Investigation of the Bottom Boundary Layer during the Georges Bank
GLOBEC Stratification Study

INVESTIGATORS: J. Churchill and A. Williams

GRANT PERIOD: October 1993 -June 1995


To examine turbulent mixing and its influence on stratification
in the bottom boundary layer over the southern flank of Georges


Our study of stratification and mixing in the bottom boundary layer on
Georges Bank began with the deployment of a BASS (Benthic Acoustic Stress
Sensor) tripod at station ST1 (40 51'N, 67 33'W) on 3 February 1995. The
BASS was equipped with 5 acoustic current meters located from 0.25 to 4.5
m above bottom. These sampled the flow at 2 Hz for 7.5 min. every half
hour. Also attached to the BASS were: 8 thermistors, 5 optical
backscatterance sensors (to estimate suspended particulate concentration),
a pressure sensor, a CTD and Benthos camera and strobe. The BASS was
scheduled for an at-sea turn-around with data recovery in mid-April.
Unfortunately this was not successful. Only one of the two buoyed
recovery lines on the BASS released upon command. Before it could be
retrieved, it wrapped around the mooring chain of the SW guard buoy at ST1
and severed on the mooring chain when recovery was attempted. On June 7
second recovery attempt using the SeaProbe Submarine also failed. A third
recovery attempt with the SeaProbe was successful and the tripod was
returned to WHOI on 11 July. The tripod sensor data, which extended till
mid-June, have been recovered and are now being processed.

To acquired further bottom boundary layer measurements during a time of
"mature" stratification, a second BASS tripod was deployed at ST1 on July
12. This was outfitted with the same complement of current meters and
thermistors as were attached to the first BASS tripod. It is slated for
recovery on 19 August.

Analysis of the tripod measurements, coupled with modelling of bottom
mixing, will be carried out over the next year.TITLE:

Tracking Biological Distributions and Investigating Small-Scale Dynamics
of the Southern Flank of Georges Bank using GPS-Tracked Drifters

INVESTIGATORS: J. Churchill and J. Manning

GRANT PERIOD: October 1993 -June 1995


(1) To track concentrated "patches" of Cod and Haddock in order
to guide the sampling of these patches; and (2) To examine small-
scale dynamics over the southern flank of Georges Bank,
particularly those influencing horizontal shear dispersion.


To meet the above objectives we deployed and tracked drifters on one
cruise in the spring of 1994 (AL9403) and on three cruises in the spring
of 1995 (SJ9503, SJ9505, and SJ9507). The drifters were drogued with a 6
m holey sock set at a mean depth of 15 m. All drifters were equipped with
GPS and VHF electronics for positioning and communication, and with an
ARGOS PPT as a backup positioning and communication system. All drifters
were also outfitted with a near-surface thermistor. In order to monitor
thermal stratification in the upper water column, additional thermistors
were set at various depths on a number of other drifters.

During all cruises, a drifter or drifters were deployed in what was
identified from "bongo" surveys as patch of cod and haddock larvae. The
subsequent surveying of the patch was guided by the real-time color
display of drifter and ship positions available on the bridge and in the
science labs. The usual procedure was to carry out repeated mocness tows
along lines perpendicular to the drifter track.

During our third 1995 cruise, drifters were deployed in closely spaced
clusters. The first of the two cluster deployments of this cruise was
carried out at the start of a bongo-net survey (10-14 May). The drifters
were set out at four of the bongo sites (Figure 1) separated by roughly 8
km. These were in the vicinity of the GLOBEC stratification mooring
array. The second cluster was set out to the southwest of the first and
centered at roughly the 65 m isobath. We deployed 5 drifters in this
cluster: four at the corners of a box measuring roughly 3.5 km per side
and the fifth near the box's center (Figure 1). A sixth drifter was added
by T. Durbin's group operating off the R/V Endeavor.position and thermistor data from all 21 drifter deployments have
been processed. All data have been, or will shortly be, posted on the
GLOBEC WWW ( Researchers may access thermistor
and drifter position files via the Mosaic server under the category of
"Process Cruises". The CTD data and shipboard sensor data that were
collected in conjunction with these drifter deployments are also available
through the Mosaic server.


