Mixing in a coastal environment: 1. A view from dye dispersion

Abstract

[1] Dye release experiments were performed together with microstructure profiling to compare the two methods of estimating diapycnal diffusivity during summer and fall stratification on the continental shelf south of New England. The experiments were done in 1996 and 1997 as part of the Coastal Mixing and Optics Experiment. During the 100 hours or so of the experiments the area of the dye patches grew from less than 1 km 2 to more than 50 km 2 [Sundermeyer and Ledwell, 2001]. Diapycnal diffusivities inferred from dye dispersion range from 10 À6 to 10 À5 m 2 /s at buoyancy frequencies from 9 to 28 cycles/hour. Diffusivities estimated from the dye and those estimated from dissipation rates in the companion paper by Oakey and Greenan [2004] agree closely in most cases. Estimates of diffusivities from towed conductivity microstructure measurements made during the cruises by Duda and Rehmann [2002] and Rehmann and Duda [2000] are fairly consistent with the dye diffusivities. The dye diffusivities would be predicted well by an empirical formula involving shear and stratification statistics developed by MacKinnon and Gregg [2003] from profiling microstructure measurements obtained at the same site in August 1996. All of the measurements support the general conclusion that the diffusivity, averaged over several days, is seldom greater than 10 À5 m 2 /s in the stratified waters at the site, and usually not much greater than 10 À6 m 2 /s. Severe storms, such as a hurricane that passed over the CMO site in 1996, can dramatically increase the mixing at the site, however.

Spring Mixing: Turbulence and Internal Waves during Restratification on the New England Shelf

“…MGa found their summer turbulence measurements to be consistent with those of Rehmann and Duda (2000) and Ledwell et al (2004). MGa found their summer turbulence measurements to be consistent with those of Rehmann and Duda (2000) and Ledwell et al (2004).…”

Horizontal dispersion in shelf seas: High resolution modelling as an aid to sparse sampling

“…For the dye release Rhodamine WT was chosen as a passive tracer due to its ability to be detectable for several days after release at a level sufficient for detection by a fluorometer (details can be found in Ledwell et al (2004)). The fluorometer was mounted in a towed Scanfish vehicle which scanned the whole water column undulating from 5 m below the surface to a depth of 10 m above the bed.…”

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

“… 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.…”