Slope Water Interaction Workshop

23-24 August, 2000

Clark Laboratory, Woods Hole Oceanographic Institution

Abstract

• Introduction
• Basic Phenmenon
  • May 1999 Slope Water Intrusion (AVHRR)
  • June 1997 Gulf Stream Intrusion (AVHRR)
  • May 1997 Detrainemnt Event (AVHRR)
  • Eddy-Continental Shelf Interaction (Model Results)
  • April 1999 Scotian Shelf Crossover Event (AVHRR)

Overviews

• 1995 Events
• 1997 Events
• 1997 Cruises
• 1999 Events
• 1999 Cruises

Observations and Models

• BIO Moorings
  • June 1997 R/V Parizeau Cruise Track
  • 1997 Southeast Flank Mooring
  • 1999 Northeast Channel Mooring
• Great South Channel Mooring Deployment Cruises
  • Mooring Positions
  • Jan 1997 (OC 296) CTD Stations
  • Aug 1997 (OC 311) CTD Stations
• Eddy Interactions with the Continental Shelf- Model Results
  • No Background Flow
  • Background Shear Flow
• May-June 1997 Detrainment
  • EN301 Cruise Track with AVHRR
• March 1997 SeaSoar- Storm Induced Exchange and Scotian Shelf Crossover
  • Pre-Storm AVHRR
  • Post-Storm AVHRR
  • Survey Chart
• June-July 1997 SeaSoar- Gulf Stream Plume Interactions
  • Survey Chart
  • Early AVHRR (streamer)
  • Mid AVHRR (streamer)
  • Late AVHRR (tendril)
• 1995-1999 Remote Sensing

2. Basic Phenomena

The Gulf Stream is a highly unstable, nonlinear western boundary current that begins to meander downstream of Cape Hatteras. This meandering grows in amplitude and complexity as the Gulf Stream flows eastward, and forms cyclonic and anticyclonic eddies east of about 70W. The anticyclonic eddies, formed from sharp northward looping meanders, contain warm Sargasso Sea water in their centers and are called "warm- core rings" (WCR's). Prior work has shown that some WCR's can push relatively warm, salty Slope Water onto the southern flank of Georges Bank (an "intrusion") and pull generally cooler, fresher shelf water off the Bank (a "detrainment"). While the surface temperature contrast between Bank and offbank waters is generally sufficient to allow detection of these features in AVHRR imagery, in-situ salinity measurements are needed to identify Slope Water intrusions.

Both WCR intrusion and detrainment events were observed during the 1995-1999 Georges Bank GLOBEC field study. Several examples will be presented next for illustration. A Slope Water intrusion onto the outer southern flank started about in mid-May 1999, as a very large eddy moved northward towards the Bank (Figure 1). In late June 1997, the Gulf Stream made a northward meander that turned eastward just south of the Bank (Figure 2). As this meander evolved and formed an intense WCR, very warm, salty water of nearly Gulf Stream origin (with S > 35.5) moved over the southern flank, reaching almost the 60-m isobath.

These two intrusions caused significant changes in the physical environment and distributions of target species southern flank that were captured in the GLOBEC in-situ measurements. A fundamental question which we hope to address is whether these and similar intrusions cause any net change in the biology of the Bank, i.e., are warm-water species brought onto the Bank and added to the Bank ecosystem when the intruded water is carried away from the southern flank. Intrusions were observed in 1995, 1997, and 1999; even though WCRs occurred in 1996 and 1998, the long-term mooring data at the southern flank (SF) site indicates that no intrusions occurred in these two years.

Detrainment events were observed in all five GLOBEC years. In early May 1997, a large WCR and Gulf Stream meander combined to carry a "streamer" of cold shelf water far off the Bank towards the southeast (Figure 3). The streamer was at least 200 km long by May 13, and the WCR moved slowly towards the west, continuing to detrain shelf water. One cruise sampled this detrainment event towards the end of its evolution. Detrainment clearly removes Bank water from the Bank, thus acting as a sink for Bank water and animals. How important are detrainment events to the mass balance and the population dynamics of the Bank remains key questions to be addressed with GLOBEC observations. A simple animation of the ability of a discrete vortex to carry water off the shelf is shown in Figure 4. Note in this animation, the water found in the nose of the detrained water originates to the west of the plume.

Bisagni et al (1996) showed that intrusions of cold, fresh Scotian Shelf Water can occur over the eastern end of Georges Bank. These intrusions, a result of the transport of water from the flanks of Browns Bank across the Northeast Channel onto Georges Bank ("cross-over" events), were first clearly recognized in AVHRR and shipboard measurements made during 1992, became a focus of later GLOBEC work, especially during the 1999 field year. A strong cross-over event occurred in late April 1999, when a band of very cold Scotian Shelf water can be seen extending from Browns Bank southwestward over the eastern end of Georges Bank (Figure 5). Preliminary analysis of moored and AVHRR data suggests that cross- overs occurred in all five GLOBEC years.