Lagrangian Measurements as Part of a Georges Bank


Kenneth H. Brink
Woods Hole Oceanographic Institution
Woods Hole, MA 02543

Robert C. Beardsley
Woods Hole Oceanographic Institution
Woods Hole, MA 02543

Richard Limeburner
Woods Hole Oceanographic Institution
Woods Hole, MA 02543


October 1, 1994 -September 30, 1996


The primary objective of the large-scale GLOBEC drifter program is to
characterize the recirculation over Georges Bank -both its spatial
and temporal structure and variability, and integrate this physical
characterization with biological observations to formulate a coherent
description of the Georges Bank ecosystem. This Lagrangian
understanding of the circulation is important because it will 1)
provide estimates of residence time over the Bank which directly
relate to the ability of the system to retain nutrients and biota, 2)
provide direct observations of where the water actually goes (i.e.,
the primary transport paths through important spawning grounds and the
preferred regions where Bank water is exported), and 3) lead to a
better understanding of the physics of the recirculation and improve
our ability to model this dynamic system.


A total of 25 drifters with drogues centered at a depth of 10 to 15 m
have been deployed over Georges Bank during the period January to
June, 1995 on cruises EN259 (P. Weibe), EN261 (T. Durbin), EN263 (C.
Miller), EN265 (J. Sibunka), and Al9506 (A. Bucklin). An additional 25
drifters (5 deployments of 5 drifters) are planned for deployment over
Georges Bank during summer and fall of 1995.


The drifter position data have been made available to the scientific
community via the GLOBEC WWW home page An animation of the
drifter trajectories was projected onto a basemap of Georges Bank that
also shows the yearday as the drifters move over the Bank. A movie
"player" is required to play the animation and information is given on
the GLOBEC home page where to obtain various free movie players with
VCR-like controls and how to configure these players to PC, Mac, or
Unix workstations. The 1995 GLOBEC drifter movie is updated monthly
and allows other researchers to observe the actual 1995 Lagrangian
near-surface circulation patterns and time scales of the variability
over Georges Bank. We plan to incorporate a windstress vector into the
drifter animation in the near future. Also, four similar animations of
drifter movements over Georges Bank during the SCOPEX program in 1988
and 1989 with drogues centered at 5 m and 50 m have are now available
on the GLOBEC home page, as well as an animation of 2 years of
near-surface drifter data in the California Current.

Preliminary analysis of the near-surface Lagrangian drifter
trajectories over Georges Bank during the first 6 months of 1995

1. Many of the drifters deployed on Georges Bank between January and
June, 1995, remained within the region defined by the 60 m isobath.
The combined tracks of all the drifters generally describe the limits
of the 60 m isobath, except over the eastern side of the Great South
Channel. (Figure 1 -available from ftp anonymous at, cd
argos, as a binary file named figure1.gif). This implies that the
Lagrangian circulation at a depth of 15 m was generally non-dispersive
within the 60 m isobath during the first six months of 1995.

2. High wind events had a strong influence on the drifter
trajectories. In early February, 1995, strong northeast winds shifting
to the northwest forced the five drifters over Georges Bank offshelf
where they became entrained in a warm-core ring. In March, strong
northwest winds forced another drifter southwestward to the 100 m
isobath. This drifter then moved westward and southwestard along the
local 100 m isobath into the Middle Atlantic Bight.

3. In February and March, 1995, the near-surface flow at the 100 m
isobath was along-shelf toward the west and southwest at about 50
cm/sec. During this same period the mean flow over the Bank 50 km to
the north was small, less than 5 cm/s.

4. In June, two drifters moved along-shelf over the 100 m isobath and
then veered back onto the south flank of the Bank within the 60 m.
However, the near-surface flow in the eastern Great South Channel was
stagnant during late May and June.

6. Some early patterns of recirculation are visible in the drifter
animation. However, we expect the recirculation patterns to strengthen
during the summer and early fall months. Future work will integrate
the biological sampling and population studies into the advective
patterns observed in the drifter trajectories.