Frontal hydrography of the northern and southern flanks of Georges Bank, March-April and June 1999: Preliminary results of R/V Endeavor Cruises 321 and 325

I. Belkin, D. Gifford, M. Sieracki, and K. Wishner

The primary goal of hydrophysical program carried out during R/V Endeavor cruises 321 and 325 was to study the frontal structure of, respectively, the northern and southern flanks of Georges Bank. Both regions share some very important similarities, first of all, strong tidal currents accountable for the maintenance of tidal mixing fronts surrounding the on-Bank's well-mixed waters.

Northern Flank (R/V Endeavor Cruise 321, March 27-April 13, 1999)

The northern flank of Georges Bank features a complex frontal system including the tidal mixing front (TMF) and the shelf-slope front (SSF). Our goal was: (1) to map SSF centered around the 150-160 m isobath; (2) to map TMF located on the Bank; (3) to sample the Scotian Shelf water's cold plume identified from satellite SST imagery. Our primary goal was to resolve TMF and SSF using high-resolution CTD sections and Scanfish tows, and to determine the excursion ranges of the fronts’ movements caused by the tide.

Main Results and Conclusions:

  1. There are two distinct fronts along the northern side of Georges Bank, the shelf-slope front (SSF) and the tidal mixing front (TMF). The SSF is associated with a vertical structure change from the relatively uniform or monotonous structure typical of the Bank, to the strongly stratified water column of the Gulf of Maine, with non-monotonous vertical distributions of T and S. The SSF is centered over the 160-m isobath that corresponds to the mid-slope of the Bank (the slope encompasses the 80-240 m depth range). The TMF separates the completely mixed waters of the Bank’s proper from the moderately stratified waters of the Bank’s periphery.

  2. Locations of both fronts vary, depending on the tidal cycle. The dominant semi-diurnal tide moves both fronts in the cross-shelf break direction. Because the shelf break and slope within our study area extend almost strictly zonally, the fronts move in the meridional direction (north-south). During the off-bank slack tide, the TMF joins the SSF to form a single front, located a few miles off-bank of the mid-slope. During the on-bank slack tide, both fronts shift accordingly. The SSF, however, never moves onto the Bank, being centered over the mid-slope or upper slope, the difference between these two is just about 1 mile. On the contrary, the TMF excursion range is many times larger than the SF's one. We determined the TMF excursion range to be about 8 miles (~15 km). Consequently, during the on-bank slack tide, a 5-to-7-mile wide zone with moderately developed monotonous stratification is observed between the TMF and SSF.

  3. The water column of the off-Bank Gulf of Maine features a deep layer of very warm and salty water (Scotian Slope water, SSW), with T up to 10°C and more, and S up to 34.82 ppt, centered at the ~150-180-m depth. The interface between the SSW and the upper layer is sometimes extremely sharp, with vertical salinity gradients up to 0.1 ppt/m. In most cases, staircase structures in T and S were distinct above the SSW layer, indicative of the double-diffusion process at work. The SSW core depth varies widely in space and time. The warm layer appears to slosh on-bank/off-bank in sync with the dominant semi-diurnal tide.

  4. The Scotian Shelf water's cold plume exerts strong influence on the hydrography of the Bank’s northern side, especially on the salinity (hence, density) field of the upper 50-m layer. In the winter of 1999, the cold plume was a persistent feature from the Scotian Shelf across Browns Bank and Northeast Channel up to the Northeast Peak. The Plume's T (~3.0°C) and S (~31.6 ppt) were much lower than the ones of the ambient waters. The Plume's proper (~ 50 m thick) was surrounded by a thin (~10 m thick) fringe, with T=3.0-4.0°C and S=31.6-32.0 ppt. The front was visible as a change in the sea roughness, with the noticeably reduced roughness within the plume.

Southern Flank (R/V Endeavor Cruise 325, June 13-30, 1999)

The southern flank of Georges Bank features two major fronts, the tidal mixing front (TMF) near the 60-m isobath, and the shelf-slope front (SSF) or shelf break front, usually located offshore of the 80-100-m isobaths. Our study was focused on the TMF. The primary goals were: (1) to investigate the TMF’s temporal variability due to the dominant semidiurnal tide, along a fixed line; in particular, the tidal excursion range of the TMF has had to be determined; (2) to resolve the fine cross-frontal structure of the TMF and its environ, using cross-frontal Scanfish tows; (3) to map the TMF along the entire southern flank of Georges Bank using high-resolution CTD sections and Scanfish tows.

Main Results and Conclusions

  1. The TMF location along the southern flank of Georges Bank is topographically controlled, being fairly close to the 55-60 m isobaths. The TMF has surface manifestations in T, S, and the sea surface roughness; the along-front convergence was always distinct, with a single or multiple line(s) of foam, streaks of slicks aligned with the front, and bands or patches of sea weeds.

  2. During several crossings, a well-defined T-min in the SST field was observed that corresponded to the TMF. The magnitude of the T-min sometimes reached 2°C.

  3. The northern edge of TMF appears as a very sharp interface between the completely vertically homogeneous (non-stratified) waters of the Bank’s proper and the moderately stratified waters of the Bank’s periphery. While crossing this discontinuity in the off-Bank direction, the onset of stratification occurs within a 400-500-m distance. A spatial offset between the above-mentioned interface and the surface manifestation of TMF in the SST and SSS fields was observed.

  4. An on-Bank outbreak of the warm, salty Slope Water (T=16-18°C, S>34 ppt) was surveyed that apparently resulted from strong interaction of two warm-core Gulf Stream rings immediately south of the Bank's proper.

  5. Comparison of the tidal prediction model’s output with the shipboard ADCP current measurements has shown that the model can and should be used as an operational tool in planning and execution of hydrographic surveys in the tidal environment of Georges Bank.