Near-surface recirculation over Georges Bank Author: Richard Limemurner and Robert C. Beardsley Author: Richard Limemurner and Robert C. Beardsley

Acronyms GOM: Gulf of Maine GOM: Gulf of Maine GB: Georges Bank GB: Georges Bank GSC: Great South Channel (South Channel) GSC: Great South Channel (South Channel) NEC: Northeast Chanel NEC: Northeast Chanel

1 Introduction Position of Georges Bank Position of Georges Bank Clockwise gyre and seasonality Clockwise gyre and seasonality Reason for the general pattern and seasonality Reason for the general pattern and seasonality -- Tides rectification -- topography -- Horizontal pressure gradient vertical stratification vertical stratification Recirculation near the southern flank (main objective of the paper) Recirculation near the southern flank (main objective of the paper) Fig.1 Summer pattern of the circulation near Georges Bank Nantuck et Island Little Georges Shoal

2 Data and methods Experiment: South Channel Ocean Productivity Experiment (SCOPEX) for 1988 and 1989 springtime Experiment: South Channel Ocean Productivity Experiment (SCOPEX) for 1988 and 1989 springtime Instrument: Low-Cost Drifters (LTD) : transmitted location, sea surface temperature, drogue tension and battery status typically 6~8 times per day via ARGOS. Instrument: Low-Cost Drifters (LTD) : transmitted location, sea surface temperature, drogue tension and battery status typically 6~8 times per day via ARGOS. 13 drifters at 5m to track the low-salinity water above the pycnocline 13 drifters at 5m to track the low-salinity water above the pycnocline 6 drifters at 50m to track the cold Maine Intermediate water below seasonal pycnocline. 6 drifters at 50m to track the cold Maine Intermediate water below seasonal pycnocline. Methods: Methods: linearly interpolated into a uniform 6h time-series linearly interpolated into a uniform 6h time-series a parabolic-linear filter with 33h half-amplitude method to eliminate tidal and inertial motions. a parabolic-linear filter with 33h half-amplitude method to eliminate tidal and inertial motions. Ancillary data: Wind data from NOAA environmental buoys near Nantucket Shoals  wind stress time-series using the neutral drag law of Large et al (1981). Ancillary data: Wind data from NOAA environmental buoys near Nantucket Shoals  wind stress time-series using the neutral drag law of Large et al (1981).

3The SCOPEX drifter experiment results — early March 1988 (Late winter) Three drifters: 70,71,72 Three drifters: 70,71,72 No 70: stopped in June No 71,72: Northeast Channel dominant southward fresh water plume was not present over Wilkinson Basin during spring of 1988

3The SCOPEX drifter experiment results — In April (early spring) of surface, 6 deep drifters 10 surface, 6 deep drifters Surface southwestern GSC outflow; Central and Eastern GSC inflow (10cm/s) Surface southwestern GSC outflow; Central and Eastern GSC inflow (10cm/s) Variable cyclonic circulation near Wilkinson Basin Variable cyclonic circulation near Wilkinson Basin deeper drifters deeper drifters slower ; 69°W slower ; 69°W Two : a stagnation region of relatively low velocity before moving towards Georges Bank. Two : a stagnation region of relatively low velocity before moving towards Georges Bank. Three : along the northern flank of the bank and at least one drifter moved eastward to the eastern tip of the bank Three : along the northern flank of the bank and at least one drifter moved eastward to the eastern tip of the bank all of them stopped transmitting before leaving the Northeast Peak region. all of them stopped transmitting before leaving the Northeast Peak region.

