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Overflows in the ocean occur when dense water flows down a continental slope into less dense ambient water. It is important to study idealized and small-scale models, which allow for confidence and control of parameters. The work presented here is a direct qualitative and quantitative comparison between physical laboratory experiments and lab-scale numerical simulations. Physical parameters are varied, including the Coriolis parameter, the inflow density, and the inflow volumetric flow rate. Laboratory experiments are conducted using a rotating square tank and high-resolution camera mounted on the table in the rotating reference frame. Video results are digitized in order to compare directly to numerical simulations. The MIT General Circulation Model (MITgcm), a three-dimensional ocean model, is used for the direct numerical simulations corresponding to the specific laboratory experiments. It was found that the MITgcm was not a good match to laboratory experiments when physical parameters fell within the high eddy activity regime. However, a more extensive resolution study is needed to understand this fully. The MITgcm simulations did provide a good qualitative and quantitative match to laboratory experiments run in a low eddy activity regime. In all cases, the MITgcm simulations had more eddy activity than the laboratory experiments.
density-driven currents, lab-scale, numerical parameters, ocean modelling, overflows
[1] Legg, S., Overflows and Convectively-driven Flows, Buoyancy-driven Flows, 2012, pp. 203–239.
[2] Girton, J.B. & Sanford, T.B., Descent and modification of the overflow plume in the Denmark strait*. Journal of Physical Oceanography, 33(7), pp. 1351–1364, 2003. https://doi.org/10.1175/1520-0485(2003)033<1351:damoto>2.0.co;2
[3] Käse, R.H., Girton, J. & Sanford, T., Structure and variability of the Denmark Strait Overflow: Model and observations. Journal of Geophysical Research: Oceans (1978– 2012), 108(C6), 2003. https://doi.org/10.1029/2002jc001548
[4] Mauritzen, C., Price, J., Sanford, T. & Torres, D., Circulation and mixing in the Faroese Channels. Deep Sea Research Part I: Oceanographic Research Papers, 52(6), pp. 883–913, 2005. https://doi.org/10.1016/j.dsr.2004.11.018
[5] Foster, T.D. & Carmack, E.C., Frontal zone mixing and Antarctic Bottom Water forma- tion in the southern Weddell Sea. In Deep Sea Research and Oceanographic Abstracts, 23(4), pp. 301–317, 1976. https://doi.org/10.1016/0011-7471(76)90872-x
[6] Gordon, A.L., et al., Energetic plumes over the western Ross Sea continental slope.
Geophysical Research Letters, 31(21), 2004. https://doi.org/10.1029/2004gl020785
[7] Muench, R., et al., A dense water outflow from the Ross Sea, Antarctica: Mixing and the contribution of tides. Journal of Marine Systems, 77(4), pp. 369–387, 2009. https:// doi.org/10.1016/j.jmarsys.2008.11.003
[8] Peters, H., et al., Mixing and entrainment in the Red Sea outflow plume. Part I: Plume structure. Journal of Physical Oceanography, 35(5), pp. 569–583, 2005. https://doi. org/10.1175/jpo2679.1
[9] Peters, H. & Johns, W.E., Mixing and entrainment in the Red Sea outflow plume. Part II: Turbulence characteristics. Journal of Physical Oceanography, 35(5), pp. 584–600, 2005. https://doi.org/10.1175/jpo2689.1
[10] Baumert, H.Z., Simpson, J. & Sündermann, J., Marine Turbulence: Theories, Observa- tions, and Models. Vol. 1, Cambridge University Press, 2005.
[11] Baringer, M.O.N. & Price, J.F., Mixing and spreading of the Mediterranean out- flow. Journal of Physical Oceanography, 27(8), pp. 1654–1677, 1997. https://doi. org/10.1175/1520-0485(1997)027<1654:masotm>2.0.co;2
[12] Ilıcak, M., Özgökmen, T.M., Peters, H., Baumert, H.Z. & Iskandarani, M., Performance of two-equation turbulence closures in three-dimensional simulations of the Red Sea overflow. Ocean Modelling, 24(3), pp. 122–139, 2008. https://doi.org/10.1016/j.oce- mod.2008.06.001
[13] Legg, S., Briegleb, B., Chang, Y., Chassignet, E.P., Danabasoglu, G., Ezer, T., Gordon, A.L., Griffies, S., Hallberg, R., Jackson, L., Large, W., Özgökmen, T.M., Peters, H., Price, J., Riemenschneider, U., Wu, W., Xu, X. & Yang, J., Improving oceanic overflow representation in climate models: the gravity current entrainment climate process team. Bulletin of the American Meteorological Society, 90(5), pp. 657–670, 2009. https://doi. org/10.1175/2008bams2667.1
[14] Snow, K., et al., Sensitivity of abyssal water masses to overflow parameterisations.
