Drifter Event F

On June 2, 1997, a second strong isolated storm passed to the south of
Georges Bank, bringing moderate winds to the southwest (0.1-0.2 N/m^2)
until the 5th when the winds veered to south and the stress increased to
0.4-0.8 N/m^2 for one day.  The winds then relaxed back to 0.1-0.2 N/m^2
towards the southsouthwest for the next 3 days.

Ten drifters were on the bank prior to the storm. During the storm,
these drifters continued to move generally towards the southwest along
the local topography, but with differing speeds (Figure F1).  The three
drifters on the eastern flank moved southwest with mean speeds of only
~7-9 cm/s (peak speeds ~18-23 cm/s), while the two drifters at the
eastern end of the southern flank moved more westward at a mean speed of
~15 cm/s (peak speed ~26-28 cm/s).  The three more southern drifters on
the southern flank moved along the shelf more rapidly, with mean speeds
of ~17-31 cm/s (peak speeds ~28-45 cm/s). The two drifters on the crest
moved slowly west. (The path of the one drifter caught in an eddy in the
Northeast Channel is intriguing. A south-southwest wind stress should
cause flow into the Gulf, with perhaps a compensating outflow through
the Northeast Channel. The rapid advection of the drifter out the
channel along its southern side may reflect such a flow.--pure
speculation)

During June 7-9, despite moderate winds (0.1-0.2 N/m^2) towards the
south, all drifters along the southern flank basically stopped, and some
even began to move south and east. Why this occurred is unclear.
Starting first on the eastern flank, these drifters start to move
southwestward along the southern flank.  The one drifter south of the
Great South Channel on June 9 remained in that area for more than 10
days before moving west.

The flow response to this southward wind stress event differs in several
aspects from that observed during the March 31 storm.  First, this
second storm was weaker, and the near-surface wind-driven flow was
weaker and only marginally coherent over the bank.  Second, the mean
wind stress during this second storm was oriented more to the west than
the March 31 storm, which may explain why seven of the eight drifters on
the southern flank moved onbank to shallower water during the storm.

Six of the southern flank drifters were working for the 17-day period
prior to the storm when there was little wind forcing.  Their paths
(panel 4, Figure F1) exhibit the classic around-bank flow, with mean
speeds varying from ~8 cm/s for the shallowest drifter on the Northeast
Peak to up to ~15-20 cm/s for the more offbank drifters. Note that four
of these drifters move off to close to the 2000-m isobath, yet remain
part of the shelf flow.  The three drifters that make the clockwise arc
offbank near 67W are thought to be part of the shelfbreak jet and
frontal system.

What happened during June 7-9 to stop the westward flow along the
southern flank of the bank remains unknown.  It is also unclear why the
around-bank flow appeared to return to normal first on the eastern end
of the bank and last south of the Great South Channel.

Figure F1. Drifter tracks and wind stress for May 29 -June 13, 1997.
Each panel shows drifter tracks over a 5-day period, starting on May 29.
The 5-day average wind stress vector is shown in the upper right of each
panel, and a 0.2 N/m^2 wind stress scale vector is shown in the lower
right for comparison.  The lower right panel shows the trajectories of
six of the drifters shown in the other panels for the period May 16-June
11.

[use gb_plotF to make figure]