Cruise Report - R/V Oceanus Voyage 309
30 July - 13 August, 1997

Coastal Mixing and Optics Experiment
Tracer and Microstructure Studies of Turbulence and Dispersion

James R. Ledwell, Neil S. Oakey, Timothy F. Duda, Harvey E. Seim,
and Miles A. Sundermeyer

The final Oceanus dye/microstructure cruise has been successfully completed. Two dye releases were performed near the 70-meter isobath, in the vicinity of 40.5 N, 70.5 W. The first was in the upper pycnocline, at 15 to 20 meters, the second was 5 meters off the bottom. Copious profiling for dissipation rates were interspersed with the tracer surveys. Special tows were performed to measure conductivity variance dissipation rates at various levels in the water column. Tracking of the dye patch was supported by 300 khz broadband ADCP, which should also provide high resolution shear and internal wave measurements. Six drogues were used to enhance the dispersion studies and to assist with tracking of the tracer patches. Although most of the moorings for CMO had been recovered, the Williams/Trowbridge 7-meter BASS tripod was redeployed to complement our experiment. The tripod was successfully recovered on 14 August 1997 in excellent condition.

For the first experiment, 100 kg of Rhodamine WT were released in a 2-km streak, on the $sigma$theta = 24.6 surface, at around 20 meters depth, and about 13 km east of the central mooring site, near the 70-meter isobath. The buoyancy frequency there was a local maximum of about 15 cph. The stratification above this level was unseasonably weak, perhaps due to a tropical storm that had passed through the area in late July. Winds were light during the experiment; tides were strong. Three good surveys were made of the patch over the 5 days of the experiment. A preliminary estimate of the diapycnal diffusivity is on the order of 0.05 $cm$2/s. Preliminary estimates of the diffusivity from microstructure profiles taken between dye surveys are of the same order of magnitude for the level of the dye patch, but increase with depth as the stratification decreases. The mean flow during this experiment was virtually nil, although the patch was stretched into a narrow streak over 10 km long, oriented across isobaths by the fourth day.

The second dye release was performed at approximately 65 meters depth, 5 meters off the bottom, approximately 1 km ESE of the BASS tripod. 100 kg of fluorescein was used, the depth being great enough for photodecay to be negligible. Again the initial streak was about 2 km long. Winds were light for the entire experiment, with tides near neap. The subtidal flow was very low at the time of the release, but became eastward during the experiment. If there was a well-mixed bottom boundary layer at the time of the release, it was confined to the bottom meter. The stratification at the level of the release was about 15 cph just above the release surface, and more than 20 cph in the 2 or 3 meters below it. The patch was sampled well 3 times over 5 days, with microstructure profiling to the bottom interspersed. The diffusivity again appeared to be less than 0.1 $cm$2/s. Inshore, the dye was spread into a smooth patch, 5 km wide along shore by 10 km long across shore, that ultimately contacted the bottom on the inshore end. Offshore, the dye extended into a 1 km wide streak, stretched 30 km to the east (along shore) at the end of 5 days.

A conventional CTD line was run at the beginning of the cruise, from the 40-meter isobath to the 123-meter isobath: (40°55' N, 70°19' W) to (40°07' N, 70°40' W), i.e., stations 1 to 19 in the repeat line of Lentz et al. A short CTD tow-yo was run at the end of the cruise from the 85-meter isobath to the 50-meter isobath: (40°18' N, 70°03' W) to (40°38' N, 70°06' W).

The cruise was extremely productive of data, aided by the calm weather, and a supportive crew and science party. The two dye/drogue releases were smooth and well controlled. The six surveys seemed to be relatively thorough. A total of 1101 EPSONDE profiles were obtained. Approximately 15 hours of towed microconductivity measurements were made at speeds of 3 to 4 knots. Although one drogue was lost just after the first deployment, the other five remained free for both experiments, and were successfully recovered.

The scientific fruit of the cruise awaits further analysis, but we anticipate accurate estimates of diaypcnal diffusivity from the dye, the microstructure profiles and the towed probe. The lateral dispersion of the dye patches offers an interesting study of the processes involved. We look forward to comparison of the data from the deep experiment with data from the BASS tripod.

The following people were in the science party:

• Russell Burgett, University of Rhode Island
• David Ciochetto, WHOI 1
• Terence G. Donoghue, WHOI
• Timothy F. Duda, WHOI
• Sharon Gillam-Locke, BIO2
• Blair Greenan, BIO
• James R. Ledwell, WHOI