(1962) computed a western boundary current 

 flowing opposite the Gulf Stream in an area 

 south of Cape Cod in 1959 to 1960. The geo- 

 strophic flow below 1,400 meters was de- 

 termined using a 2,000-meter reference level. 

 Volkmann concluded that the westward flow 

 of water may not be continous but may consist 

 of a series of eddies or transients. He also con- 

 cluded from transport calculations that large 

 amounts of water flowing westward between 

 Cape Cod and Cape Hatteras are recirculated 

 by the Gulf Stream, and that the amount of 

 water recirculated can be influenced by large- 

 scale transcients. Direct measurements of sub- 

 surface currents were made by Barrett (1955) 

 off of Cape Hatteras in 1962. He found that the 

 inshore westward flowing current appeared to 

 be continous along the steep continental slope 

 from depths of several hundred meters to 

 depths greater than 2,500 meters. 



Assuming geostrophic conditions were dom- 

 inant in the water column over a depth of 500 

 meters or greater, several inferences concern- 

 ing a counterflow west of the Gulf Stream can 

 be drawn from the density profiles. East of 

 73 ""W longitude, counterflow was as shallow as 

 100 meters and within 40 miles of the edge 

 (100 meters depth) of the continental shelf 

 (figs. 41, 42, 44, and 53). Through sections 4 

 and 7 in September and section 2 in December 

 a northeastwai-d flow below 150 meters was 

 indicated between the counterflow and the con- 

 tinental slope (figs. 44, 47, and 52). Since this 

 flow was not indicated through adjacent sec- 

 tions, it may represent an eddy in the counter- 

 flow. No counterflow was indicated in the upper 

 500 meters along section 9 in September. It is 

 possible that a counterflow may have existed 

 deeper than 500 meters along this section, or 

 it may have been entrained in the Gulf Stream. 



Niiler and Spiegel (1968) presented a nu- 

 mercial treatment of formation of a quasi- 

 geostropic jet along a shoaling coast. They 

 assume coastal regions to be shallow on a scale 

 comparable to the width of the boundary 

 current and the current is assumed to be of 

 constant potential vorticity. The significant 

 features of the numerical solution are "(i) the 

 appearance of a countercurrent, (ii) a pocket 

 of warm water above the ledge where the shelf 

 drops oflf into the deep ocean, and (iii) a pocket 

 of cold water on the ledge." These features ap- 



pear to be generally in good agreement with 

 the conditions found during September. 



Niiler and Spiegel's numerical treatment in- 

 dicates the dependence of the current upon the 

 topographical features of the shelf edge. As 

 the shelf edge changes from a sharp ledge to a 

 gradual rise, the countercurrent amplitude is 

 decreased and the width is increased. They 

 noted that the numerical analysis is valid only 

 for a limited distance downstream of where 

 the countercurrent begins. This is because the 

 influx of water increases the horizontal density 

 gradient to a point where the quasigeostrophic 

 approximation is invalidated. This phenomena 

 may explain the apparent discontinuity of the 

 countercurrent indicated by the ICNAF data. 



Shelfwater — Slope Water Boundary 



A striking feature of the data collected dur- 

 ing September and December 1967 was the 

 evidence of dynamic processes occurring at the 

 shelf water-slope water interface. This inter- 

 face represents a boundary between markedly 

 different temperatures and salinities, and be- 

 tween deep and shallow water regimes. This 

 boundary is subject to the dynamic processes 

 of wind mixing, tides, internal waves and cur- 

 rents which may be strongly influenced by 

 transients. Due to these conditions and shal- 

 lowness of the shelf edge, the assumptions of 

 geostrophic flow were not applied to infer 

 water motion in this area. 



The physical properties of the shelf water- 

 slope water boundary were strongly influenced 

 by seasonal variations in 1967. The marked 

 change in the density structure between 30 and 

 50 meters depth over the shelf edge from Sep- 

 tember to December indicated a strong seasonal 

 variation in vertical mixing. The vertical lines 

 of constant sigma-t in December (figs. 51 

 through 56) suggest intense mixing, probably 

 due to strong wind mixing and convection re- 

 sulting from surface cooling. In September, 

 these mixing processes were absent and the den- 

 sity of this layer is well stratified (figs. 41-50) . 



CONCLUSIONS 



A continuous core of relatively cold water 

 existed in September along the shelf edge from 

 Cape Cod to the offing of the Chesapeake Bay. 

 Evidence of "calving" in which a large bubble 

 of cold water separates from the core and 



