US GLOBEC - Eastern Front News

| ## News From the Eastern Front Peter C. Smith, and Don Lawrence / Bedford Institute of Oceanography Richard Limeburner, and Bob Beardsley / Woods Hole Oceanographic Institution Canadian drifters are invading the eastern front of Georges Bank at an unprecedented rate this year. These drifters are drogued at 10 m and tracked by satellite. They are being launched in sets of three from the southern end of Browns Bank on a roughly biweekly schedule. The first set of two (one failed) was deployed on Jan. 28 and, following conventional wisdom, hiked northwestward past Yarmouth, N.S., and into the northern Gulf of Maine (Fig. 1a). Ten days later (Feb. 8, Fig.1b), a trio set out from roughly the same locations and two shot directly across the Northeast Channel (NEC), while the third "wimped out" and went east. One of the two invaders has since turned tail also, but the first penetrated to the heart of Georges Bank (60-m isobath) !! Figure 1: 1/28 and 2/8 Deployments Figure 2: 2/12 and 2/13 Deployments During the BIO cruise to the area that same week, three drifters were set across the density front associated with a Slope/Gulf Stream intrusion on the eastern side of the NEC. They immediately jumped across the Channel, then were drawn off into the slope water as the front receded (Feb. 12, Fig. 2a). One day later (Feb 13, Fig. 2b), two more drifters set along the northeast flank demonstrated their independence by splitting into Georges Bank and Basin. Again the Bank drifter crossed the 60-m isobath. Finally, a deployment on Feb. 26 (Fig. 3b) has sent another envoy across the NEC to Georges Bank, where it is gliding along the southern flank. The other two drifters appear to be in danger of being drawn offshore by a warm-core ring. Here too, the contrast with conventional wisdom (Jan 28 deployment, Fig. 3a) is striking. Figure 3: 1/28 and 2/26 Deployments Given this remarkable set of data, we are left with important questions, such as: What should be our "conventional view" of the Lagrangian circulation in this area and time of year ?? Are Scotian Shelf cross-overs the norm in winter / spring, or is this year anomalous ?? At least two facts argue for the latter option: Historical hydrographic data from Browns Bank and the Northeast Peak of Georges Bank clearly show a surface salinity contrast of order 1 psu between October and May. This difference could not be maintained if significant volumes of SSW crossed directly to Georges in this period. Satellite SST imagery has clearly shown a sequence of SSW cross-over phenomena this season, starting with a prolonged event from mid-November to mid-December. Thus it appears that we are experiencing a season of anomalous exchanges on the eastern front. What an opportune time to be studying them. Last updated: 26 March, 1999 Please address any comments, suggestions or questions to: GLOBEC Webmaster