Near-surface recirculation over Near-surface recirculation over Georges BankGeorges Bank

Author: Richard Limemurner and Robert Author: Richard Limemurner and Robert C. BeardsleyC. Beardsley

AcronymsAcronyms

GOM: Gulf of MaineGOM: Gulf of Maine GB: Georges BankGB: Georges Bank GSC: Great South Channel (South GSC: Great South Channel (South

Channel)Channel) NEC: Northeast ChanelNEC: Northeast Chanel

1 Introduction 1 Introduction Position of Position of Georges BankGeorges Bank

Clockwise gyre Clockwise gyre and seasonality and seasonality

Reason for the Reason for the general general pattern and pattern and seasonalityseasonality

-- Tides -- Tides rectificationrectification

-- topography-- topography-- Horizontal -- Horizontal

pressure pressure gradientgradient

vertical vertical stratificationstratification

Recirculation Recirculation near the near the southern flank southern flank (main objective (main objective of the paper)of the paper)

Fig.1 Summer pattern of the circulation near Georges Bank

Nantucket Island

Little Georges Shoal

2 Data and methods2 Data and methods Experiment: South Channel Ocean Productivity Experiment: South Channel Ocean Productivity

Experiment (SCOPEX) for 1988 and 1989 springtime Experiment (SCOPEX) for 1988 and 1989 springtime

Instrument: Low-Cost Drifters (LTD) : transmitted Instrument: Low-Cost Drifters (LTD) : transmitted location, sea surface temperature, drogue tension and location, sea surface temperature, drogue tension and battery status typically 6~8 times per day via ARGOS. battery status typically 6~8 times per day via ARGOS.

13 drifters at 5m to track the low-salinity water above 13 drifters at 5m to track the low-salinity water above the pycnocline the pycnocline

6 drifters at 50m to track the cold Maine Intermediate 6 drifters at 50m to track the cold Maine Intermediate water below seasonal pycnocline.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 a parabolic-linear filter with 33h half-amplitude

method to eliminate tidal and inertial motions. method to eliminate tidal and inertial motions.

Ancillary data: Wind data from NOAA environmental Ancillary data: Wind data from NOAA environmental buoys 44010 near Nantucket Shoals buoys 44010 near Nantucket Shoals wind stress wind stress time-series using the neutral drag law of Large et al time-series using the neutral drag law of Large et al (1981). (1981).

33 The SCOPEX drifter experiment The SCOPEX drifter experiment resultsresults

— early March 1988 (Late winter)— early March 1988 (Late winter) Three drifters: Three drifters:

70,71,7270,71,72

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

33 The SCOPEX drifter experiment The SCOPEX drifter experiment resultsresults

— In April (early spring) of 1988— In April (early spring) of 1988 10 surface, 6 deep drifters10 surface, 6 deep drifters SurfaceSurface southwestern GSC southwestern GSC

outflow; Central and outflow; Central and Eastern GSC inflow Eastern GSC inflow (10cm/s) (10cm/s)

Variable cyclonic Variable cyclonic circulation near Wilkinson circulation near Wilkinson BasinBasin

deeper driftersdeeper drifters slower ; 69°Wslower ; 69°W Two : a stagnation region of Two : a stagnation region of

relatively low velocity relatively low velocity before moving towards before moving towards Georges Bank. Georges Bank.

Three : along the northern Three : along the northern flank of the bank and at flank of the bank and at least one drifter moved least one drifter moved eastward to the eastern tip eastward to the eastern tip of the bankof the bank

all of them stopped all of them stopped transmitting before leaving transmitting before leaving the Northeast Peak region.the Northeast Peak region.

