Bibliography - Sonya Legg

1. Griffies, Stephen, Alistair Adcroft, Ventakramani Balaji, Robert W Hallberg, Sonya Legg, T Martin, and A Pirani, et al., February 2009: Sampling Physical Ocean Field in WCRP CMIP5 Simulations: CLIVAR Working Group on Ocean Model Development (WGOMD) Committee on CMIP5 Ocean Model Output, International CLIVAR Project Office, CLIVAR Publication Series No. 137, 56pp.
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2. Green, J A M., J H Simpson, Sonya Legg, and M R Palmer, 2008: Internal waves, baroclinic energy fluxes and mixing at the European shelf edge. Continental Shelf Research, 28(7), doi:10.1016/j.csr.2008.01.014.
   [ Abstract ]

   The energy flux in internal waves generated at the Celtic Sea shelf break was estimated by (i) applying perturbation theory to a week-long dataset from a mooring at 200 m depth, and (ii) using a 2D non-hydrostatic circulation model over the shelf break. The dataset consisted of high resolution time-series of currents and vertical stratification together with two 25-h sets of vertical profiles of the dissipation of turbulent kinetic energy. The observations indicated an average energy flux of 139 W m⁻¹, travelling along the shelf break towards the northwest. The average energy flux across the shelf break at the mooring was only 8 W m⁻¹. However, the waves propagating onshelf transported up to 200 W m⁻¹, but they were only present 51% of the time. A comparison between the divergence of the baroclinic energy flux and observed dissipation within the seasonal thermocline at the mooring showed that the dissipation was at least one order of magnitude larger. Results from a 2D model along a transect perpendicular to the shelf break showed a time-averaged onshelf energy flux of 153–425 W m⁻¹, depending on the magnitude of the barotropic forcing. A divergence zone of the energy flux was found a few kilometre offshore of the location of the observations in the model results, and fluxes on the order of several kW m⁻¹ were present in the deep waters further offshelf from the divergence zone. The modelled fluxes exhibited qualitative agreements with the phase and hourly onshelf magnitudes of the observed energy fluxes. Both the observations and the model results show an intermittent onshelf energy flux of 100–200 W m⁻¹, but these waves could only propagate 20–30 km onshore before dissipating. This conclusion was supported by a 25-h dataset sampled some 180 km onto the shelf, where a weak wave energy flux was found going towards the shelf break. We therefore conclude that shelf break generated internal waves are unlikely to be the main source of energy for mixing on the inner part of the shelf.

3. Jackson, L, Robert W Hallberg, and Sonya Legg, May 2008: A Parameterization of shear-driven turbulence for ocean climate models. Journal of Physical Oceanography, 38(5), doi:10.1175/2007JPO3779.1.
   [ Abstract ]

   This paper presents a new parameterization for shear-driven, stratified, turbulent mixing that is pertinent to climate models, in particular the shear-driven mixing in overflows and the Equatorial Undercurrent. This parameterization satisfies a critical requirement for climate applications by being simple enough to be implemented implicitly and thereby allowing the parameterization to be used with time steps that are long compared to both the time scale on which the turbulence evolves and the time scale with which it alters the large-scale ocean state. The mixing is expressed in terms of a turbulent diffusivity that is dependent on the shear forcing and a length scale that is the minimum of the width of the low Richardson number region (Ri = N²/|u_z|², where N is the buoyancy frequency and |u_z| is the vertical shear) and the buoyancy length scale over which the turbulence decays [L_b = Q^1/2/N, where Q is the turbulent kinetic energy (TKE)]. This also allows a decay of turbulence vertically away from the low Richardson number region over the buoyancy scale, a process that the results show is important for mixing across a jet. The diffusivity is determined by solving a vertically nonlocal steady-state TKE equation and a vertically elliptic equilibrium equation for the diffusivity itself. High-resolution nonhydrostatic simulations of shear-driven stratified mixing are conducted in both a shear layer and a jet. The results of these simulations support the theory presented and are used, together with discussions of various limits and reviews of previous work, to constrain parameters.

4. Legg, Sonya, L Jackson, and Robert W Hallberg, 2008: Eddy-resolving modeling of overflows In Ocean Modeling in an Eddying Regime, Geophysical Monograph 177, M. W. Hecht, and H. Hasumi, eds., Washington, DC, American Geophysical Union, 63-82.
5. Legg, Sonya, and J Klymak, September 2008: Internal hydraulic jumps and overturning generated by tidal flow over a tall steep ridge. Journal of Physical Oceanography, 38(9), doi:10.1175/2008JPO3777.1.
   [ Abstract ]

