CHAPTER 3 



Feb ru o r y 



Mof c h 



"T" 



Sc 



ole: i =2x10'' 

 I d yne 5 / c m ^ 



Moy 



1 



June 



1 



Resultant surface wind stress vectors; 

 values centered at 40''N, 73°W. 



Februo ry 



March 



April 



May 



June 



~r 



"T" 



"T" 



1 



Mean vertical motion into surface Ekman layer; 

 values centered at 40''N, 73''W, positive upward. 



FIGURE 3-11. — Variation of monthly (a) mean surface wind stress over New York Bight (computed by using 10-day 

 wmd vector averages) and (b) mean vertical motion at bottom of surface Ekman layer. February-June 1976. 



An estimate of the mean vertical motion through the 

 bottom of the surface Ekman layer was calculated from 

 the linearized steady state vorticity equation (see Mc- 

 Lellan 1965) and is given by 



<^T, 



p.f\d 





/ 



(2) 



where t, and Xy are, respectively, the zonal and meridional 

 components of the stress vector, p„ is the water density, 

 /is the Coriolis parameter, and p is the change in /with 

 latitude. 



Except for March, the vertical motion induced by the 

 curl of the wind stress is positive. This suggests that 

 ("open-ocean") upwelling may have been prevalent in the 

 Bight throughout the analysis period. 



A more appropriate indicator of the vertical motion 

 field for the inner shelf is the upwelling index of Ekman 

 mass transport given by: 



M<?l = Tjf 



'gk 



■rJf 



(3) 



(4) 



where t,,, t, and / are defined above. Figure 3-12 shows 

 that the integrated mass transport in the surface mixed 

 layer had a strong offshore component from Februarv to 

 June. The sustained cross-isobath flow implies that on- 

 welling of deeper water onto the continental shelf off New 

 Jersey may have been anomalously strong and established 

 2 to 3 months sooner than normal. 



71 



