66 



ATKINSON 



w 10 - 



E 

 o 



a 



c 



0) 



o 



270 



1200 



0.15 



1300 1400 1500 

 Time of day (h] 



1600 1700 



1.5 



Tide height (m) 



Fig. 10 Change in current speed as a function of tide helgfit 

 near Runit on successive days. Average current speed plotted 

 at midpoint of time interval. 



HIGH 



> 



< 



3 

 O 



> 



< 



L^ 1 I I . 



10 20 30 



DISTANCE (km) 



40 



Fig. 11 Increase in cumulative net input over the windward 

 reef flat as a function of distance from the north end of the 

 lagoon. 



360 - 



-o 



090 



O 



UJ 



180 - 



270 



10 20 



DISTANCE (km; 



Fig. 12 Change in direction of drogues suspended at 38 m 

 as a function of distance from the north end of the lagoon. 



The observed pattern of wind-driven currents resembles 

 in many ways the pattern predicted by Ekman for an 

 enclosed sea in which the following conditions apply: (1) 

 impermeable, closed boundary; (2) constant, unidirectional 

 windstress over the entire surface; (3) homogeneous water; 

 (4) uniform depth; and (5) constant eddy viscosity. At 

 Enewetak these conditions are only partially met. The 

 lagoon rim is closed neither to leeward nor windward; in 

 particular, large quantities of water are introduced along 

 the windward edge (Table 2). 



In a fully enclosed sea, the Ekman flow integrated over 

 depth is zero at every point. In a lagoon such as Enewetak 

 this will not be the case, but the detailed effects of the 

 "leaky" boundary and the irregular bathymetry have not 

 been estimated from present data. The remarkably shallow 

 spiral pattern of currents is a new finding which should be 

 further investigated and modeled. 



Surface current speeds are 5 to 20 cm s~', approxi- 

 mately 2% of the wind speed. The surface drift is generally 

 downwind and seems responsive to the wind direction of 

 the previous 6 to 12 hours. The mid-depth upwind current 

 speeds are about one-half of the surface current speeds. 



These wind-driven currents would cause the surface 

 water to overturn in 5 to 10 days if there was no vertical 

 mixing. Von Arx (1948) estimated approximately the same 

 time for turnover at Bikini. Von Arx (1949), Munk et al. 

 (1949), and Ford (1949) suggested that the surface water 

 at Bikini sinks in the western portion of the lagoon and 

 upwells in a small band in the eastern portion of the 

 lagoon. No direct evidence of upwelling has been found at 

 Enewetak. Upwelling, if it exists as such, will be largely 

 intermittent, because of the intermittent (tidal) pulsing of 



