Project Summary

Robert Beardsley, Richard Limeburner, Charlie Flagg, Peter Smith, Charles Hannah, John Loder

U.S. GLOBEC: Frontal Exchange Processes over Eastern Georges Bank

I. Introduction:

This project is designed to focus on frontal processes which occur over eastern Georges Bank in winter through spring. In particular, we want to examine (a) the flow of water and biota onto the Bank along the northeastern flank driven by the strong local semidiurnal tidal currents (which can reach ~1.5 m/s during spring tide), winter storms, and the seasonally-evolving density field, and (b) the advection of Scotian Shelf water across the Northeast Channel onto the Bank associated with the shelf/slope front and episodic events such as winds and buoyancy pulses. While pre-GLOBEC work identified the Northeast Peak as the dominant site for cod and haddock egg production during the January--April period, recent GLOBEC work suggests that the Gulf of Maine is the prime source for Calanus finmarchicus which reach the Bank via the northeastern flank, and that both copepods and cod and haddock larvae from Browns Bank can reach the Bank via Scotian Shelf "cross-over" events. Thus, understanding the physical processes which establish the frontal structures over the eastern flank and their influences on the distribution and behavior of the target species is important to achieving the overall goals of the Georges Bank Study.

II. Proposed Research:

We propose both field work and modeling. The field work includes a moored array deployment, the release of satellite-tracked drifters, and one rapid response survey of a cross-over. The model effort includes both process-oriented studies and construction of circulation fields for 1999 using data assimilation. These components will be described next.

A. Moored Array

The revised moored array combines WHOI and BNL moorings on Georges Bank and BIO moorings on Browns Bank and in Northeast Channel. In particular, the WHOI and BNL moorings are designed to investigate frontal processes with high vertical resolution over the northern flank of Georges Bank near 66.5 W and monitor the upper ocean currents and water properties over the eastern flank. The northern flank moorings (NFS,NFD) form a cross-isobath transect spanning the 90-m to 150-m isobaths, along which other GLOBEC investigators plan to make physical and biological measurements during late winter/early spring. The NFD site includes a bottom-mounted narrow-band ADCP, set to profile current and zooplankton biomass, and a vertical array of SeaCat temperature and conductivity (salinity) recorders. The NFS site features a wind recorder and a vertical array of Vector-Measuring Current Meters (VMCMs), SeaCats, and temperature recorders (T-pods) to obtain high-quality current and water property measurements with good vertical resolution throughout the water column. The eastern flank site (EF) supports several VMCMs, SeaCats, and T-pods to observe flows in the upper 40-m over the outer shelf. All sites feature a near-surface (2-m) SeaCat to detect the low salinity signature of Scotian Shelf water. This array combined with the long-term Northeast Peak mooring (NEP) (to be deployed by J. Irish) should determine when and where SSW crosses onto Georges Bank, and whether it penetrates to mid-shelf over the eastern Bank.

The BIO moorings are designed to monitor the source region for SSW cross-overs along the southwest flank of Browns Bank and in Northeast Channel. The bank moorings (BBI, BBO) are located along the 120-m isobath at the edge of Browns Bank, near the axis of the Scotian Shelf water that flows into the Gulf of Maine. These sites feature four Aanderaa current meters (AaCMs) at depths of 10, 30, 50, and 100-m. The channel moorings (NECE, NECW) are located at the sites of earlier long-term moorings designed to monitor in- and outflows to the Gulf on the eastern and western sides, respectively. These moorings support five AaCMs at depths of 10, 30, 50, 100, and 150-m, as well as a near-surface SeaCat. The subarray of BBO, BBI, and NECE is intended to provide evidence of cross- and along-channel frontal circulation at scales (10-25km) consistent with those of mesoscale features found in SST images of cross-over events. The western channel mooring (NECW) is placed to monitor first arrivals of SSW surface water on Georges Bank.

The BIO moorings will be deployed in September 1998 and recovered in June 1999 using DFO research vessels. The WHOI and BNL moorings will be deployed in late November 1998 and recovered in August 1999 using Oceanus or Endeavor.

B. Drifters

We propose to release 3 satellite-tracked drifters drogued at 10-m bimonthly in a cross-isobath line over the southeastern flank of Browns Bank between the 80-120m isobaths, the region hypothesized as the upstream source of SSW cross-over water. We propose a total of 7 deployments of 3 drifters bimonthly during the four-month January-April 1999 period. These deployments should provide a detailed characterization of the various pathways for near-surface SSW from this upstream source region and help determine to what degree the transport of SSW to Georges Bank during late winter/early spring is episodic and/or weak but persistent.

C. Rapid-reponse Survey

Because of the infrequent occurrence and variable duration of SSW cross-overs, a strategy must be developed to detect their initial stages on Georges Bank and to mount rapid surveys to explore their full impact in a timely fashion. Fortunately, SSW is distinctly fresher and cooler in winter and spring than normal bank water. Past experience suggests that real-time monitoring of the near-surface T-S properties at the NECW and EF mooring sites should provide about 5-7 days of lead time needed to organize a "rapid response" survey of the more mature stages of a cross-over on the bank.

We plan to transmit the 2-m SeaCat T-S data via ARGOS to WHOI/BIO and when alerted by this and/or AVHRR SST data (from J. Bisagni), use a Canadian Coast Guard search-and-rescue (SAR) vessel and the BIO Moving Vessel Profiler (MVP) to map the SSW cross-over in progress over the bank and seed the core of the cross-over with satellite-tracked near-surface drifters. The MVP measures temperature, conductivity, fluorescence, and plankton concentration size spectrum versus depth (with a laser optical plankton counter). The drifter trajactories will provide a Lagrangian description of the spatial and temporal evolution of the SSW as the cross-over event progresses.

A short SAR cruise is planned for January 1999 to a) test the MVP system and b) provide initial hydrographic fields from the western Scotian Shelf for the proposed model studies. This cruise will be scheduled to coincide with the January 1999 Georges Bank broad-scale survey, so that we will have (the first) two-ship quasi-synoptic mapping of the northern flank frontal system. The main SAR cruise would occur during the first detected cross-over event in 1999.

D. Modeling

The modeling component will be directed towards an improved description and understanding of the entire Northern Flank Frontal System (NFFS) during winter, spring and early summer. We will evaluate the frontal system as a function of position along the northern and eastern edge of Georges Bank and as a function of season (before and after the onset of stratification). The focus will be on the mechanisms for the transport of water properties, nutrients and biota from the Gulf of Maine and Scotian Shelf to the Northeast Peak and southern flank of Georges Bank during winter-spring.

The modeling will have three primary components: a) analysis and process interpretation of the NFFS using seasonal-mean and tidal circulation fields; b) construction and analysis of circulation fields for the NFFS (including Northeast Channel) representing pre-onset of stratification in 1999; c) process studies of potential dynamical triggers for Scotian Shelf cross-overs, and of the cross-overs' spatial structure and water property fluxes.