Turbulence and Mixing Studies in GLOBEC on Georges Bank

INVESTIGATORS:            

Neil S. Oakey
Bedford Inst. of Oceanography
Halifax, Nova Scotia
Canada

D.L. Hebert
University of Rhode Island
Graduate School of Oceanography
Narragansett, RI

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

STATEMENT OF OBJECTIVES:

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.

STATEMENT OF WORK:

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 6 to June 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 collaborations.

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) 

Milestones 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)

SUMMARY OF KEY FINDINGS:

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-diurnal 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.