Cross-Frontal Exchange and Scotian Shelf Cross-over Workshops

10-12 October, 2000

Holiday Inn, Falmouth, Massachusetts



Cross-Frontal Exchange

Presentations Discussion Topics Upcoming North Sea Study (LIFECO) (St. John)

Scotian Shelf Cross-over

Presentations Discussion Questions Synthesis Topics Appendix


Observations of Scotian Shelf Cross-overs During Winter/Spring, 1999

Peter C. Smith (BIO)
Bob Beardsley, Dick Limeburner, Jim Irish (WHOI)
Charlie Flagg (BNL)

Moored measurements (Fig. A1), drogued (10 m) drifters and satellite-derived Sea Surface Temperature (SST) indicate that the winter/spring season of 1999 was a period of frequent cross-overs of Scotian Shelf waters (SSW) from Browns Bank to the Northeast Peak of Georges. Seasonal mean (Jan.-June) currents in 1999 are deflected westward across Northeast Channel relative to their historical means, particularly near the surface, and near-surface salinities (and temperatures) measured on Georges are often well below normal.

Figure A1. Mooring positions for the Scotian Shelf cross-over array in Northeast Channel and on southwestern Browns and northeastern Georges Banks. Sentinel moorings transmitted near-surface T,S to shore in real-time via ARGOS. Triangles indicate nominal positions for biweekly drifter deployments.

Analysis of historical hydrographic data from the western Scotian Shelf and Gulf of Maine indicates that a climatological salinity contrast exists between the surface waters (0-20 m) on Browns Bank and those on eastern Georges for the period, December to June. [In summer/fall, the contrast disappears because of the arrival of the annual freshwater pulse on the Northeast Peak.] This fact was used to establish a near-surface salinity criterion, S<32.0, that served to indicate the presence of SSW on Northeast Peak. The near-surface records (Fig.A2) clearly depict significant periods of cross-over in Nov/Dec., 1998, and in mid-Feb., mid-Mar., early Apr., late-Apr/early-May, and late-May, 1999. The duration of these events is maximal at the eastern flank (EF) and western Northeast Channel (NECW, not shown) moorings, and minimal at the northern flank shallow site (NFS).

The trajectories of the 10-m drogued drifters, deployed regularly on the southern and western flanks of Browns Bank, show a high degree of variability in drift pathways (Fig. A3), and a significant frequency of cross-overs of near-surface SSW. Overall statistics of the pathways reveal that 13% of the drifters penetrated across the 60-m isobath to the cap region of the Bank, 25% crossed the 100-m isobath to the Northeast Peak region, and 50% crossed the 200-m isobath on the southeastern flank of the Bank, while 40% of the drifters were ultimately entrained into the offshore region. However, closer inspection reveals that only drifters that impinge on the northern flank of the Bank cross the 100-m or 60-m isobaths, thereby tracing surface waters that have a direct impact on the important Bank ecosystems. On the other hand, all of the drifters crossing the 200-m isobath on the southeast flank never crossed the 100-m contour, but were either entrained to the offshore or passed through the system into the Mid-Atlantic Bight. Figure A2 also shows the periods during which drifters deployed on Browns both reached and crossed the northern flank to the Northeast Peak and/or cap regions of Georges. There appears to be a clear correlation with some of the cross-over periods identified in the moored records.

Of the potential dynamical processes driving the cross-over phenomenon, wind forcing does not appear to be the primary player, based on a close inspection of the wind stress records from the Georges Bank met. buoy (440011). However, the proximity of offshore frontal features (e.g. Gulf Stream meanders, Warm Core Rings) to the mouth of Northeast Channel appears to play a role in particular events, such as the cross-overs identified in mid-March and early April. Furthermore, analysis indicates that the northward positions of the Gulf Stream and shelf/slope water front on longitudes 65o and 66oW are positively correlated with the NAO index, with a one- to two-year time lag. This observation may provide a key to the long-term variability in winter/spring Scotian Shelf cross-over events.