3The SCOPEX drifter experiment results — early June near-surface drifters and 6 deep drifters 13 near-surface drifters and 6 deep drifters 7 characteristics  7 characteristics  western GSC outflow western GSC outflow Cape Cod near-shore Cape Cod near-shore Cape Cod offshore Cape Cod offshore northern flank of Georges Bank near 68°W( three paths with two split path at east) northern flank of Georges Bank near 68°W( three paths with two split path at east) Southern flank of the bank (two paths) Southern flank of the bank (two paths) Deep drifters: (One in Wilkinson, one into GOM, two in eastern flank) Deep drifters: (One in Wilkinson, one into GOM, two in eastern flank)

3The SCOPEX drifter experiment results — early June 1989

4 Discussion During the SCOPEX field program eight drifters with drogues centered near surface at 5m made a total of 16 complete circuits. During the SCOPEX field program eight drifters with drogues centered near surface at 5m made a total of 16 complete circuits. The drifters generally followed narrower track over western than eastern of the flank The drifters generally followed narrower track over western than eastern of the flank None of the drifters passed into the shallow central region of the bank. None of the drifters passed into the shallow central region of the bank. It also appears that some drifters deviates up to 50km from the main track and make a loop or eddy before returning to the main track. It also appears that some drifters deviates up to 50km from the main track and make a loop or eddy before returning to the main track.

The recirculation periods for the 16 circuits don ’ t change significantly with time during the recirculation season The recirculation periods for the 16 circuits don ’ t change significantly with time during the recirculation season 4 Discussion

The drifters generally transited the eastern/southern flank of the bank more slowly than the western/northern flank of the bank. The drifters generally transited the eastern/southern flank of the bank more slowly than the western/northern flank of the bank.

The radial distance of the recirculation varied from 50km (southern and northern) to 150km (western and eastern) with its largest of 200km occurred around northeastern flank. The radial distance of the recirculation varied from 50km (southern and northern) to 150km (western and eastern) with its largest of 200km occurred around northeastern flank. The Subtidal speed was lower near eastern and western flank and greatest over northeastern flank and the width of the circulation was greatest in the eastern flank due to the multiple circulation paths. The Subtidal speed was lower near eastern and western flank and greatest over northeastern flank and the width of the circulation was greatest in the eastern flank due to the multiple circulation paths. 4 Discussion

some influence of wind events on drifter velocity as seen in some eastern and southern flank of the bank. some influence of wind events on drifter velocity as seen in some eastern and southern flank of the bank. 42 No 43 Yes 51 Yes 47 No

4 Discussion other factors besides wind effect on the drifter velocity field as other factors besides wind effect on the drifter velocity field as warm-core rings, entrainment features warm-core rings, entrainment features frontal instabilities frontal instabilities topographic waves should be examined in future study. topographic waves should be examined in future study. 56 No 58 No 73 No

5 Summary In this paper the author analyzed the drifter data near the GSC in late spring in 1988 and In this paper the author analyzed the drifter data near the GSC in late spring in 1988 and (1) many near-surface drifters entrained into the clockwise gyre around Georges Bank and eight drifters completed sixteen circuits around the bank. (1) many near-surface drifters entrained into the clockwise gyre around Georges Bank and eight drifters completed sixteen circuits around the bank. (2) The recirculation period were about 48days and the average drifter speed is around 10 cm/s. (2) The recirculation period were about 48days and the average drifter speed is around 10 cm/s. (3) No clear variation in the strength of the clockwise gyre over the bank during the stratification season, which was different from the results of some moored current data in other years. (3) No clear variation in the strength of the clockwise gyre over the bank during the stratification season, which was different from the results of some moored current data in other years. (4) The drifters generally followed a narrow track around much of bank, while in the eastern flank there were at least three preferred paths, which is probably due the northwest-southeast oriented sand ridge. (4) The drifters generally followed a narrow track around much of bank, while in the eastern flank there were at least three preferred paths, which is probably due the northwest-southeast oriented sand ridge. (5) No drifters entrained the shallow central area of the Bank. (5) No drifters entrained the shallow central area of the Bank. (6) Strong wind appeared to influence some of the drifters ’ trajectory especially during in the eastern and southern flank of the Bank. (6) Strong wind appeared to influence some of the drifters ’ trajectory especially during in the eastern and southern flank of the Bank.

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