Cruise Report - R/V Oceanus Voyage 309
30 July - 13 August, 1997
Coastal Mixing and Optics Experiment
Tracer and Microstructure Studies of Turbulence and
James R. Ledwell, Neil S. Oakey, Timothy F. Duda, Harvey
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
For the first experiment, 100 kg of Rhodamine
WT were released in a 2-km streak, on the 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 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 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:
1WHOI = Woods Hole
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
Scott Madin, WHOI
Neil S. Oakey, BIO
Christopher Rehmann, WHOI
Robert Ryan, BIO
Harvey E. Seim, Skidaway Institute of Oceanography
Cynthia J. Sellers, WHOI
Miles A. Sundermeyer, WHOI
Stewart C. Sutherland, Lamont-Doherty Earth Observatory
David Walsh, Dalhousie University
2BIO = Bedford
Institute of Oceanography
© Woods Hole Oceanographic Institution