Mixing in a coastal environment: 2. A view from microstructure measurements

Abstract

During the Coastal Mixing and Optics Experiment in 1996 and 1997, an integrated dye and microstructure experiment was done to measure and compare mixing rates on the continental shelf. The results of the dye experiment are presented in the companion paper by Ledwell et al. [2004]. In this paper, we explore the results from microstructure measurements using a vertical profiling instrument. We measure temperature and velocity microstructure and, along with simultaneous measurements of salinity and temperature as well as a shipboard acoustic Doppler current profiler (ADCP), are able to estimate the vertical diffusivities of heat, mass, and momentum. In three of four dye injections performed, we were able to make a comparison of the diffusivity from both dye and microstructure measurements. Although the mixing rates were quite small (vertical diffusivity of heat, KT < 10−5 m2 s−1), the two techniques yielded consistent results. A comparison of the vertical diffusivities KT and Kρ (the vertical diffusivity for density) allowed us to determine a flux Richardson number of Rf = 0.16 ± 0.03. Rf showed little dependence on either the buoyancy frequency, N, or gradient Richardson number, Ri. A clear relationship was found between the ratio of diffusivities, Km/KT and Ri consistent with Km/KT = 5 Ri. Turbulence levels were extremely low, with Cox numbers in one experiment of about 20 and in the other three of about 5 (i.e., KT about 20 and 5 times molecular diffusion, respectively).