Coastal Mixing & Optics Experiment

Tracer Studies of Mixing in Stratified Coastal Waters

A study of stirring and mixing in density-stratified waters on the continental shelf was conducted as part of the ONR funded Coastal Mixing and Optics Experiment. Fluorescent dye and drogues were released, and their subsequent dispersion measured to determine rates of diapycnal diffusion, lateral dispersion, and lateral homogenization over 3- to 5-day periods.

Temperature and conductivity were measured simultaneously with the dye at 2-cm resolution along with the dye measurements from a towed body in order to characterize the spatial scale of patches of enhanced gradients. Dissipation rates of temperature variance were inferred from these measurements and compared with the diapycnal diffusivity estimated from the dye dispersion in order to test the Osborn-Cox relation proposed between the two. Shear was also measured by shipborne ADCP throughout the experiment at 2-m resolution.

Dissipation rates of temperature variance and of turbulent kinetic energy were measured directly by profiler deployments in work proposed separately by Neil Oakey of Bedford Institute of Oceanography, on alternate watches with the dye surveys to test the Osborn-Cox relation, as well as the relation between diapycnal diffusion and kinetic energy dissipation proposed by Osborn. Detailed hydrographic measurements and measurements of the Reynolds stress and an approximation of the heat flux were made continuously throughout the experiment from a tripod-based array of instruments by Trowbridge and Williams. These experiments were further embedded in the mooring array proposed by Lentz, et al., which provided a 9-month time series of the meteorological forcing and of the current field on 30-km scale, with 5 m vertical resolution.

The Dye-Release Experiments

Fluorescent dye and drogues were released in order to determine rates of diapycnal diffusion, lateral dispersion, and lateral homogenization over 3- to 5-day periods. Five releases were performed in late summer when a pycnocline was well established. Four of these were at mid-depths and the fifth just above the bottom boundary layer (see table and figures below).

-	dates	dye	injection lat, lon	target density (kg/m3)	target depth (m)	sampling surveys
experiment 1	Sept 1995	rhodamine	40.44 N, 70.49 W	25.20	40	2
experiment 2	Sept 1996	rhodamine	40.46 N, 70.32 W	24.063	35	3
experiment 3	Sept 1996	fluorescein	40.47 N, 70.32 W	24.30	46	3
experiment 4	Aug 1997	rhodamine	40.48 N, 70.33 W	24.6	18	3
experiment 5	Aug 1997	fluorescein	40.48 N, 70.49 W	26.14	62	3

 

The study site was approximately 100 km south of Martha's Vineyard, MA in the region known as the "mud patch" (see figures). For each experiment, a single streak of dye was released along an isopycnal surface using a towed injection system. Along with the dye, three to six drogues were also released at the target depth and tracked via ARGOS transmitters. The drogue positions were used along with near real-time shipboard ADCP observations both to assist in locating the dye patch and as an aid to planning sampling strategies during the dye surveys. ADCP data were also logged for later analysis.

After each dye release, three-dimensional surveys of the dye patch were conducted using a sampling sled that was tow-yo'd behind the ship at 2-4 kts. The winch operated at speeds between 30-60 m/min, which provided a vertical profile every 100-200 m along the ship track. The sampling system included either a rhodamine or fluorescein fluorometer (depending on the type of dye released), a chlorophyll fluorometer, and a Seabird CTD. These instruments sampled at rate of 6 Hz, providing an average vertical resolution of 10 cm.

Hydrographic sections extending across the shelf were made at the beginning and sometimes the end of each cruise. These stations provided detailed descriptions of local conditions and were used to determine where to inject the dye and to help plan subsequent dye sampling strategies. Meteorological data were also logged for each of the cruises using the shipboard Improved Meteorological (IMET) package. In addition, data were obtained from nearby buoys of the National Oceanic and Atmospheric Administration / National Data Buoy Center (NOAA / NDBC).

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