A guiding premise of the GLOBEC stratification experiment was that the
onset of stratification may significantly influence larval dynamics and
the chances of larvae survival. Data from the thermistors attached to the
drifters offered a rare Lagrangian view of the thermal structure of tagged
water parcels. Most remarkable were the rapid changes seen in the thermal
stratification of the upper water column. Over a period of less than a
day we observed solar warming of a few degrees C in the surface 15 m of
the water column. The attendant thermal stratification broke down on an
equally short time scale.

The time series of drifter positions (which were sampled at 5 min
intervals) gave a detailed, albeit short-term, view of the current field
over the southern flank of the bank. Using the methods outlined by
Churchill (in preparation), we converted the position time series to time
series of alongbank and onbank velocity. These were also separated into
tidal and subtidal frequency bands. As expected, drifter velocities were
dominated by oscillations of the semidiurnal tide. These oscillations
were 50-100 cm/s in magnitude and somewhat stronger (by about 25 %) in the
across-bank direction. The subtidal flow sampled by the drifters tended
to be related to the surface wind (measured by the NOAA buoys) in the
manner expected. An alongbank wind directed to the southwest was usually
accompanied by an onbank deflection of the drifters and an alongbank
acceleration of the drifters to the southwest (i.e., classical upwelling
circulation). The reverse was usually true when the alongbank wind
component was directed to the northeast.

Drifter tracks from the first cluster deployment gave evidence of a strong
jet-like flow at the shelf-edge which was not wind-driven. Within a
couple of tidal periods after its deployment, the drifter set out at the
southeast corner of the deployment array (labeled drifter 1a in Figure 1)
migrated to the shelf-edge and then was rapidly carried southwestward at a
speed in excess of 40 cm/s. The other drifters, which remained onbank of
the shelf-edge, moved relatively slowly (at rates of < 25 cm/s) to the
southwest. Records from the thermistors on the "renegade" drifterthat it resided at the shelf-edge front during its charge to the
southwest. The shelf-edge jet in which this drifter was evidently caught
may have been related to the two Gulf Stream rings which were positioned
off the southern flank at the time. When they become available, we will
examine records from the GLOBEC moorings to examine the extent and timing
of the shelf-edge jet in relation to the passage of the rings.

Data from the second cluster allowed us to examine small-scale dispersion
over the southern flank. An important result is deduced by comparing the
time records of the cluster's mean position with the variance of drifter
locations about the mean position (Figure 2). While tidal oscillations
are prominent in the mean position records, they make relatively minor
contributions to changes in position variance. This implies that the tide
varies on a spatial scale much larger than the cluster size. The variance
plot further reveals a significant stretching of the cluster in the
along-bank direction and a compression of the cluster in the on-bank
direction. The rate of increase in along-bank variance of drifter
position, gives an along-bank horizontal diffusion coefficient of 3 x 10^6
cm^2/s. This is relatively small when compared with estimates of
diffusion coefficients in near-shore environments. For example, it is
roughly 1/10 of the alongshelf diffusion coefficient determined by Davis
(1985) from drifter data acquired off the California coast. In future
work we will compare dynamical properties determined from the cluster data
(i.e., correlation time-scales, velocity convergence, etc.) with that
determined from the moored instrument records.

Churchill, J. H. (in preparation) Use of GPS-tracked drifters in
coastal circulation studies.

Davis, R. E. (1985) Drifter observations of coastal surface
currents during CODE: The statistical and dynamical views.
J. Geophys. Res., 90, 4756-4772.

Figure 1. Summary of drifter deployments on SJ9507. The first
cluster deployment (1b,2b,4c,6b,7a), etc.

Figure 2. The top plot show the long-bank and on-bank components
of the mean position of the 2nd drifter cluster. The lower plot
shows the long-bank and on-bank components of the variance of the
drifter positions about the mean cluster position.