Ocean Science www.ocean-sci.net 1812-0784 1812-0792 6 2 2010 10.5194/os-6-563-2010 http://www.ocean-sci.net/6/563/2010/ http://www.ocean-sci.net/6/563/2010/os-6-563-2010.html http://www.ocean-sci.net/6/563/2010/os-6-563-2010.pdf 563 572 2010-06-08 On the numerical resolution of the bottom layer in simulations of oceanic gravity currents N. Laanaia A. Wirth achim.wirth@hmg.inpg.fr J. M. Molines B. Barnier J. Verron LEGI/MEOM/CNRS, Laboratoire des Ecoulements Géophysiques et Industriels, UMR 5519, BP 53, 38041 Grenoble cedex 9, France The role of an increased numerical vertical resolution, leading to an explicit resolution of the bottom Ekman layer dynamics, is investigated. Using the hydrostatic ocean model NEMO-OPA9, we demonstrate that the dynamics of an idealised gravity current (on an inclined plane), is well captured when a few (around five) sigma-coordinate levels are added near the ocean floor. Such resolution allows to considerably improve the representation of the descent and transport of the gravity current and the Ekman dynamics near the ocean floor, including the important effect of Ekman veering, which is usually neglected in today's simulations of the ocean dynamics. <br><br> Results from high resolution simulations (with &sigma; and z-coordinates) are compared to simulations with a vertical resolution commonly employed in today's ocean models. The latter show a downslope transport that is reduced by almost an order of magnitude and the decrease in the along slope transport is reduced six-fold. We strongly advocate for an increase of the numerical resolution at the ocean floor, similar to the way it is done at the ocean surface and at the lower boundary in atmospheric models. Coleman, G. N., Ferziger, J. H., and Spalart, P. R.: A numerical study of the turbulent Ekman layer, J. Fluid. Mech., 213, 313–348, 1990. Ezer, T.: Entrainment, diapycnal mixing and transport in three-dimensional bottom gravity current simulations using the Mellor-Yamada turbulence scheme, Ocean Model., 9, 151–168, 2005. Ezer, T. and Mellor, G. L.: A generalized coordinate ocean model and a comparison of the bottom boundary layer dynamics in terrain-following and in z-level grids, Ocean Model., 6, 379–403, 2004. Ezer, T. and Weatherly, G. L.: A numerical study of the interaction between a deep cold jet and the bottom boundary layer of the ocean? J. Phys. Oceanogr., 20, 801–816, 1990. Griffiths, R. W.: Gravity currents in rotating systems, Ann. Rev. Fluid Mech., 18, 59–89, 1986. Jungclaus, J. H.: A three-dimensional simulation of the formation of anticyclonic lenses (meddies) by the instability of an intermediate depth boundary current, J. Phys. Oceanogr., 29, 1579–1598, 1999. Jungclaus, J. H., Hauser, J., and Käse, R. H.: Cyclogenesis in the Denmark Strait overflow plume, J. Phys. Oceanogr., 31, 3214–3229, 2001. Killworth, P. D. and Edwards, N. R.: A turbulent bottom boundary layer code for use in numerical ocean models, J. Phys. Oceanogr., 29, 1221–1238, 1999. Legg, S., Hallberg, R. W. and Girton, J. B.: Comparison of entrainment in overflows simulated by z-coordinate, isopycnal and non-hydrostatic models, Ocean Model., 11, 69–97, 2006. Legg, S., Jackson, L., and Hallberg, R. W.: Eddy resolving modeling of overflows, Geophys. Monog. Series, 117, 63–81, 2008. Madec, G.: Nemo ocean Engine, Institut Pierre Simon Laplace (IPSL), France, http://www.nemo-ocean.eu/About-NEMO/Reference-manuals, last access: June~2010, No~27, ISSN~No~1288-1619, 2008. Pedlosky, J.: Ocean Circulation Theory, Springer, 453~ps. ISBN:~3-540-60489-8, 1998. Whitehead, J. A., Stern, M. E., Flierl, G. R., and Klinger, B. A.: Experimental observations of baroclinic eddies on a sloping bottom, J. Geophy. Res., 95, 9585–9610, 1990. Willebrand, J., Barnier, B., Böning, C., Dieterich, C., Killworth, P., Le Provost, C., Jia, Y., Molines, J. M., and New, A. L.: Circulation characteristics in three eddy-permitting models of the North Atlantic, J. Prog. Oceanogr., 48, 123–162, 2001. Winton, M., Hallberg, R., and Gnanadesikan, A.: Simulation of density-driven frictional downslope flow in z-coordinate ocean modelset, J. Phys. Oceanogr., 28, 2163–2174, 1998. Wirth, A.: A non-hydrostatic flat-bottom ocean model entirely based on Fourier expansion, Ocean Model., 9, 71–87, 2004. Wirth, A.: On the basic structure of oceanic gravity currents, Ocean Dynam., 59, 551–563, doi:10.1007/s10236-009-0202-9, 2009. Wirth, A.: Estimation of Friction Parameters in Gravity Currents by Data Assimilation in a Model Hierarchy, submitted to Ocean Dynam., 2010. Wirth, A. and Verron, J.: Estimation of Friction Parameters and Laws in 1.5D Shallow-Water Gravity Currents on the f-Plane, by Data Assimilation Ocean Dynam., 58, 247–257, doi:10.1007/s10236-008-0151-8, 2008. Wirth, A. and Sommeria, J.: Gravity current experiments on the Coriolis platform, Technical report EPSHOM, 2007.