Ocean Modelling, 89, pp. 84–103. https://doi.org/10.1016/j.ocemod.2015.03.004
[15] Wang, H., Legg, S.A. & Hallberg R.W., Representations of the Nordic Seas overflows and their large scale climate impact in coupled models. Ocean Modelling, 86, pp. 76–92, 2015. https://doi.org/10.1016/j.ocemod.2014.12.005
[16] Riemenschneider, U. & Legg, S., Regional simulations of the Faroe Bank Channel overflow in a level model. Ocean Modelling, 17(2), pp. 93–122, 2007. https://doi. org/10.1016/j.ocemod.2007.01.003
[17] Cenedese, C., Whitehead, J.A., Ascarelli, T.A. & Ohiwa, M., A dense current flowing down a sloping bottom in a rotating fluid. Journal of Physical Oceanography, 34(1), pp. 188–203, 2004. https://doi.org/10.1175/1520-0485(2004)034<0188:adcfda>2.0.co;2
[18] Etling, D., Gelhardt, F., Schrader, U., Brennecke, F., Kühn, G., Chabert d’Hieres, G. & Didelle, H., Experiments with density currents on a sloping bottom in a rotating fluid. Dynamics of Atmospheres and Oceans, 31(1–4), pp. 139–164, 2000. https://doi.org/10.1016/s0377-0265(99)00031-7
[19] Sommeria, J. & Decamp, S., Scaling properties of dense water overflows on a continental slope. 18ème Congrès Fran\ccais de Mécanique (Grenoble 2007), 2007.
[20] Hallberg, R., Time integration of diapycnal diffusion and Richardson number- dependent mixing in isopycnal coordinate ocean models. Monthly Weather Review, 128(5), pp. 1402–1419, 2000. https://doi.org/10.1175/1520-0493(2000)128<1402:tiod da>2.0.co;2
[21] Davies, P.A., KÄse, R.H. & Ahmed, I., Laboratory and numerical model studies of a negatively-buoyant jet discharged horizontally into a homogeneous rotating fluid. Geophysical & Astrophysical Fluid Dynamics, 95(1–2), pp. 127–183, 2001. https://doi.org/10.1080/03091920108203417
[22] Song, Y. & Haidvogel, D., A semi-implicit ocean circulation model using a generalized topography-following coordinate system. Journal of Computational Physics, 115(1), pp. 228–244, 1994. https://doi.org/10.1006/jcph.1994.1189
[23] Wobus, F., Shapiro, G.I., Maqueda, M.A.M. & Huthnance, J.M., Numerical simulations of dense water cascading on a steep slope. Journal of Marine Research, 69(2), pp. 391–415, 2011. https://doi.org/10.1357/002224011798765268
[24] Adcroft, A., et al., Overview of the Formulation and Numerics of the MIT GCM, in Proceedings of the ECMWF Seminar Series on Numerical Methods, Recent Developments in Numerical Methods for Atmosphere and Ocean Modelling, pp. 139–149, 2004.
[25] Ravela, S., et al., A realtime observatory for laboratory simulation of planetary flows. Experiments in Fluids, 48(5), pp. 915–925, 2009. https://doi.org/10.1007/s00348-009- 0752-0
[26] Berntsen, J., Xing, J. & Alendal, G., Assessment of non-hydrostatic ocean models using laboratory scale problems. Continental Shelf Research, 26(12–13), pp. 1433–1447, 2006. https://doi.org/10.1016/j.csr.2006.02.014
[27] Legg, S., Hallberg, R.W. & Girton, J.B., Comparison of entrainment in overflows simulated by z-coordinate, isopycnal and non-hydrostatic models. Ocean Modelling, 11(1–2), pp. 69–97, 2006. https://doi.org/10.1016/j.ocemod.2004.11.006
[28] Illari, L., et al., WEATHER IN A TANK – Exploiting laboratory experiments in the teaching of meteorology, oceanography, and climate. Bulletin of the American Meteorological Society, 90(11), pp. 1619–1632, 2009. https://doi.org/10.1175/2009bams2658.1
[29] Orlanski, I., A simple boundary condition for unbounded hyperbolic flows. Journal of Computational Physics, 21(3), pp. 251–269, 1976. https://doi.org/10.1016/0021- 9991(76)90023-1
[30] Reckinger, S.M., Petersen, M.R. & Reckinger, S.J., A study of overflow simulations using MPAS-Ocean: Vertical grids, resolution, and viscosity. Ocean Modelling, 96, Part 2, pp. 291–313, 2015. https://doi.org/10.1016/j.ocemod.2015.09.006