PROJECT TITLE: Turbulence and Mixing Studies in GLOBEC on
Georges Bank

INVESTIGATORS: Neil S. Oakey, Bedford Inst. of Oceanography
D.L. Hebert, University of Rhode Island

GRANT PERIOD: Oakey: October 1, 1994 to September 30, 1996
Hebert: October 1, 1995 to September 30, 1996


The objectives of the study are to understand (a) the physical
processes that control stratification on Georges Bank and (b) the
relationships between stratification variability and the critical
life parameters for the local zooplankton and larval fish
populations. In particular the specific objective of the mixing
studies is to understand the importance of turbulence in the
development of stratification.


One of the most frequently cited oceanographic features
contributing to high biological production on Georges Bank are
enhanced vertical mixing rates at the Bank edges resulting in an
enhanced flux of nutrients over the sides of the Bank. Estimates
of mixing can be obtained from the measurement of temperature and
velocity microstructure (turbulence) and calculating vertical
diffusivities of heat, mass and momentum. Two sites were chosen on
the southern flank of Georges Bank for the field study, one in the
"mixed" region of the bank in a depth of 44 meters and a second in
much more stratified region near the site of the main
stratification mooring, ST1, in a depth of 76 meters. Each site
was studied on two cruises in the spring of 1995 separated by about
six weeks to observe the development of stratification. This gives
us turbulent mixing data in the framework of the larger scale
picture on the southern flank of Georges Bank to complement the
results that we have from similar studies done in 1988 on the
northern side of Georges Bank.

The principle measurements of the small-scale turbulence
studies were a series of microstructure profiles from near the
surface to the bottom using the profiler EPSONDE while at anchor
for several semi-diurnal tidal periods at each of the two sites.
Both sites were studied on two cruises from the R/V Seward Johnson,
the first from April 25 to May 3, 1995 and the second from June 6June 16, 1995. The microstructure measurements were obtained
using the profiler EPSONDE. This is a tethered-free-fall
microstructure profiler approximately 2.2 meters long by 0.15
meters in diameter which measures temperature microstructure using
fast thermistors and thin-film thermometers and velocity
microstructure (turbulence) using shear probes. The instrument is
deployed from a ship using a thin tether line which also acts as
the data link and obtains useful data in a profile from about five
to eight meters from the surface to within 10 cm from the bottom
where the instrument is stopped by a guard which protects the
probes. It also measures temperature, conductivity and depth.
Using standard analysis techniques these data yield dissipation and
Chi-Theta and derived quantities such as Cox-Number, turbulent heat
flux, and diffusivities.

In addition to the microstructure studies, we made almost
continuous measurements of water velocity using two broad band
ADCPs (a 600 kHz and a 150 kHz) while on station. This will
provide the velocity framework to help interpret the mixing
measurements. Also, frequent CTD profiles, with a fluorometer and
transmissometer were made on both cruises. During the second
cruise, Lew Incze took chlorophyll-a samples for calibration of the
fluorometer. As well, during the second cruise, Mark Berman and
Jack Green made vertical profiles of zooplankton abundance using
their acoustical TAPS system mounted on the CTD. Numerous pump
stations were made by Berman, Green and Incze to identify and
measure the vertical distribution of zooplankton. These
concomitant physical and biological measurements will provide a
starting point for interesting interdisciplinary scientific

The responsibility for program has been divided between Oakey
and Hebert. Oakey has taken the lead in the field program to
obtain turbulence data. The analysis will be done between Oakey
and Hebert and Hebert will take the lead role (along with a
student) in merging the mixing data and ADCP data to provide a
complete picture of mixing.

Milestones Completed:
1994 EPSONDE and ancillary equipment were prepared for
the field season; sensors were built and calibrated
and field supplies were purchased. (Oakey)
1995 Two field programs were carried out on the R/V
Seward Johnson: SJ9506 from April 25 to May 3, and
SJ9508 from June 6 to June 16. (Oakey and Hebert) to be Completed:
1995 Start the raw spectral analysis of the data; apply
corrections and calibrations. (Hebert and Oakey)
1996 Complete the data analysis to provide a data base of
dissipation, Chi-Theta, and other microstructure
quantities as a function of depth and time over the
anchor stations. Merge the data with density gradient,
temperature gradient, and velocity shear to provide depth
and time averaged derived quantities such as
diffusivities, turbulent heat flux and Cox Number.
Interpret the results and interpret the data in the
context of the larger scale data sets. Prepare
manuscript(s) and data report(s) on the physical
oceanographic results. Collaborate with the biological
oceanographers to incorporate the mixing results into a
better understanding of biological processes on Georges
Bank. (Hebert and Oakey)


At this time, shortly after our two cruises, none of the data
is available via the data management system.

Little can be said at this point regarding "key findings" in
our studies. Field work has been only recently completed and final
analysis of the data has not yet been done. We have, nevertheless,
a large number of profiles (1800 from the two field programs) and
a preliminary analysis indicates that the data are of good quality.
The mixed site on the Bank showed very little change between the
two experimental periods separated by six weeks. The site is
almost completely unstratified and clearly dominated by tidally
generated turbulence at the bottom boundary layer. We see very
high levels of dissipation often in excess of 10^-6 W/kg, strongest
at the bottom and decreasing in intensity from the bottom. These
results will be explored to examine tidally generated turbulence
and possible effects of bottom roughness. For the site on the
slope of the Bank adjacent to the main stratification mooring (ST1)
stratification had started to develop over the six week interval
between cruises with the evolution of a strong thermocline at about
20 meters depth. In the second field study a particularly long
anchor station (107 hours) allowed us to observe the response of
the water column turbulence and mixing over almost nine semi-tidal cycles. As at the mixed site the major forcing is
the tide with high dissipation, often greater than 10^-6 W/kg near
the bottom and decreasing to levels less than 10^-9 W/kg at the
base of the thermocline, where the increased density gradient
appears to decrease the turbulence.
This minimum coincides with the base of a surface mixed layer that
was isolated from the bottom mixed layer.