US GLOBEC: Frontal Exchange Processes over Eastern Georges Bank > Background Logos

US GLOBEC: Frontal Exchange Processes over Eastern Georges Bank

Figure 1- Norhteast Peak mooring locations.
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