pus. Iflfif)). In II I'ccpnt Coast Guard Oceano- 

 {rrapliii" Unit Teclinicul l{fp()it. Bisliop (197;')) 

 develops an operationally oriented technique to 

 estimate these steady coastal currents. Tni)ut 

 parameters to tlie model are tiie surface wind 

 stres? and mean vertically-averaged signia-t 

 •rradient. 



On the August 1974 cruise, measurements of 

 sigma-t indicated a strong (i.e., ;5xl<l '"gm cm*) 

 cross-.shelf gradient in tiie veiticaliy averaged 

 sigma-t field. This is geneially tiie typical sum- 

 mer density structure as contrasted to tiie weakly 

 stratified (i.e.. 1x10 '"gm cnr*) winter shelf 

 water. The summer wind field exhibits mean 

 stress values of the order of 10 - dynes/ciir- 

 toward the northeast wiiile winter mean stress is 

 in the 1 dyne cm- range toward tiie southwest 

 according to data the for ."i^ squaie centered at 

 37.r)°X 72.r)°W as presented in HidaUa (19.58). 

 Roth in smnmer and winter a south to south- 

 west mean drift is derived from drift card data 

 (Humpus. 1969). It seems straightforward that 

 this velocity field (approximately ('(lual in mag- 

 nitude for each season) is maintained in the 

 sunnner months \>y the well developed density 

 field, and in the winter l)y the mean wind stress. 



Note added in proof. Ilccoiit ciiinpiitations of tin- 

 mean winter wind stress in slu-lf waters sliows tli<> 

 stress to lie toward tiie nortlieast rather tliau tlie 

 soutiiwest. A paper liy Heardsiey aiui Hutnian (19741 

 sugKest tliat aloii^' siiore pressure ;.'radieiit may lie a 

 sit'iiilicant factor in niaintaiiiiii;; a mean soutiiwest 

 drift a;;ainst tlie opposiiijr mean wind stre.ss. 



Measured values of this mean vertically aver- 

 aged sigma-t gradient, obtained on this cruise. 

 were used in calculations to estimate surface 

 coastal drift based on the above mentioned 

 analytical model (wind stress was neglected). 

 The result indicated a shelf circulation (fig. fia) 

 generally setting toward tlie southwest with 

 maximum surface velocities near the siielf l)reak 

 of approximately of 20 cm/sec. This calculation 

 approximates estimates of surface drift on the 

 Mid-Atlantic shelf (Humptis and Lauz'ier. 196.")). 



Comparison of the shelf circulation derived 

 from this model (fig. 6a) with that derived from 

 dynamic topography (fig. 61)) shows that the 

 two are in general agieement but ditler in de- 

 tails. The differences are probably related 

 e(|nally to ditferences in the governing e(|uations 

 (liishop includes friction in his model) and to 

 differences in applying the data (Bishop uses a 



mean sigma-t gradient for each section: the dy- 

 namic method uses tiie dynamic height for indi- 

 vidual stations). 



In waters seaward of the slope, contours of 

 dynamic heights referenced to 1000 meters (fig. 

 6b) indicate the presence of two anticyclonic 

 eddies with a trough Iwtween them. The slope 

 circulation is dominated by the two eddies, the 

 only othei- feature present being the trough. 

 Maximum geostrophic speeds in the southern 

 eddy are approximately 40 cm sec ^ 



An .\nticyclonic Eddy in the Slope Water 



One of the interesting features found during 

 this cruise was the anticyclonic eddy located 

 about 11.") nmi southwest of Cape May, New 

 .Jersey (fig. 61)). Eddies such as this are a com- 

 mon feature in the slope water along the conti- 

 nental slope of the New York Bight. Infrared 

 satellite imagery shows that there is a continual 

 progression of such anticyclonic eddies through 

 the Bight. They commonly have a diameter of 

 ."lO to 110 nmi with a spacing of about 110 to 220 

 nmi between eddies. The eddies seem to form 

 from meanders in the North Atlantic Current in 

 the northwest .\tlantic. generally east of 6.")°W, 

 and from there drift westward and southwest- 

 ward along the continental slope until they reach 

 the vicinity of Cape Hatteras where they rejoin 

 the Gulf Stream (fig. .")). 



The eddy southwest of (\ipe Maj' appears on 

 the temperature, salinity, and densit}' sections 

 as a core of warm saline water which is le-ss 

 dense than the suriounding watei' (figs. 7, 13, 

 and 19). This core has a temperature of 1.5° to 

 16°C, a salinity of 36.1 to 36.2Voo, and a <t, of 

 26.80 to 27.00. 



Evidence of a second eddy located about 120 

 mni south of Block Island was found on sections 

 E and F (fig. 6). The center of the northern 

 eddy was seaward of the axailable observations, 

 and no conclusions can be drawn comparing the 

 two eddies. 



The circulation pattern aroimd both eddies 

 was anticyclonic. as indicated on the dynamic 

 topography chart. The dynamic topography 

 chart showed geostrophic speeds in the southern 

 eddy of up to about 40 cm sec '. 



Following the survey of the smaller eddy, a 

 surface current drogue (fig. 26) was deployed 



8 



