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Long-Term Observations on the Southern Flank of Georges Bank. Part I: A Description of the Seasonal Cycle of Currents, Temperature, Stratification, and Wind Stress

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  • 1 U.S. Geological Survey, Woods Hole, MA 02543
  • | 2 Woods Hole Oceanographic Institution, Woods Hole, MA 02543
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Abstract

Nearly continuous current measurements at 45 and 75 m were made from May 1975 to March 1979 at 40°51′N, 67°24′W on the southern flank of Georges Bank in water 85 m deep. Measurements at 15 and 84 m were made less often. The mean flow at 45 and 75 m was southwestward at approximately 8.5 and 3.7 em s−1, respectively. At 45 m the monthly along-bank flow ranged from 2 to 17 cm s−1, and the average seasonal change was approximately 6 cm s−1; strongest southwestward flow was in September and weakest flow was in March. Most of this seasonal change was driven by the seasonal change in the cross-bank density field. At 75 m there was no significant seasonal change in the monthly mean along-bank flow. In winter, only about 21 percent of the along-bank flow at 45 m can be explained by tidal rectification, the density field, and wind stress. In contrasts, in late summer almost all of the flow at 45 m can be explained by these three driving mechanisms. The monthly averaged cross-bank flow was very weak, confidence limits were too large to determine any statistically significant vertical shear in the seasonal mean cross-bank flow. The current observations on the southern flank and additional measurements made at other locations around the perimeter of Georges Bank suggest that, although a monthly mean subsurface clockwise circulation around the bank exists throughout the year, the flow was strongest in late summer and early fall and that recirculation around Georges Bank may be most likely in late summer. The flow was weakest and most variable in winter.

Abstract

Nearly continuous current measurements at 45 and 75 m were made from May 1975 to March 1979 at 40°51′N, 67°24′W on the southern flank of Georges Bank in water 85 m deep. Measurements at 15 and 84 m were made less often. The mean flow at 45 and 75 m was southwestward at approximately 8.5 and 3.7 em s−1, respectively. At 45 m the monthly along-bank flow ranged from 2 to 17 cm s−1, and the average seasonal change was approximately 6 cm s−1; strongest southwestward flow was in September and weakest flow was in March. Most of this seasonal change was driven by the seasonal change in the cross-bank density field. At 75 m there was no significant seasonal change in the monthly mean along-bank flow. In winter, only about 21 percent of the along-bank flow at 45 m can be explained by tidal rectification, the density field, and wind stress. In contrasts, in late summer almost all of the flow at 45 m can be explained by these three driving mechanisms. The monthly averaged cross-bank flow was very weak, confidence limits were too large to determine any statistically significant vertical shear in the seasonal mean cross-bank flow. The current observations on the southern flank and additional measurements made at other locations around the perimeter of Georges Bank suggest that, although a monthly mean subsurface clockwise circulation around the bank exists throughout the year, the flow was strongest in late summer and early fall and that recirculation around Georges Bank may be most likely in late summer. The flow was weakest and most variable in winter.

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