Cross-Frontal Exchange and Scotian Shelf Cross-over Workshops

10-12 October, 2000

Holiday Inn, Falmouth, Massachusetts

 


Overview

Cross-Frontal Exchange

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

Scotian Shelf Cross-over

Presentations Discussion Questions Synthesis Topics Appendix

 

Discussion Question #3

#3.     What drives interannual variability in Scotian Shelf Cross-overs?

Facilitator: Peter Smith
Rapporteur: Charles Hannah

Peter Smith suggested three potential physical mechanisms for driving interannual variability in Scotian Shelf cross-overs:

Variability in freshwater flux on the Scotian Shelf,

Variability in the wind forcing, and

Variability in the position and/or intensity of offshore fronts (e.g. the shelf/slope front or Gulf Stream)

There is roughly a 9-11 month lag between the April-May peak in freshwater discharge into the St. Lawrence estuary and the arrival of the resulting salinity pulse off Cape Sable on the Scotian Shelf. Time series of annual mean and springtime (Mar-June) mean runoff for the 1990's indicate that '91,'93,'97,'98 were particularly high runoff years in the March-June period. Thus if high if St. Lawrence river outflow were important, then '92,'94,'98,'99 should be high cross-over years. Bob Houghton questioned which is more important: the change in the freshwater volume flux or the location of the shelf-slope front? He also suggested the need to consider changes in fresh water flux from further north. Missing fresh water flux due to Labrador Current and changes in the water properties on the Labrador Shelf can confound correlations between the St. Lawrence runoff and freshwater flux on the Scotian Shelf.

The winter mean wind stress over the Gulf of Maine is from the NW, which suggests mean Ekman transport would aid SSW surface water cross-over from Browns to Georges Bank. However, Bob Beardsley demonstrated that there were no strong connections between wind events and the currents at the EF mooring (see Presentation D above). Nevertheless, it has been argued that cross-overs are not a change in local circulation, but are due to perturbations in the upstream flow (Charles Hannah), so that the effects of wind forcing may be applied remotely, rather than locally. There is a need to look at this problem more carefully with a realistic circulation model.

It has been suggested that the frequency and strength of cross-overs are enhanced by the proximity of the shelf/slope front and/or Gulf Stream warm-core rings to the shelf break and particularly the mouth of Northeast Channel. In fact, there are documented cases of cross-over events driven by frontal eddies that have penetrated into the Channel (e.g. see Presentation E above). Furthermore, the northward positions of the shelf/slope front and the Gulf Stream on longitudes 65o and 66oW are positively correlated with the NAO index, suggesting a large-scale connection. Dick Limeburner showed movies of 3-day average SST for the period '97-'99. Dick also showed a summary of the locations of rings on a monthly basis for 1997-99, which indicated a high concentration of rings in the slope water, generally closer to the shelfbreak, in 1999. One ring was stationary near mouth of NE Channel between Feb and April, 1999. These data suggest that 1999 had about twice as many rings as 1997 and 1998. Bob Houghton wondered whether salinity on Browns Bank was lower in high cross-over years, but no direct answer was forthcoming.