. However, this result is largely fortuitous, as this scaling does not account for two processes that are fundamental to the estuarine circulation. The skill of this scaling results because of the compensatory relationship between the contribution of the advective acceleration terms and the suppression of turbulence due to density stratification. Both of these processes, neither of which is accounted for in the linear scaling, increase the residual estuarine circulation but have an opposite dependence on tidal amplitude and, consequently, strength of stratification.
    Description: This research was supported by the Beacon Institute for Rivers and Estuaries—Woods Hole Oceanographic Institution postdoctoral fellowship program, as well as NSF Grants OCE-0452054 and OCE-0451740.
    Keywords: Advection ; Estuarine circulation ; Friction ; Density currents ; Baroclinic flows
    Repository Name: Woods Hole Open Access Server
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  • 7
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    Direct breaking of the internal tide near topography : Kaena Ridge, Hawaii (2010)
    American Meteorological Society
    Details
    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): 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.
    Keywords: Pacific Ocean ; Topographic effects ; Internal waves ; Barotropic flows ; Baroclinic flows
    Repository Name: Woods Hole Open Access Server
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  • 8
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    Vertical structure of dissipation in the nearshore (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    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): 1764-1777, doi:10.1175/jpo3098.1.
    Description: The vertical structure of the dissipation of turbulence kinetic energy was observed in the nearshore region (3.2-m mean water depth) with a tripod of three acoustic Doppler current meters off a sandy ocean beach. Surface and bottom boundary layer dissipation scaling concepts overlap in this region. No depth-limited wave breaking occurred at the tripod, but wind-induced whitecapping wave breaking did occur. Dissipation is maximum near the surface and minimum at middepth, with a secondary maximum near the bed. The observed dissipation does not follow a surfzone scaling, nor does it follow a “log layer” surface or bottom boundary layer scaling. At the upper two current meters, dissipation follows a modified deep-water breaking-wave scaling. Vertical shear in the mean currents is negligible and shear production magnitude is much less than dissipation, implying that the vertical diffusion of turbulence is important. The increased near-bed secondary dissipation maximum results from a decrease in the turbulent length scale.
    Description: Funding was provided by NSF and ONR.
    Keywords: Turbulence ; Kinetic energy ; Ocean
    Repository Name: Woods Hole Open Access Server
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  • 9
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    Effect of sea surface temperature-wind stress coupling on baroclinic instability in the ocean (2010)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    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): 1092–1097, doi:10.1175/JPO3045.1.
    Description: The impact of the observed relationship between sea surface temperature and surface wind stress on baroclinic instability in the ocean is explored using linear theory and a nonlinear model. A simple parameterization of the influence of sea surface temperature on wind stress is used to derive a surface boundary condition for the vertical velocity at the base of the oceanic Ekman layer. This boundary condition is applied to the classic linear, quasigeostrophic stability problem for a uniformly sheared flow originally studied by Eady in the 1940s. The results demonstrate that for a wind directed from warm water toward cold water, the coupling acts to enhance the growth rate, and increase the wavelength, of the most unstable wave. Winds in the opposite sense reduce the growth rate and decrease the wavelength of the most unstable wave. For representative coupling strengths, the change in growth rate can be as large as ±O(50%). This effect is largest for shallow, strongly stratified, low-latitude flows.
    Description: This work was supported by the Office of Naval Research Grant N00014-05-1-0300.
    Keywords: Wind stress ; Instability ; Sea surface temperature ; Baroclinic flows ; Ocean dynamics
    Repository Name: Woods Hole Open Access Server
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  • 10
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    Rossby wave instability and apparent phase speeds in large ocean basins (2010)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    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): 1177-1191, doi:10.1175/jpo3054.1.
    Description: The stability of baroclinic Rossby waves in large ocean basins is examined, and the quasigeostrophic (QG) results of LaCasce and Pedlosky are generalized. First, stability equations are derived for perturbations on large-scale waves, using the two-layer shallow-water system. These equations resemble the QG stability equations, except that they retain the variation of the internal deformation radius with latitude. The equations are solved numerically for different initial conditions through eigenmode calculations and time stepping. The fastest-growing eigenmodes are intensified at high latitudes, and the slower-growing modes are intensified at lower latitudes. All of the modes have meridional scales and growth times that are comparable to the deformation radius in the latitude range where the eigenmode is intensified. This is what one would expect if one had applied QG theory in latitude bands. The evolution of large-scale waves was then simulated using the Regional Ocean Modeling System primitive equation model. The results are consistent with the theoretical predictions, with deformation-scale perturbations growing at rates inversely proportional to the local deformation radius. The waves succumb to the perturbations at the mid- to high latitudes, but are able to cross the basin at low latitudes before doing so. Also, the barotropic waves produced by the instability propagate faster than the baroclinic long-wave speed, which may explain the discrepancy in speeds noted by Chelton and Schlax.
    Description: PEI was supported by a postdoctoral grant from the Norwegian Research Council, JHL was supported under the Norwegian NOCLIM II program, and JP was partly supported by NSF OCE 0451086.
    Keywords: Rossby waves ; Ocean models ; Barotropic flows ; Baroclinic flows
    Repository Name: Woods Hole Open Access Server
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