Churchill#2 Report

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

    INVESTIGATORS:

    J. Churchill
    Woods Hole Oceanographic Institution
    Woods Hole, MA

    J. Manning
    National Marine Fisheries Service
    Woods Hole, MA

    GRANT PERIOD: October 1993 - June 1995

### STATEMENT OF OBJECTIVES:

- To track concentrated "patches" of Cod and Haddock in order to guide the sampling of these patches;

- To examine small-scale dynamics over the southern flank of Georges Bank, particularly those influencing horizontal shear dispersion.

### STATEMENT OF WORK:

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.

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

Drifter 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 (http://globec.whoi.edu). 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.

### SUMMARY OF KEY FINDINGS:

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" drifter indicate that 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.

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.

### REFERENCES:

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.