资源环境科技发展态势分析平台

The efficiency of the conversion of mechanical to potential energy, often expressed as the flux Richardson number, Ri(f), is an important determinant of vertical mixing in the ocean. To examine the dependence of Ri(f) on the buoyancy Reynolds number, Re-B, we analyze three sets of data: microstructure profiler data for which mixing is inferred from rates of dissipation of turbulent kinetic energy (epsilon) and temperature variance () measured in the open ocean, time series of spectrally fit values of epsilon and covariance-derived buoyancy fluxes measured in nearshore internal waves, and time series of spectrally fit values of epsilon and measured in an energetic estuarine flow. While profiler data are well represented by Ri(f)approximate to 0.2 for 1

Plain Language Summary The efficiency of turbulent mixing, the rate at which mechanical energy of turbulence is converted into changes in potential energy, plays a central role in determining the dynamics of ocean circulation and the transport of carbon. This efficiency is commonly assumed to be constant, although there is evidence that it can depend on the strength of the turbulence, measured by the rate of dissipation of the turbulence energy, and the strength of the density stratification. Notably, ocean circulation models with variable efficiency produce different flows than do ones with constant efficiency. Using three oceanic data sets, including the large body of microstructure data acquired over the past 20years, we analyze how efficiency varies with turbulence strength. Consideration of these data, and other published data, shows that mixing efficiency is, as commonly assumed, likely to be constant in much of the ocean. In contrast, in places that are highly turbulent, for example, very energetic currents, it might be relatively low. We show that at present, what controls the transition from constant to variable efficiency remains uncertain, although reanalysis of existing microstructure data in energetic parts of the ocean should help clarify the extent to which mixing efficiency varies in these regions.