Editorial Type:
Article
Article Type:
Research Article

Residual Transport of Suspended Material by Tidal Straining near Sloping Topography

Kirstin Schulz Leibniz-Institute for Baltic Sea Research, Warnemünde, Germany

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Lars Umlauf Leibniz-Institute for Baltic Sea Research, Warnemünde, Germany

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Print Publication:
01 Jul 2016
DOI:
https://doi.org/10.1175/JPO-D-15-0218.1
Page(s):
2083–2102
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Abstract

Tidal straining is known to have an important impact on the generation of residual currents and the transport of suspended material in estuaries and the coastal ocean. Essential for this process is an externally imposed horizontal density gradient, typically resulting from either freshwater runoff or differential heating. Here, it is shown that near sloping topography, tidal straining may effectively transport suspended material across isobaths even if freshwater runoff and differential heating do not play a significant role. A combined theoretical and idealized modeling approach is used to illustrate the basic mechanisms and implications of this new process. The main finding of this study is that, for a wide range of conditions, suspended material is transported upslope by a pumping mechanism that is in many respects similar to classical tidal pumping. Downslope transport may also occur, however, only for the special cases of slowly sinking material in the vicinity of slopes with a slope angle larger than a critical threshold. The effective residual velocity at which suspended material is transported across isobaths is a significant fraction of the tidal velocity amplitude (up to 40% in some cases), suggesting that suspended material may be transported over large distances during a single tidal cycle.

Corresponding author address: Kristin Schulz, Department of Physical Oceanography, Leibniz-Institute for Baltic Sea Research, Seestraße 15, 18119 Warnemünde, Germany. E-mail: kirstin.schulz@io-warnemuende.de

Abstract

Tidal straining is known to have an important impact on the generation of residual currents and the transport of suspended material in estuaries and the coastal ocean. Essential for this process is an externally imposed horizontal density gradient, typically resulting from either freshwater runoff or differential heating. Here, it is shown that near sloping topography, tidal straining may effectively transport suspended material across isobaths even if freshwater runoff and differential heating do not play a significant role. A combined theoretical and idealized modeling approach is used to illustrate the basic mechanisms and implications of this new process. The main finding of this study is that, for a wide range of conditions, suspended material is transported upslope by a pumping mechanism that is in many respects similar to classical tidal pumping. Downslope transport may also occur, however, only for the special cases of slowly sinking material in the vicinity of slopes with a slope angle larger than a critical threshold. The effective residual velocity at which suspended material is transported across isobaths is a significant fraction of the tidal velocity amplitude (up to 40% in some cases), suggesting that suspended material may be transported over large distances during a single tidal cycle.

Corresponding author address: Kristin Schulz, Department of Physical Oceanography, Leibniz-Institute for Baltic Sea Research, Seestraße 15, 18119 Warnemünde, Germany. E-mail: kirstin.schulz@io-warnemuende.de
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