33 The SCOPEX drifter experiment The SCOPEX drifter experiment resultsresults

— early June 1989— early June 1989 13 near-surface 13 near-surface

drifters and 6 deep drifters and 6 deep drifters drifters

7 characteristics7 characteristics western GSC outflowwestern GSC outflow Cape Cod near-shoreCape Cod near-shore Cape Cod offshoreCape Cod offshore northern flank of northern flank of

Georges Bank near Georges Bank near 68°W( three paths 68°W( three paths with two split path with two split path at east)at east)

Southern flank of Southern flank of the bank (two paths)the bank (two paths)

Deep drifters: (One Deep drifters: (One in Wilkinson, one in Wilkinson, one into GOM, two in into GOM, two in eastern flank)eastern flank)

33 The SCOPEX drifter experiment The SCOPEX drifter experiment resultsresults

— early June 1989— early June 1989

4 Discussion4 Discussion During the 1988-1989 During the 1988-1989 SCOPEX field SCOPEX field program eight program eight drifters with drogues drifters with drogues centered near surface centered near surface at 5m made a total of at 5m made a total of 16 complete circuits.16 complete circuits.

The drifters generally The drifters generally followed narrower followed narrower track over western track over western than eastern of the than eastern of the flankflank

None of the drifters None of the drifters passed into the passed into the shallow central shallow central region of the bank. region of the bank.

It also appears that It also appears that some drifters some drifters deviates up to 50km deviates up to 50km from the main track from the main track and make a loop or and make a loop or eddy before returning eddy before returning to the main track. to the main track.

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

4 Discussion4 Discussion

4 Discussion4 Discussion

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

The radial distance of the recirculation varied from 50km (southern The radial distance of the recirculation varied from 50km (southern and northern) to 150km (western and eastern) with its largest of and northern) to 150km (western and eastern) with its largest of 200km occurred around northeastern flank. 200km occurred around northeastern flank.

The Subtidal speed was lower near eastern and western flank and The Subtidal speed was lower near eastern and western flank and greatest over northeastern flank and the width of the circulation was greatest over northeastern flank and the width of the circulation was greatest in the eastern flank due to the multiple circulation paths. greatest in the eastern flank due to the multiple circulation paths.

4 Discussion4 Discussion

4 Discussion4 Discussion some some

influence of influence of wind events wind events on drifter on drifter velocity as velocity as seen in some seen in some eastern and eastern and southern flank southern flank of the bank. of the bank.

42 No

43 Yes

51 Yes

47 No

4 Discussion4 Discussion other factors other factors

besides wind besides wind effect on the effect on the drifter drifter velocity field velocity field as as

warm-core warm-core rings, rings, entrainment entrainment featuresfeatures

frontal frontal instabilities instabilities

topographic topographic waves waves should be should be examined in examined in future study.future study.

56 No

58 No

73 No

5 Summary5 Summary In this paper the author analyzed the drifter data In this paper the author analyzed the drifter data

near the GSC in late spring in 1988 and 1989. near the GSC in late spring in 1988 and 1989. (1) many near-surface drifters entrained into the (1) many near-surface drifters entrained into the

clockwise gyre around Georges Bank and eight clockwise gyre around Georges Bank and eight drifters completed sixteen circuits around the bank. drifters completed sixteen circuits around the bank.

(2) The recirculation period were about 48days and (2) The recirculation period were about 48days and the average drifter speed is around 10 cm/s. the average drifter speed is around 10 cm/s.

(3) No clear variation in the strength of the clockwise (3) No clear variation in the strength of the clockwise gyre over the bank during the stratification season, gyre over the bank during the stratification season, which was different from the results of some moored which was different from the results of some moored current data in other years.current data in other years.

(4) The drifters generally followed a narrow track (4) The drifters generally followed a narrow track around much of bank, while in the eastern flank there around much of bank, while in the eastern flank there were at least three preferred paths, which is probably were at least three preferred paths, which is probably due the northwest-southeast oriented sand ridge. due the northwest-southeast oriented sand ridge.

(5) No drifters entrained the shallow central area of (5) No drifters entrained the shallow central area of the Bank. the Bank.

(6) Strong wind appeared to influence some of the (6) Strong wind appeared to influence some of the drifters’ trajectory especially during in the eastern drifters’ trajectory especially during in the eastern and southern flank of the Bank.and southern flank of the Bank.

ThanksThanks！！