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Figure 1- Norhteast Peak mooring locations.
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Principal Investigators
Robert Beardsley, Charles Flagg, Peter Smith and Richard Limeburner
Brief Outline
The Northeast Peak field program studied frontal exchange
processes over the eastern flank of Georges Bank and the exchange of
water from Browns Bank to Georges Bank. The field program featured
two main components, moored arrays deployed by WHOI and Brookhaven NAtional Laboratory (BNL)
investigators on Georges Bank and by Bedford Institute of Oceanography (BIO) investigators on Browns Bank
and in the Northeast Channel, and the release of satellite-tracked
surface drifters over the southeastern flank of Browns Bank. In
addition, a synoptic hydrographic survey was conducted during a
'cross-over' event.
The WHOI and BNL array consisted of surface and subsurface moorings
deployed at three sites on the eastern flank of Georges Bank
(Northeast Flank Deep, NFD; Northeast Flank Shallow, NFS; Eastern
Flank, EF) in November 1998. The NFS and EF surface buoys dragged
during high NW winds and seas in January, and were replaced with
larger surface buoys with heavier mooring chain and anchors. The NFD
surface buoy was found adrift and replaced, and the instrumentation
deployed under the first NFD buoy was recovered by dragging. All
WHOI and BNL moorings were successfully recovered in August 1999.
The BIO array consisted of surface and subsurface moorings deployed
on Browns Bank and on both sides of the Northeast Channel. These
moorings were set in fall 1998 and successfully recovered in
September 1999.
A total of 26 WOCE-style surface drifters with drogues centered at 15
m were released on the southeastern flank of Browns Bank. Some of
these drifters moved around Browns Bank and entered the Gulf of
Maine, but several drifters moved directly across the Northeast
Channel and onto Georges Bank, indicative of Scotian Shelf Water
'cross-over' events, while other drifters were entrained into the
Slope Water by Gulf Stream warm-core rings and eddies.
Preliminary processing of the moored array, drifter, and hydrographic
data has been completed and posted on the primary GLOBEC website, and
some initial results will be published shortly in a special section
in the Journal of Geophysical Research - Oceans devoted to the
physical oceanography of Georges Bank.
This program provided direct support and data for two postdoctoral
investigators and one undergraduate. It also supported collaboration
with other GLOBEC investigators in three primary ways: helping
organize and document open program workshops on specific processes
critical to Georges Bank; ongoing studies with Changsheng Chen
involving numerical circulation model studies of physical processes;
and funding participation on the national U.S. GLOBEC scientific
steering committee and its executive committee.
The NEP mooring deployment times are shown below.
Major Findings
1. Scotian Shelf Water cross-over (SSC) events appear to be
associated with circulation anomalies (e.g., Gulf Stream rings) in
the Slope Water, which influence the near- surface circulation in and
around the Northeast Channel (NEC)• Wind effects do not appear to
play a major role in SSC dynamics.
2. 3-D seasonal mean circulation models indicate (a) a direct near-
surface drift pathway from outer Browns Bank to Georges Bank, but
particles tend to remain outside the 100-m isobath on Georges Bank,
and (b) closed near-surface streamlines around Georges Bank, Georges
Basin, and Browns Bank, indicative of a sensitive 'saddle point' in
the NEC. Intermittent perturbations of the saddle point by
circulation anomalies can lead to significant SSCs, i.e. events where
the Scotian Shelf Water reaches Northeast Peak
3. Pathways for SSCs are highly variable as evidenced by drifter
paths and moored records. The dominant pathway for low salinity
Scotian Shelf waters to impact Georges Bank is directly across the
Northeast Channel. An additional pathway onto the Bank is from the
north after the cyclonic circuit of Georges Basin but the impact is
muted, perhaps as a result of the added distance traveled and mixing
with ambient waters.
4. Cross-over plume depths are typically more than 15-m deep,
sometimes extending more than 35 m. The duration of intrusion events
onto the Northeast Peak observed during 1999 ranged from several days
to one month. The spatial extent varied widely but on at least one
occasion the SSC plume occupied an area of more than 75 km x 75 km
with a vertical extent of 15 to 20 m so that at least 110 km^3 of
water on the Northeast Peak were dramatically affected by the influx
of water with Scotian Shelf characteristics.
To read the full proposal (PDF format), click here.
This project is funded by the National Science Foundation.
Website Updated February 7, 2007. R. Limeburner
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