   Recent observations from the Hawaiian Ridge indicate episodes of overturning and strong dissipation coupled with the tidal cycle near the top of the ridge. Simulations with realistic topography and stratification suggest that this overturning has its origins in transient internal hydraulic jumps that occur below the shelf break at maximum ebb tide, and then propagate up the slope as internal bores when the flow reverses. A series of numerical simulations explores the parameter space of topographic slope, barotropic velocity, stratification, and forcing frequency to identify the parameter regime in which these internal jumps are possible. Theoretical analysis predicts that the tidally driven jumps may occur when the vertical tidal excursion is large, which is shown to imply steep topographic slopes, such that dh/dxN/ω > 1. The vertical length scale of the jumps is predicted to depend on the flow speed such that the jump Froude number is of order unity. The numerical results agree with the theoretical predictions, with finite-amplitude internal hydraulic jumps and overturning forming during strong offslope tidal flow over steep slopes. These results suggest that internal hydraulic jumps may be an important mechanism for local tidally generated mixing at tall steep topography.

6. Riemenschneider, U, and Sonya Legg, 2007: Regional simulations of the Faroe Bank Channel overflow in a level model. Ocean Modelling, 17(2), doi:10.1016/j.ocemod.2007.01.003.
   [ Abstract ]

   The work presented in this paper is part of an effort to understand and improve the representation of overflows in large scale, coarse resolution ocean climate models. To this end we developed a regional model of the Faroe Bank Channel overflow using the MITgcm (Massachusetts Institute of Technology General Circulation Model), a typical global ocean model using discrete levels as the vertical co-ordinate. In order to isolate the numerical diffusion resulting from the advection of tracers, the model is run without any turbulence closure schemes, without convective adjustment or any other physically based parameterization of mixing. Comparison between the model results and recent observations of the Faroe Bank Channel plume allows assessment of the model performance, including its ability to correctly represent the mixing and the downslope transport in the plume. It is found that at the highest resolution used in this paper (2.5 km – horizontal and 25 m – vertical) the structure of the modeled plume and the magnitude of the entrainment is comparable to the observed plume. The dependence of the mixing on various model parameters, such as vertical and horizontal resolution, vertical viscosity, drag coefficient and inflow forcing, is tested extensively. The numerical mixing in the model is found to be most sensitive to changes in the horizontal resolution, and to a lesser extent on vertical resolution and vertical viscosity. The inflow forcing and drag coefficient show only a very minor effect on the mixing. The results presented in the paper identify the shortcomings of the model at coarser resolutions which need to be addressed when attempting to represent such overflows realistically in large scale climate and ocean models.

7. Legg, Sonya, Robert W Hallberg, and J B Girton, 2006: Comparison of entrainment in overflows simulated by z-coordinate, isopycnal and non-hydrostatic models. Ocean Modelling, 11(1-2), doi:10.1016/j.ocemod.2004.11.006.
   [ Abstract ]

   A series of idealised numerical simulations of dense water flowing down a broad uniform slope are presented, employing both a z-coordinate model (the MIT general circulation model) and an isopycnal coordinate model (the Hallberg Isopycnal Model). Calculations are carried out at several different horizontal and vertical resolutions, and for a range of physical parameters. A subset of calculations are carried out at very high resolution using the non-hydrostatic variant of the MITgcm. In all calculations dense water descends the slope while entraining and mixing with ambient fluid. The dependence of entrainment, mixing and down-slope descent on resolution and vertical coordinate are assessed. At very coarse resolutions the z-coordinate model generates excessive spurious mixing, and dense water has difficulty descending the slope. However, at intermediate resolutions the mixing in the z-coordinate model is less than found in the high-resolution non-hydrostatic simulations, and dense water descends further down the slope. Isopycnal calculations show less resolution dependence, although entrainment and mixing are both reduced slightly at coarser resolution. At intermediate resolutions the z-coordinate and isopycnal models produce similar levels of mixing and entrainment. These results provide a benchmark against which future developments in overflow entrainment parameterizations in both z-coordinate and isopycnal models may be compared.

8. Legg, Sonya, and K M H Huijts, 2006: Preliminary simulations of internal waves and mixing generated by finite amplitude tidal flow over isolated topography. Deep-Sea Research, Part II, 53(1-2), doi:10.1016/j.dsr2.2005.09.014.
   [ Abstract ]

   Much recent observational evidence suggests that energy from the barotropic tides can be used for mixing in the deep ocean. Here the process of internal-tide generation and dissipation by tidal flow over an isolated Gaussian topography is examined, using two-dimensional numerical simulations employing the MITgcm. Four different topographies are considered, for five different amplitudes of barotropic forcing, thereby allowing a variety of combinations of key nondimensional parameters. While much recent attention has focused on the role of relative topographic steepness and height in modifying the rate of conversion of energy from barotropic to baroclinic modes, here attention is focused on parameters dependent on the flow amplitude. For narrow topography, large amplitude forcing gives rise to baroclinic responses at higher harmonics of the forcing frequency. Tall narrow topographies are found to be the most conducive to mixing. Dissipation rates in these calculations are most efficient for the narrowest topography.

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