Cross-Frontal Exchange and Scotian Shelf Cross-over Workshops

10-12 October 2000

Holiday Inn, Falmouth, Massachusetts


Cover Page


Cross-Frontal Exchange

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

Scotian Shelf Cross-over

Presentations Discussion Questions Synthesis Topics Appendix



A workshop on Cross-Frontal Exchange was held jointly with a workshop on Scotian Shelf Crossovers from 10-12 October 2000 at the Holiday Inn in Falmouth, Massachusetts. These workshops were sponsored by the U.S. GLOBEC Northwest Atlantic/Georges Bank Program. The Cross-Frontal Exchange workshop was convened by Karen Wishner, Jim Ledwell, and Bob Houghton. The Crossover workshop was convened by Peter Smith, Dave Townsend and Dave Mountain. Chip Clancy and Bob Groman helped tremendously with organization.

The primary motivation for these meetings was to inform all interested GLOBEC investigators of progress to date in the individual projects and to exchange information and data related to future collaboration and synthesis of results. The first day was devoted to discussions of exchange across the tidal mixing front, especially as revealed by the 1999 field work. As an aside, Michael St. John, from the Danish Institute for Fisheries Research, summarized a new study of frontal exchange and recruitment success that will be undertaken by the European Community in the North Sea over the next 3 years. It is called LIFECO: Linking Hydrographic Frontal activity and Ecosystem Dynamics in the North Sea & Skagerrak: Impact on Fish Stock Recruitment. The second day of the workshop was devoted to a discussion of Scotian Shelf crossover events, both in 1999 and in previous years. The third day was devoted to a summary and to future directions, especially to a discussion of specific topics to be addressed in the synthesis phase of the Northwest Atlantic GLOBEC program.

Overview of Day One: Cross-Frontal Exchange

The first day of the workshops, dedicated to cross-frontal exchange began with overviews of field work. Jack Barth summarized results from the SeaSoar and the URI neutrally buoyant float. Bob Houghton summarized the various dye and drogue studies that had been done. Karen Wishner summarized process studies of biological/physical interactions and Lou Incze summarized process studies of fish larvae and zooplankton. Later in the day, Ron Schlitz summarized the current meter observations on the south flank.

After the morning overviews, several questions about cross frontal exchange were discussed. Jim Bisagni led a discussion on the definition of the tidal mixing front. He showed satellite observations, criteria for locating the front from space, and listed sources of data available to GLOBEC investigators. Bob Houghton showed how the front looks to a tow-yowed vehicle. It was pointed out that it would be advantageous to all to have a compilation of all the observations of the front during the 1999 effort. Dave Townsend later offered definition of the front in terms of primary productivity.

Jim Ledwell led a discussion of the influence of air-sea forcing on transport across the front. Jim Churchill showed surface drogues driven by easterly winds across the front on the south flank of the bank. There are many such wind events during the year, but it is not clear that any important species would be driven across the front in the surface layer.

Bob Houghton then led a discussion of the secondary circulation and isopycnal mixing, showing the transport of dye in the bottom boundary layer across the front and onto the bank. Ledwell showed horizontal spreading of a dye patch offshore of the tidal mixing front. Charles Hannah showed that at times neither secondary circulation nor isopycnal mixing is required for material to cross the front but rather that water regularly crosses the front along the north flank, especially during seasons of weak stratification on the north flank.

Overview of Day Two: Scotian Shelf Cross-overs (SSC)

The first portion of the second day features a series of presentations related to observations and modeling of the cross-over phenomenon. Peter Smith presented 1999 observations from moorings, surface drifters and satellite SST; then Charles Hannah followed with a lucid interpretation of cross-overs in terms of the background seasonal circulation fields from numerical model results. Charlie Flagg presented results from the Northern Flank and Northeast Peak moorings, and Bob Beardsley examined evidence for wind forcing of cross-overs. Rounding out the physical oceanography, Jim Bisagni discussed the seasonal SST frontal climatologies on Georges from satellite imagery; Bob Houghton raised an issue about reconciling observations of the salinity distribution on the cap of the Bank with SSC events; and Andy Dale highlighted modifications of the internal wave climate on the northern flank caused by SSC.

Shifting the focus to biological impacts of SSC, Karen Wishner examined the abundances of both total Calanus and particular life stages, within and outside of cross-over waters sampled in 1997 and ’99. Diane Gifford then compared growth rates of Calanus nauplii in the same water masses and examined various factors (e.g. temperature) affecting growth. Maureen Taylor presented evidence for cross-overs during 1997-’99 based on the surface salinity fields from the broadscale cruises, and Ann Bucklin and Dennis McGillicuddy showed compelling evidence for the episodic delivery of P. Newmani to the Bank via cross-over events, in contrast to the more stable advective life history of indigenous P. moultoni.

The latter part of the Day Two was devoted to discussion of four particular questions/topics related to SSC:

#1. What is the biological definition of a Scotian Shelf Cross-over?

#2. What is the SST signature of a Scotian Shelf Cross-over?

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

#4. What are the impacts of Scotian Shelf Cross-overs on the target species in the Georges Bank ecosystem?

Overview of Day Three: Synthesis

The first task of the morning was to construct a time line of observations and recognized cross-over events during the 1997 and 1999 field years. Observations included broadscale, process, mooring and NMFS monitoring cruises, whereas cross-over events were identified in salinity time series from moored instruments and particular events in the surface drifter and SST data.

The remainder of the morning was devoted to further discussion of synthesis topics related to the following questions:

1) What drives the interannual variability of Scotian Shelf cross overs? What variables have reflect such variations and how do they co vary with other parameters?. What indications are there for local or remote forcing of Scotian Shelf cross overs?

2) What is the impact of the Scotian Shelf cross overs on the target species within the Georges Bank ecosystem?

3) In the tidal mixing zone, what is the role of cross frontal exchange in terms of the distribution, abundance, and rate processes affecting the target species on the Bank? This applies both to the role of the exchange in maintaining the Bank's ecosystem as well as to perturbations of the system.

The workshops adjourned shortly after midday.