Ocean Science www.ocean-sci.net 1812-0784 1812-0792 4 2 2008 10.5194/os-4-115-2008 http://www.ocean-sci.net/4/115/2008/ http://www.ocean-sci.net/4/115/2008/os-4-115-2008.html http://www.ocean-sci.net/4/115/2008/os-4-115-2008.pdf 115 132 2008-05-06 Descent and mixing of the overflow plume from Storfjord in Svalbard: an idealized numerical model study I. Fer ilker.fer@gfi.uib.no B. Ådlandsvik Geophysical Institute, University of Bergen, Bergen, Norway Bjerknes Centre for Climate Research, Bergen, Norway Institute of Marine Research, Bergen, Norway Storfjorden in the Svalbard Archipelago is a sill-fjord that produces significant volumes of dense, brine-enriched shelf water through ice formation. The dense water produced in the fjord overflows the sill and can reach deep into the Fram Strait. For conditions corresponding to a moderate ice production year, the pathway of the overflow, its descent and evolving water mass properties due to mixing are investigated for the first time using a high resolution 3-D numerical model. An idealized modeling approach forced by a typical annual cycle of buoyancy forcing due to ice production is chosen in a terrain-following vertical co-ordinate. Comparison with observational data, including hydrography, fine resolution current measurements and direct turbulence measurements using a microstructure profiler, gives confidence on the model performance. The model eddy diffusivity profiles contrasted to those inferred from the turbulence measurements give confidence on the skill of the Mellor Yamada scheme in representing sub-grid scale mixing for the Storfjorden overflow, and probably for gravity current modeling, in general. The Storfjorden overflow is characterized by low Froude number dynamics except at the shelf break where the plume narrows, accelerates with speed reaching 0.6 m s^−1, yielding local Froude number in excess of unity. The volume flux of the plume increases by five-fold from the sill to downstream of the shelf-break. Rotational hydraulic control is not applicable for transport estimates at the sill using upstream basin information. To the leading order, geostrophy establishes the lateral slope of the plume interface at the sill. This allows for a transport estimate that is consistent with the model results by evaluating a weir relation at the sill. Aagaard, K., Coachman, L. K., and Carmack, E.: On the halocline of the Arctic Ocean, Deep-Sea Res., 28A, 529–545, 1981. Anderson, L. G., Falck, E., Jones, E. P., Jutterström, S., and Swift, J. H.: Enhanced uptake of atmospheric CO₂ during freezing of seawater: A field study in Storfjorden, Svalbard, J. Geophys. Res., 109, C06004, doi:06010.01029/02003JC002120, 2004. Arneborg, L., Fiekas, V., Umlauf, L., and Burchard, H.: Gravity currents and entrainment- a process study based on observations in the Arkona Basin, J. Phys. Oceanogr., 37, 2094–2113, 2007. Baines, P. G. and Condie, S.: Observations and modelling of Antarctic downslope flows: a review, in: Ocean, ice and atmosphere: interactions at the Antarctic continental margin, Antarctic Research Series, American Geophysical Union, 29–49, 1998. Baines, P. G.: Mixing in flows down gentle slopes into stratified environments, J. Fluid Mech., 443, 237–270, 2001. Canuto, V. M., Dubovikov, M. S., and Cheng, Y.: Entrainment: Local and non-local turbulence models with double diffusion, Geophys. Res. Lett., 32, L22604, doi:22610.21029/22005GL023771, 2005. Cenedese, C., Whitehead, J. A., Ascarelli, T. A., and Ohiwa, M.: A dense current flowing down a sloping bottom in a rotating fluid, J. Phys. Oceanogr., 34, 188–203, 2004. Conkright, M. E., Locarnini, R. A., Garcia, H. E., O'Brien, T. D., Boyer, T. P., Stephens, C., and Antonov, J. I.: World Ocean Atlas 2001: Objective Analyses, Data Statistics, and Figures, CD-ROM Documentation. National Oceanographic Data Center, Silver Spring, MD, 17 pp., 2002. Dickson, R. and Brown, J.: The production of North-Atlantic deep water – sources, rates and pathways, J. Geophys. Res., 99, 12 139–12 341, 1994. Ellison, T. H. and Turner, J. S.: Turbulent entrainment in stratified flows, J. Fluid Mech., 6, 423–448, 1959. ETOPO2: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Geophysical Data Center. 