Drifter Event A.

The first set of five drifters were deployed within the 60-m isobath on
Georges Bank on January 14-15, 1995. Although these drifters moved
towards the southwest with speeds of 15-20 cm/s on the 18th in response
to a short (1-day) but strong (0.4 N/m^2) southwest wind event, all five
drifters remained within the 60-m isobath for the next 9 days (Figure
A1). Sustained winds (0.2-0.4 N/m^2) towards the southeast during the
24-30th drove all the drifters southward at speeds of 5-15 cm/s, with
three drifters crossing the 60-m isobath on the southern flank.  During
the next five days, the wind stress was more variable in direction and
the drifters moved generally westward along the southern flank. A gale
hit the bank on Febuary 4, with sustained winds (0.6 N/m^2) towards the
east- southeast for the next 4 days.  The drifters responsed quickly and
moved towards the east and then turning toward south at speeds of 18-28
cm/s. By the 9th, four of the five drifters had moved to or crossed the
100-m isobath. The drifters continued to be driven southward into deeper
water over the next four days by two short but strong eastward wind
events (0.2-0.4 N/m^2) with the result that four became entrained into a
Gulf Stream warm-core ring by Febuary 14, with the fifth drifter
following the next day (Figure A2).

This example demonstrates one response of 10-m drifters over the bank to
strong wind forcing during very weak stratification.  In particular, the
drifters tended to move down wind initially and then veer clockwise
during very strong, persistent winds. The near-surface water surrounding
the drifters must have moved southward too, so that there must be on-
bank flow on the northern side of the bank by continuity.  While other
examples of similar wind-forced drifter motion will be described, this
is the only example during the GLOBEC field program that we observed all
drifters located within the 60-m isobath being driven southward and off
the bank in roughly 20 days.  We think this is due to the rapid
succession of strong east to southeast wind events over this 20-day
period.  None of the winter storms observed in GLOBEC were able
individually to drive a drifter from the crest of the bank to off the
southern flank.  The succesion of storms in January-February 1995
provided sufficiently persistant and strong east to southeast winds to
move these five drifters off the bank.

Fig. A1. Drifter tracks and wind stress for January 20 - February 9,
1995. Each panel shows the tracks of five drifters over a 5-day period,
starting on January 20.  The 5-day average wind stress vector is shown
in the upper right of each panel. An eastward 0.2 N/m^2 wind stress
vector is shown in the lower righr corner for comparison. [gb_plotA.m]

Fig. A2.  The track of drifter 5 for the period January 15 to February
16, when it has been entrained into a warm-core ring. The 2-day vector-
averaged wind stress is plotted every two days along the track.  An
eastward 0.2 N/m^2 wind stress vector is shown in the upper right for
comparison. [gb_plot_track(5,15,47,1995).m]