Anchor Stations 



Velocity vectors obtained by direct measure- 

 ment at the two anchor stations (fig. 24) did not 

 display the characteristics of an "Ekman Spi- 

 ral." Theoretically, wind stress on infinitely 

 deep water will produce an "Ekman Spiral" dis- 

 tribution of current vectors that decreases in 

 magnitude and spiral to the right with depth 

 (in the northern hemisphere) . Instead, the cur- 

 rents observed generally increased in magnitude 

 from the surface to the bottom and had no reg- 

 ular progression with depth. For example, at 

 anchor station 1 the current speed 10 m. above 

 the bottom (17 cm., sec) was greater than that 

 at the 25 m. level (13 cm./sec). 



Two possible explanations exist: either the 

 deeper currents observed were not wind driven, 

 or the winds were strong enough to induce a 

 nearly uniform current to the bottom (100 m). 

 Neumann and Pierson (1966) demonstrated that 

 there is not much difference between wind 

 driven currents in deep water and those in water 

 that is 1.25 times deeper than the depth of fric- 

 tional influence. These observations were made 

 in water 100 m. deep. Thus, if the depth of fric- 

 tional influence was 80 m. or less, the wind 

 driven currents should have displayed the char- 

 acteristics of an "Ekman Spiral." The formula 

 for depth of frictional influence (Sverdrup et al., 

 1942) indicates that at a latitude of 47°N winds 

 of 17.5 knots would be required to develop a 

 depth of frictional influence of 80 m. Fredholm's 

 work (Ekman, 1905) shows that at 47°N a wind 

 driven current would be close to its fully devel- 

 oped state within about 36 hours. In this region 

 the wind was from the southwest for 96 hours 

 preceding the observations. While the wind 

 speed reached 20 knots, it averaged 12 knots for 

 the entire period and averaged only 8 knots dur- 

 ing the 24 hour period immediately preceding 

 the observations. Temperature and salinity dis- 

 tribution from oceanographic stations adjacent 

 to the anchor stations (figs. 25-28) are isother- 

 mal and isohaline to depths varying from 35 m. 

 to 65 m. Sharp thermoclines and haloclines ex- 

 isted, indicating that wind mixing did not reach 

 below these depths. Thus it is unlikely that the 

 deeper currents observed were of wind driven 

 origin. The currents could have been due to 

 tidal effects or the distribution of mass. 



Geostrophic currents at 25, 50, and 75 m. were 

 derived from the dynamic topography relative 



to the 1000 decibar surface. When geostrophic 

 currents were compared with direct measure- 

 ments (fig. 24), there was reasonably close 

 agreement at anchor station 1, but very poor 

 agreement at anchor station 2. In the case of 

 anchor station 1, the velocities of the geostro- 

 phic current were only slightly (<2 cm./sec) 

 smaller in magnitude and differed by about 50 

 degrees in direction from those obtained at the 

 same depth by direct measurement. Considering 

 the short duration of the direct measurements 

 (30 minutes at each depth) and the possible 

 temporal variations, such as local eddies or tidal 

 currents, this agreement is judged reasonably 

 close. In the case of anchor station 2, the agree- 

 ment was very poor. The geostrophic currents 

 are questionable here because the dynamic con- 

 tours are nearly parallel to the line of hydro- 

 graphic stations. Thus, drawing the contours 

 was even more subjective than usual. The pos- 

 sibility exists that the directly observed current 

 included tidal effects that were not detected by 

 the geostrophic method. Direct measurements 

 made over longer periods could be used to filter 

 out tidal effects and possibly would provide bet- 

 ter agreement with the geostrophic method. 



In these comparisons the accuracy of the cal- 

 culated geostrophic currents was limited by : 



• The subjectivity involved in contouring 

 near the edge of the data field. 



• The method used to calculate dynamic 

 heights in water shallower than the reference 

 level. 



• The fact that the geostrophic approxima- 

 tion itself may not be valid in shallow water 

 because of frictional effects. 



Because of these limitations, the disagreement 

 between the direct measurements and the geo- 

 strophic method is not surprising. 



Taut-Line Instrumented Arrays 



All five of the current meters emplaced meas- 

 ured cyclic variations in the current with a 

 strong semidiurnal constituent as shown by the 

 plots of north-south and east-west components 

 of the velocity vs. time (figs. 29b, 29d, 30b, and 

 31b). This semidiurnal component was super- 

 posed on a low frequency carrier with a period 

 of six to eight days. The exact period of this 

 carrier component was not determined, because 

 the power density spectral estimates were only 



