Drifter Event E

On March 31, 1997, a strong low pressure system passed to the south of
Georges Bank, bringing winds above 15-17 m/s (0.5-0.8 N/m^2) first
towards the southwest then veering to south for the next three days
through April 3. Winds on the bank were significantly weaker (less than
5-10 m/s) both before and after this storm, so that this strong
sustained southward wind stress appears as an isolated forcing event.

When the storm first struck the bank, 15 drifters were located either on
the bank or in the Gulf of Maine near the northwest flank of the bank
(Figure E1). Prior to the storm, these drifters moved relatively slowly
(with mean speeds less than ~10 cm/s except for one moving with a mean
speed of ~ 17 cm/s on the eastern tip of the bank), and exhibited little
spatial coherence, due in part to the weak wind forcing during this pre-
storm period.

During the four-day storm (March 31 to April 4), all drifters moved
towards the southwest, with mean speeds that varied from a minimum of ~
10 cm/s for the drifter in Wilkenson Basin to a maximum of ~ 35 cm/s for
the drifter that crossed the Great South Channel and moved westward
south of Nanucket Shoals.  This last drifter started in the eastern side
of the Great South Channel, very quickly accelerated to a peak speed of
~73 cm/s towards the southwest on late April 1, then slowed as it turned
west.  The five drifters initially located on the southern and eastern
flank moved along the southern flank at mean speeds ~18 to 34 cm/s, with
peak speeds from ~24 to 57 cm/s.

During the next four days, with little to no wind forcing, the drifters
located along the northeastern and northern flank of the bank reverse
their storm-driven motion and move towards the northeast, in the
direction of the normal clockwise circulation around the bank.  One of
these drifters started on the crest and was carried west into the
current that flows along the northern side of Great South Channel and
clockwise around Little Georges Shoal.  The five drifters on the
southern flank continued to move westward along the shelf but at
different speeds.  The two drifters furthest off the shelf south of the
Great South Channel continued to move rapidly down the shelf, with mean
speeds of ~32 and 38 cm/s and peak speeds of ~52 and 46 cm/s.  The two
drifters near the 100-m isobath moved more slowly, with mean speeds of
~15 and 19 cm/s and peak speeds of ~35 and 36 cm/s.  The one mid-shelf
drifter was even slower (mean speed ~11 cm/s) and started to reverse
direction on April 7.

This example shows that a sufficiently strong and persistent southward
wind stress can drive the near-surface Lagrangian flow towards the
southwest over the entire bank.  In particular, the normally
northeastward along-bank flow on the northern flank can be reversed and
the normally southwest flow on the southern flank can be enhanced.
During this particular storm, the three drifters initially located on
the southern flank were advected west to the New England shelf in
roughly 8 days.  It is not clear why these three drifters continued to
move rapidly after the storm passed, except that it seems likely that
the two outermost drifters became entrained in the shelfbreak jet. [[is
there any satellite avhrr imagery that would confirm that drifter 89 is
still in shelfwater]]

Fig. E1. Drifter tracks and wind stress for March 27 - April 8, 1997.
Each panel shows drifter tracks over a 4-day period, starting on March
27.  The 4-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.

[use gb_plotE to make figure]