Ocean Dynamics

, Volume 61, Issue 10, pp 1697–1717 | Cite as

Tidal mixing in sill regions: influence of sill depth and aspect ratio

Part of the following topical collections:
  1. Topical Collection on Joint Numerical Sea Modelling Group Workshop 2010


A non-hydrostatic model in cross-sectional form with an idealized sill is used to examine the influence of sill depth (h s) and aspect ratio upon internal motion. The model is forced with a barotropic tide and internal waves and mixing occurs at the sill. Calculations using a wide sill and quantifying the response using power spectra show that for a given tidal forcing namely Froude number F r as the sill depth (h s) increases the lee wave response and vertical mixing decrease. This is because of a reduction in across sill velocity U s due to increased depth. Calculations show that the sill Froude number F s based on sill depth and across sill velocity is one parameter that controls the response at the sill. At low F s (namely F s ≪ 1) in the wide sill case, there is little lee wave production, and the response is in terms of internal tides. At high F s, calculations with a narrow sill show that for a given F s value, the lee wave response and internal mixing increase with increasing aspect ratio. Calculations using a narrow sill with constant U s show that for small values of h s, a near surface mixed layer can occur on the downstream side of the sill. For large values of h s, a thick well-mixed bottom boundary layer occurs due to turbulence produced by the lee waves at the seabed. For intermediate values of h s, “internal mixing” dominates the solution and controls across thermocline mixing.


Internal waves Sill Mixing 



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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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  1. 1.National Oceanographic CentreLiverpoolUK

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