Abstract
The role of water depth and bottom boundary layer turbulence upon lee-wave generation in sill regions is examined. Their effect upon vertical mixing is also considered. Calculations are performed using a non-hydrostatic model in cross-section form with a specified tidal forcing. Initial calculations in deeper water and a sill height such that the sill top is well removed from the surrounding bed region showed that downstream lee-wave generation and associated mixing increased as bottom friction coefficient k increased. This was associated with an increase in current shear across the sill. However, for a given k, increasing vertical eddy viscosity A v reduced vertical shear in the across sill velocity, leading to a reduction in lee-wave amplitude and associated mixing. Subsequent calculations using shallower water showed that for a given k and A v, lee-wave generation was reduced due to the shallower water depth and changes in the bottom boundary layer. However, in this case (unlike in the deepwater case), there is an appreciable bottom current. This gives rise to bottom mixing which in shallow water extends to mid-depth and enhances the mid-water mixing that is found on the lee side of the sill. Final calculations with deeper water but small sill height showed that lee waves could propagate over the sill, thereby reducing their contribution to mixing. In this case, bottom mixing was the major source of mixing which was mainly confined to the near bed region, with little mid-water mixing.
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References
Adcroft A, Hill C, Marshall J (1997) Representation of topography by shaved cells in a height coordinate ocean model. Mon Weather Rev 125:2293–2315
Afanasyev YD, Peltier WR (2001a) On breaking internal waves over the sill in Knight Inlet. Proc Roy Soc London A457:2799–2825
Afanasyev YD, Peltier WR (2001b) Reply to comment on the paper “On breaking internal waves over the sill in Knight Inlet”. Proc Roy Soc London A457:2831–2834
Baines PG (1995) Topographic effects on stratified flows: Cambridge Monographs on Mechanics. Cambridge University Press, Cambridge
Berntsen J, Xing J, Alendal G (2006) Assessment of non-hydrostatic ocean models using laboratory scale problems. Cont Shelf Res 26:1433–1447
Berntsen J, Xing J, Davies AM (2009) Numerical studies of flow over a sill: sensitivity of the non-hydrostatic effects to the grid size. Ocean Dynamics 59:1043–1059
Cummins PF, Vagle S, Armi L, Farmer D (2003) Stratified flow over topography: upstream influence and generation of nonlinear waves. Proc Roy Soc London A459:1467–1487
Davies AM, Jones JE (1990) Application of a three-dimensional turbulence energy model to the determination of tidal currents on the northwest European shelf. J Geophys Res 95:18143–18162
Davies AM, Xing J (2004) Modelling processes influencing wind induced internal wave generation and propagation. Cont Shelf Res 24:2245–2271
Davies AM, Xing J (2005) The effect of a bottom shelf front upon the generation and propagation of near-inertial internal waves in the coastal ocean. J Phys Oceanogr 6(35):976–990
Davies AM, Xing J (2007) On the influence of stratification and tidal forcing upon mixing in sill regions. Ocean Dynamics 57:431–451
Dewey R, Richmond C, Garrett R (2005) Stratified tidal flow over a bump. J Phys Oceanogr 35:1911–1927
Farmer DM, Armi L (2001) Stratified flow over topography: models versus observations. Proc Roy Soc London A457:2827–2830
Gerkema T (2001) Internal and interfacial tides: beam scattering and local generation of solitary waves. J Mar Res 59:227–251
Gerkema T (2002) Application of an internal tide generation model to baroclinic spring-neap cycles. Journal of Geophysical Research 107(C9):3124. doi:10.1029/2001JC001177
Gerkema T, Zimmerman JTF (1995) Generation of non-linear internal tides and solitary waves. J Phys Oceanogr 25:1081–1094
Gillibrand PA, Amundrud TL (2007) A numerical study of the tidal circulation and buoyancy effects in a Scottish fjord: Loch Torridon. J Geophys Res 112:C05030. doi:10.1029/2006JC003806
Hosegood P, Var Haren H (2004) Near bed solibores over the continental slope in the Faeroe-Shetland Channel. Deep-Sea Res 51:2943–2971
Inall ME, Cottier FR, Griffiths C, Rippeth TP (2004) Sill dynamics and energy transformation in a jet fjord. Ocean Dynamics 54:307–314
Inall ME, Rippeth TP, Griffiths C, Wiles P (2005) Evolution and distribution of TKE production and dissipation within stratified flow over topography. Geophys Res Lett 32:L08607. doi:10.1029/2004GL022289
