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| Turbulence Production |
Zoologically-Generated Turbulent Mixing (ZOOM): Swimming marine organisms may contribute as much as 1 TW to the generation of turbulence in the ocean, comparable to surface tide dissipation. This may well be an overestimate and much of it is likely lost dissipatively in surface waters that are already well-mixed by atmospheric forcing. With the oceanic biomass concentrated near the surface, if biologically-generated turbulent mixing plays any role in the ocean at all, it is in the transport of nutrients into the euphotic zone and transfer of gases between the surface mixed-layer and underlying pycnocline. Such mixing will be even more intermittent than that generated by internal wave shear which occupies about 10% of stratified ocean interior. Measurements in the open ocean have thus far only found turbulence associated with internal-wave shear. Measurements in Saanich Inlet, where internal wave shear is weak, find a correlation between vertical migration of the acoustic backscatter layer (krill swarms) at dusk and dawn, and elevated turbulence (Fig. 1) but not all the time.
Fig. 1: Profile time-series of turbulent kinetic energy dissipation rate (upper panel) and acoustic backscatter (lower panel) during dusk 28 April 2006. Dissipation rates exceeding 10–9 W kg–1 are shown in red. During daylight hours, dissipation rates are typically 10–9 W kg–1. Around sunset, they approach 10–5-10–4 W kg–1 for a 15- minute burst (from Kunze et al. 2006 Science).
Fig. 2: Image showing typical upward migration of the acoustic backscatter layer at dusk.
But turbulence does not always accompany diel vertical migration (Fig. 2) for reasons as yet unknown. It may be that krill (Fig. 3) can generate turbulence only when their swarms are sufficiently dense or when they swim en masse.
Fig. 3: 1-2 cm long krill (Euphasia pacifica) individual.
Intense biologically-generated turbulence need not produce mixing. Intense turbulent dissipation rates associated with schools of sardines or anchovies in Monterey Bay show no accompanying microscale temperature signals, indicating weak mixing efficiency. Intense turbulence also appears to only occasionally coincide with swimming schools and swarms of marine organisms.
The relationship between krill migration and turbulence production is part of ongoing (Shani Rousseau) and future research efforts.
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| Eric Kunze, kunze@uvic.ca |