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

Turbulence in shelf seas strongly affects the spread of pollution (such as oil spills) as well as the distribution of sediment and phytoplankton blooms. Turbulence is known to be generated intermittently close to the sea bed, 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 laboratory and numerical 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.