Jeans DRG, Sherwin TJ (2001a) The evolution and energetics of large amplitude non-linear internal waves on the Portuguese shelf. J Mar Res 59:327–353
Jeans DRG, Sherwin TJ (2001b) The variability of strongly non-linear solitary internal waves observed during an upwelling season on the Portuguese shelf. Cont Shelf Res 21:1855–1878
Legg S (2004a) Internal tides generated on a corrugated slope. Part I: Cross-slope barotropic forcing. J Phys Oceanogr 34:156–173
Legg S (2004b) Internal tides generated on a corrugated continental slope. Part II. Along-slope barotropic forcing. J Phys Oceanogr 34:1824–1838
Legg S, Adcroft A (2003) Internal wave breaking at concave and convex continental slopes. J Phys Oceanogr 33:2224–2246
Marshall J, Hill C, Perelman L, Adcroft A (1997a) Hydrostatic, quasi-hydrostatic and nonhydrostatic ocean modelling. J Geophys Res 102:5733–5752
Marshall J, Adcroft A, Hill C, Perelman L, Heisey C (1997b) A finite-volume incompressible Navier Stokes model for studies of the ocean on parallel computers. J Geophys Res 102:5753–5766
Moum JN, Nash JD (2000) Topographically induced drag and mixing at a small bank on the continental shelf. J Phys Oceanogr 30:2049–2054
New AL, Pingree RD (1990) Evidence for internal tidal mixing near the shelf break in the Bay of Biscay. Deep-Sea Res 37:1783–1803
Rippeth TP, Palmer MR, Simpson JH, Fisher NR, Sharples J (2005) Thermocline mixing in summer stratified continental shelf seas. Geophys Res Lett 32:L05602. doi:10.1029/2004GL022104
Saenko OA (2006) The effect of localized mixing on the ocean circulation and time-dependent climate change. J Phys Oceanogr 36:140–160
Saenko OA, Merrifield WJ (2005) On the effect of topographically enhanced mixing on the global ocean circulation. J Phys Oceanogr 35:826–834
Samelson RM (1998) Large scale circulation with locally enhanced vertical mixing. J Phys Oceanogr 28:712–726
Spall MA (2001) Large scale circulations forced by localized mixing over a sloping bottom. J Phys Oceanogr 31:2369–2384
Stigebrandt A (1999) Resistance to barotropic tidal flow in straits by baroclinic wave drag. J Phys Oceanogr 29:191–197
Van Haren H (2004) Spatial variability of deep-ocean motions above an abyssal plain. J Geophys Res 109:C12014. doi:10.1029/2004JC002558
Van Haren H, Howarth J (2004) Enhanced stability during reduction of stratification in the North Sea. Cont Shelf Res 24:805–819
Vlasenko V, Stashchuk N (2006) Amplification and suppression of internal waves by tides over variable bottom topography. J Phys Oceanogr 36:1959–1973
Vlasenko V, Stashchuk N, Hutter K (2005) Baroclinic tides: theoretical modelling and observational evidence. Cambridge University Press, Cambridge
Xing J, Davies AM (2001) A three-dimensional baroclinic model of the Irish Sea: formation of the thermal fronts and associated circulation. J Phys Oceanogr 31:94–114
Xing J, Davies AM (2005) Influence of a cold water bottom dome on internal wave trapping. Geophys Res Lett 32:L03601. doi:10.1029/2004GL021833
Xing J, Davies AM (2006a) Internal wave trapping and mixing in a cold water dome. J Geophys Res 111:C07002. doi:10.1029/2005JC003417
Xing J, Davies AM (2006b) Processes influencing tidal mixing in the region of sills. Geophys Res Letters 33:L04603. doi:10.1029/2005GL025226
Xing J, Davies AM (2006c) Influence of stratification and topography upon internal wave spectra in the region of sills. Geophys Res Lett 33:L23606. doi:10.1029/ 2006GL028092
Xing J, Davies AM (2007) On the importance of non-hydrostatic processes in determining tidal induced mixing in sill regions. Cont Shelf Res 27:2162–2185
Xing J, Davies AM (2009) Influence of bottom frictional effects in sill regions upon lee wave generation and implications for internal mixing. Ocean Dynamics 59:837–861
Xing J, Davies AM (2010) The effects of large and small-scale topography upon internal waves and implications for tidally induced mixing in sill regions. Ocean Dynamics 60:1–25
Zhai X, Greatbatch RJ, Zhao J (2005) Enhanced vertical propagation of storm-induced near-inertial energy in an eddying ocean channel model. Geophys Res Lett 32:L18602. doi:10.1029/2005GL023643
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The authors are indebted to E. Ashton and L. Parry for typing the text and R. A. Smith for help in figure preparation.
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Xing, J., Davies, A.M. Effect of water depth and the bottom boundary layer upon internal wave generation over abrupt topography. Ocean Dynamics 60, 597–616 (2010). https://doi.org/10.1007/s10236-010-0280-8
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DOI: https://doi.org/10.1007/s10236-010-0280-8