Notes in Biomixing in Monterey Bay, a presentation by M. Gregg (08/25/09)

Summary

In Monterey Bay, fish schools trigger a dissipation rate 10 to 1000 times larger than in the background but, given an estimate of the mixing efficiency 100 times lower than that used in previous studies, their contribution to diapycnal diffusivity is as large as the background one.

In more biologically productive region, however, biomixing could surpass background mixing.

Notes

• Dewar‘06 bulk estimate: 1% of the total 62.7 TW available would give a diapycnal diffusivity of 2e-5 m2/s.
• Kunze et al.‘06 observations of turbulence due to shallowing school of krill in an inlet, isolated from other physical processes: diapycnal diffusivity increases from 2e-6 to 4-40e-4 m2/s. To calculate this, a mixing efficiency of 0.2 was used. If this was true, a strong diurnal cycle should have been in observations of turbulence, which is not the case (Rippeth et al.‘07). Not published further observations by Kunze showed an increase by only a factor of 2.
• This talk presents observations of turbulence in Monterey Bay, along the edges of the Monterey Canyon. D
• Dissipation rate up to 1e-5 W/kg was observed, as in previous studies.
• However, no overturns were observed. The reason is that overturns are seen only at the 10-cm vertical scale, much lower than the resolution of Seabird CTD measurements.
• Thorpe and Ozmidov scales are calculated from the data and both scales are found to be equal outside fish schools, as expected from theory, while the Thorpe scale is much lower than the Ozmidov scale within fish schools.
• The estimation of the mixing efficiency is revisited and is found to be in the range of previous values used (0.1-0.3) outside fish schools but 100 times lower within fish schools. Thus, although disspation rate are high with fish school, their contribution to diapycnal diffusivity is not parallely high.
• The reason why fishes would not contribute largely to diapycnal mixing would be that the vortices that are created in the lee are mostly vertical and do not generated overturns in the vertical.
• These results cannot necessarily be generalized because the region is not too productive biologically and only one species of fish was present.