OCEANOGRAPHY 



arc released into northern lagoon water, which has 

 residence times well above the average for the whole 

 lagoon. The concentrations of radionuclides in the water 

 column decrease from the northern end of the atoll to the 

 southern end, by a factor of 2 to 5 (Noshkin et al., 1974). 

 This horizontal gradient reflects the general increase in 

 flushing rate in the south end of the lagoon, as well as hor- 

 izontal diffusion from the north end. 



The water column is vertically well mixed in terms of 

 temperature and salinity. However, in the central lagoon 

 the horizontal diffusion rates for certain materials may be 

 greater in the surface water than in deep water. Near the 

 windward reef, where both surface and deep currents 

 respond to the cross-reef currents, vertical transport may 

 be greater than in the central lagoon, and there may be no 

 difference in horizontal diffusion rates between surface 

 water and deep water. 



VON ARX MODEL FOR BIKINI 



Von Arx's (1948) model conceptualizes lagoon circula- 

 tion by linking two basic patterns: a "primary circulation" 

 and a "secondary circulation." 



The primary circulation consists of wind-driven surface 

 water moving downwind, sinking, and then returning 

 upwind to the windward (eastern) side of the atoll lagoon 

 as deep water. 



The secondary circulation consists of horizontal recircu- 

 lation of deep water. Von Arx reported that the volume 

 transport of the eastern flowing deep current is greater 

 than the volume transport of the surface current. He con- 

 cluded that some of the deep water is shoaled upward or 

 "upwelled" in the eastern part of the lagoon, becoming the 

 surface current. The remaining portion of the deep water 

 diverges at the leeward edge of the windward reef. Some 

 water moves northward following the bathymetric contour 

 of the basin. The deep water circulation forms two 

 counter-rotating bodies of water, the northern one moving 

 counterclockwise and the southern one moving clockwise. 



Von Arx estimated that the exchange of lagoon water 

 through all channels and passes during winter is approxi- 

 mately 3.8% of the total lagoon volume per tidal cycle. At 

 a 30% exchange efficiency, von Arx estimated the winter 

 Bikini lagoon flushing to be 39 days. The summer flushing 

 time was estimated to be twice as long as that in the 

 winter. 



The conspicuous feature of von Arx's model for deep 

 atoll lagoon circulation is the deep return flow toward the 

 windward side of the atoll. This return flow connects the 

 primary circulation with the secondary circulation. The 

 model for the circulation system of Enewetak has some 

 similarities to the model proposed by von Arx for Bikini. 

 The primary circulation system consisting of an overturning 

 wind-driven surface current is the same in terms of speed 

 and volume transjxjrt. 



The secondary system, or deep circulation, is not the 

 same as that proposed by von Arx. The deep current at 



Enewetak flows southward, toward the channel having net 

 outflow. Von Arx described a horizontally recirculating 

 deep current with a volume transport greater than the sur- 

 face current, hence upwelling on the windward side. At 

 Bikini the large open channel (Enyu Channel) is at the 

 southeastern end of the lagoon. A net transport toward 

 this channel would create an eastward flowing deep 

 current. The eastward mid-depth current and the "pass- 

 ward" deep current would then appear to be a single deep 

 current with a mass transport greater than the surface 

 current. The excess volume transport of von Arx's deep 

 current might largely be balanced by net outflow through 

 Bikini's southeastern channel. Von Arx did not report a 

 large net outflow; however, recalculation of his data sug- 

 gests net outflow through the Enyu Channel. Outflow was 

 also shown in the distribution of indigenous zooplankton 

 (Johnson, 1949) and was observed in surface radionuclide 

 patterns (Noshkin et al., 1974). To reach the Enyu Chan- 

 nel, the deep water in Bikini Lagoon must move east- 

 wards. In Enewetak Lagoon the only effective exit is at the 

 southernmost part of the atoll; therefore, the deep water 

 must move southward. 



Note that in the model derived from Enewetak, the 

 deep motion is primarily controlled by the location of the 

 major exit points from the lagoon. Water flow through 

 other atoll lagoons seems to be regulated by atoll morphol- 

 ogy and local wave and tidal conditions (Milliman, 1967; 

 Gallagher et al., 1971; Henderson et al., 1978; Ludington, 

 1979). Studies of deep currents in other deep lagoons 

 could be valuable in testing this interpretation. 



CONCLUSION 



Windward and leeward cross-reef currents, channel 

 currents, and tidal flow are the major factors influencing 

 the exchange of water between atoll lagoons and the sur- 

 rounding ocean. Because these factors are specific to local 

 wave climate, tidal conditions, and atoll morphology, atoll 

 lagoons have widely varying flush characteristics. Wind- 

 driven circulation, a pervasive feature of lagoons, con- 

 tributes primarily to internal circulation rather than flush- 

 ing. Upwelling on the windward side of lagoons may occur 

 as a summation of the above phenomena but does not 

 seem to be a generalizable feature of deep lagoon circula- 

 tion. Deep water flow appears to orient itself toward the 

 channels of net water output. 



ACKNOWLEDGMENTS 



This chapter is based on the final report of EXDE con- 

 tract number EY-77-5-08-1529, Water CiTculation of 

 Enewetak Atoll Lagoon and Circulation of Enewetak Atoll 

 Lagoon, by M. J. Atkinson, S. V. Smith, and E. D. Stroup. 

 Parts of the research were done under the auspices of the 

 Mid Pacific Research Laboratory. Thanks to S. V. Smith 

 for chemical data and review of the manuscript. 



