Biologically Generated Mixing in the Ocean.

This article assesses the contribution to ocean mixing by the marine biosphere at both high and low Reynolds numbers Re= uℓ/ ν. While back-of-the-envelope estimates have suggested that swimming marine organisms might generate as much high-Reynolds-number turbulence as deep-ocean tide- and wind-generated internal waves, and that turbulent dissipation rates of O(10-5 W kg-1) (Re ∼ 105) could be produced by aggregations of organisms ranging from O(0.01 m) krill to O(10 m) cetaceans, comparable to strong wind and buoyancy forcing near the surface, microstructure measurements do not find consistently elevated dissipation associated with diel vertically migrating krill. Elevated dissipation rates are associated with schools of O(0.1- 1 m) fish but with low mixing coefficients ( γ ∼ 0.002-0.02, as compared with γ ∼ 0.2 for geophysical turbulence). Likewise, viscously induced drift at low Reynolds numbers produces little mixing of temperature, solutes, dissolved nutrients, and gases when realistic swimmers and molecular scalar diffusion are taken into account. The conclusion is that, while the marine biosphere can generate turbulence, it contributes little ocean mixing compared with breaking internal gravity waves.

Climate Process Team on Internal Wave-Driven Ocean Mixing.

Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.

Observations of biologically generated turbulence in a coastal inlet.

Measurements in a coastal inlet revealed turbulence that was three to four orders of magnitude larger during the dusk ascent of a dense acoustic-scattering layer of krill than during the day, elevating daily-averaged mixing in the inlet by a factor of 100. Because vertically migrating layers of swimming organisms are found in much of the ocean, biologically generated turbulence may affect (i) the transport of inorganic nutrients to the often nutrient-depleted surface layer from underlying nutrient-rich stratified waters to affect biological productivity and (ii) the exchange of atmospheric gases such as CO2 with the stratified ocean interior, which has no direct communication with the atmosphere.

From tides to mixing along the Hawaiian ridge.

The cascade from tides to turbulence has been hypothesized to serve as a major energy pathway for ocean mixing. We investigated this cascade along the Hawaiian Ridge using observations and numerical models. A divergence of internal tidal energy flux observed at the ridge agrees with the predictions of internal tide models. Large internal tidal waves with peak-to-peak amplitudes of up to 300 meters occur on the ridge. Internal-wave energy is enhanced, and turbulent dissipation in the region near the ridge is 10 times larger than open-ocean values. Given these major elements in the tides-to-turbulence cascade, an energy budget approaches closure.