Limeburner Report

### Lagrangian Measurements as Part of a Georges Bank Study

    INVESTIGATORS:

    Kenneth H. Brink
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543
    kbrink@whoi.edu

    Robert C. Beardsley
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543
    rbeardsley@whoi.edu

    Richard Limeburner
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543
    rlimeburner@whoi.edu

    GRANT PERIOD:  October 1, 1994 - September 30, 1996

#### STATEMENT OF OBJECTIVES:

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.

#### STATEMENT OF WORK:

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.

#### SUMMARY OF KEY FINDINGS:

The drifter position data have been made available to the scientific community via the Georges Bank GLOBEC WWW home page

    		http://gb1.whoi.edu/globec-dir/misc-data.html.

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 indicate:

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). 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.

5. 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.