Swell as turbulence source in shallow water

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Two field investigations in an exposed shallow area outside Lofoten, western Norway, were conducted in 1995 and 1996. Direct measurements of turbulence were conducted from an underwater tower with acoustic current meters 6 m above the bottom. The tidal energy in this area is low and the wind conditions during the experiment were mostly weak. Nevertheless, the turbulent kinetic energy dissipation rate at 20-25 m depth was in the range 10-7-10-5 W/kg. The only other possible energy source was long swell of wavelength 100-200 m that rolled in from the open sea. Analysis shows that the wave related water motion intermittently becomes unstable, inducing strong turbulent patches in parts of the wave orbit. The mechanism behind this process is not clear, but possible explanations may be local energy concentration or interaction of waves with different frequency. In many areas and situations this energy source is equal in force to tidal and wind generated turbulence. Simultaneous measurement of vertical profiles of zooplankton and fish larvae from a nearby location enables us to discuss swell-induced turbulence in an ecological context.