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Surface effects of bottom-generated turbulence in a shallow tidal sea

Nature volume 400, pages 251–254 (1999)Cite this article

Abstract

Turbulence in shelf seas strongly affects the spread of pollution (such as oil spills1) as well as the distribution of sediment2 and phytoplankton blooms3. Turbulence is known to be generated intermittently close to the sea bed4, but little is known of its evolution through the water column, or to what extent it affects the surface. Here we present observations of the surface effects of bottom-generated turbulence in a tidally influenced and well mixed region of the North Sea, as derived from acoustic and visual images. Although the sea bed in the area is flat, we find that at any one time, 20–30% of the water surface is affected by boils—circular regions of local upwelling—of diameter 0.9±0.2 times the water depth. The signature of individual boils persists for at least 7 minutes and, in accordance with laboratory5,6 and numerical7 studies, shows the appearance of eddies. The boils contribute to the replacement of surface waters from depth in unstratified waters, and may therefore enhance the fluxes of gases between atmosphere and ocean.

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Figure 1: Sonar and video images of boils at the sea surface.
Figure 2: Video images showing details of boils.
Figure 3: CASI false-colour images of an oil patch (in red).
Figure 4: Sonograph images of strongly scattering features (shown as dark) in winds of different directions.

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Acknowledgements

We thank T. Lunnel (AEA Tech. plc) for providing the video of the oil slick and the environmental data for the CASI images. We also thank the Environment Agency for supplying the CASI images, and V. Byfield for calibrating them; and A. Hall for help in collecting the sonar data. The observations in the North Sea were funded by an EEC MAST contract. W.A.M.N.S. is supported by NERC.

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  1. School of Ocean and Earth Science, Southampton Oceanography Centre, European Way, SO14 3ZH, Southampton, UK

    W. A. M. Nimmo Smith, S. A. Thorpe & A. Graham

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  1. W. A. M. Nimmo Smith
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  3. A. Graham
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Correspondence to W. A. M. Nimmo Smith.

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Smith, W., Thorpe, S. & Graham, A. Surface effects of bottom-generated turbulence in a shallow tidal sea. Nature 400, 251–254 (1999). https://doi.org/10.1038/22295

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