2-minute gridded Global Relief Data (ETOPO2), http://www.ngdc.noaa.gov/mgg/fliers/01mgg04.html, 2001. Ezer, T., Arango, H., and Shchepetkin, A. F.: Developments in terrain-following ocean models: inter-comparisons of numerical aspects, Ocean Modell., 4, 249–267, 2002. Ezer, T.: Entrainment, diapycnal mixing and transport in three-dimensional bottom gravity current simulations using the Mellor-Yamada turbulence scheme, Ocean Modell., 9, 151–168, 2005. Fer, I., Lemmin, U., and Thorpe, S. A.: Cascading of water down the sloping sides of a deep lake in winter, Geophys. Res. Lett., 28, 2093–2096, 2001. Fer, I., Skogseth, R., Haugan, P. M., and Jaccard, P.: Observations of the Storfjorden overflow, Deep-Sea Res. I, 50, 1283–1303, 2003. Fer, I., Skogseth, R., and Haugan, P. M.: Mixing of the Storfjorden overflow (Svalbard Archipelago) inferred from density overturns, J. Geophys. Res., 109, C01005, doi:01010.01029/02003JC001968, 2004. Fer, I.: Scaling turbulent dissipation in an Arctic fjord, Deep-Sea Res. II, 53, 77–95, 2006. Fernando, H. J. S.: Turbulent mixing in stratified flows, Annu. Rev. Fluid Mech., 23, 455–493, 1991. Gill, A. E.: The hydraulics of rotating-channel flow, J. Fluid Mech., 80, 641–671, 1977. Girton, J. B., Pratt, L. J., Sutherland, D. A., and Price, J. F.: Is the Faroe Bank Channel overflow hydraulically controlled?, J. Phys. Oceanogr., 36, 2340–2349, 2006. Gordon, A. L., Visbeck, M., and Huber, B.: Export of Weddell Sea deep and bottom water, J. Geophys. Res., 106, 9005–9017, 1998. Griffiths, R. W.: Gravity currents in rotating systems, Annu. Rev. Fluid. Mech., 18, 59–89, 1986. Haarpaintner, J., Gascard, J.-C., and Haugan, P. M.: Ice production and brine formation in Storfjorden, Svalbard, J. Geophys. Res., 106, 14 001–14 013, 2001. Jungclaus, J. H., Backhaus, J. O., and Fohrmann, H.: Outflow of dense water from the Storfjord in Svalbard: A numerical model study, J. Geophys. Res., 100, 24 719–24 728, 1995. Killworth, P. D.: Mixing of the Weddell Sea continental slope, Deep-Sea Res., 24, 427–448, 1977. Killworth, P. D.: On the rate of descent of overflows, J. Geophys. Res., 106, 22 267–22 275, 2001. Lane-Serff, G. F. and Baines, P. G.: Eddy formation by dense flows on slopes in a rotating fluid, J. Fluid Mech., 363, 229–252, 1998. Legg, S., Hallberg, R. W., and Girton, J. B.: Comparison of entrainment in overflows simulated by z-coordinate, isopycnal and non-hydrostatic models, Ocean Modell., 11, 69–97, 2006. Monaghan, J. J.: Gravity current interaction with interfaces, Annu. Rev. Fluid Mech., 39, 245–261, 2007. Osborn, T. R.: Estimates of the local rate of vertical diffusion from dissipation measurements, J. Phys. Oceanogr., 10, 83–89, 1980. Pratt, L. J. and Helfrich, K. R.: Generalized conditions for hydraulic criticality of oceanic overflows, J. Phys. Oceanogr., 35, 1782–1800, 2005. Price, J. F. and Baringer, M. O.: Outflows and deep water production by marginal seas, Prog. Oceanogr., 33, 161–200, 1994. Quadfasel, D., Rudels, B., and Kurz, K.: Outflow of dense water from a Svalbard fjord into the Fram Strait, Deep-Sea Res., 35, 1143–1150, 1988. Riemenschneider, U. and Legg, S.: Regional simulations of the Faroe Bank Channel overflow in a level model, Ocean Modell., 2007, 93–122, 2007. Rudels, B. and Quadfasel, D.: Convection and deep water formation in the Arctic Ocean – Greenland Sea system, J. Mar. Syst., 2, 435–450, 1991. Schauer, U. and Fahrbach, E.: A dense bottom water plume in the western Barents Sea: downstream modification and interannual variability, Deep-Sea Res., 46, 2095–2108, 1999. Shchepetkin, A. F. and McWilliams, J. C.: A method for computing horizontal pressure-gradient force in an oceanic model with a non-aligned vertical coordinate, J. Geophys. Res., 108, 1–34, 2003. Shchepetkin, A. F. and McWilliams, J. C.: The regional oceanic modeling system (ROMS): a split-explicit, free-surface, topography-following-coordinate oceanic model, Ocean Modell., 9, 347–404, 2005. Simpson, J. E.: Gravity currents: in the environment and the laboratory, Ellis Horwood series in environmental science, edited by: Scorer, R. S., Ellis Horwood Limited, Chichester, 244 pp., 1987. Skogseth, R., Haugan, P. M., and Haarpaintner, J.: Ice and brine production in Storfjorden from four winters of satellite and in situ observations and modelling, J. Geophys. Res., 109, C10008, doi:10010.11029/12004JC002384, 2004. Skogseth, R., Fer, I., and Haugan, P. M.: Dense-water production and overflow from an Arctic coastal polynya in Storfjorden, in: The Nordic Seas: An Integrated Perspective, edited by: Drange, H., Dokken, T. M., Furevik, T., Gerges, R., and Berger, W., AGU Geophysical Monograph, 158, 73–88, 2005a. Skogseth, R., Haugan, P. M., and Jakobsson, M.: Water mass transformations in Storfjorden, Cont. Shelf Res., 25, 667–695, 2005b. Song, T. and Haidvogel, D.: A semi-implicit ocean circulation model using a generalized topography-following coordinate system, J. Comput. Phys., 115, 228–244, 1994. Sternberg, R. W., Aagaard, K., Cacchione, D., Wheatcroft, R. A., Beach, R. A., Roach, A. T., and Marsden, M. A. H.: Long-term near-bed observations of velocity and hydrographic properties in the northeast Barents Sea with implications for sediment transport, Cont. Shelf Res., 21, 509–529, 2001. Turner, J. S.: Turbulent entrainment: the development of the entrainment assumption, and its application to geophysical flows, J. Fluid Mech., 173, 431–471, 1986. Whitehead, J. A., Leetmaa, A., and Knox, R. A.: Rotating hydraulics of strait and sill flows, Geophys. Fluid Dyn., 6, 101–125, 1974. Whitehead, J. A.: Topographic control of oceanic flows in deep passages and straits, Rev. Geophys., 36, 423–440, 1998.