Chapter 5 



Oceanographx; of Enewetak Atoll 



MARLIN J. ATKINSON 



Zoology Department. Uniuersiti; of Western Australia 

 Nedlands, Western Australia 



INTRODUCTION 



Enewetak and Bikini lagoons are large, deep atoll 

 lagoons. The circulation systems of both lagoons are dom- 

 inated by wind-driven currents (von Arx, 1948, for Bikini, 

 Atkinson et al., 1981, for Enewetak). However, the full 

 dynamics of the Enewetak circulation system is explained 

 by a combination of wind-driven currents, slope currents 

 from water input by waves, and tidal currents. The findings 

 at Enewetak indicate that the internal circulation and flush- 

 ing of deep atoll lagoons is affected by atoll morphology 

 and local wave and tidal conditions, features which in gen- 

 eral control circulation in shallow atoll lagoons (Mllliman, 

 1967; Gallagher et al., 1971; Henderson et al., 1978; 

 Ludington, 1979). 



This chapter begins with the general oceanography of 

 the northern Marshall Islands and then concentrates on the 

 oceanography of Enewetak Lagoon. The oceanography of 

 Bikini Lagoon and vicinity have been well studied com- 

 pared to Enewetak. In this chapter frequent comparisons 

 are made between Enewetak and Bikini. 



NORTH EQUATORIAL CURRENT 



Enewetak resides in the North Equatorial Current. In 

 the region of the Marshall Islands, the current Is between 

 6° to 8° and 15° to 17° north latitude. The southern 

 boundary of the current moves northward with the sun 

 during northern hemisphere summer and shifts back 

 toward the south In winter. The current has a general 

 westward drift between 20 to 50 cm s"^ Surface water is 

 isothermal to 75 m and varies seasonally between 26° and 

 29°C. The main thermocline is between 150 and 300 m 

 with a temperature of 10°C at 300 m. By 1500 m the 

 temperature drops to 3°C. 



Between the region of 3° to 11°N the salinity Is rela- 

 tively low (34.1 to 34.5 °/oo) reflecting the annual net 



rainfall in the region and the eastern flow of the Equatorial 

 Countercurrent. Higher salinities occur to the north of 11° 

 (the latitude of Enewetak) due to increased evaporation. 

 Isohalines show development of Intermediate Water about 

 11°N. Figure 1 shows the temperature-salinity relation- 

 ships of the Western North Pacific Central Water and 

 Pacific Equatorial Water in the region of the northern 

 Marshall Islands (Barnes et al., 1948). The solid lines In 

 the diagram indicate the temperature-salinity correlations 

 at different latitudes; at 20°N the water is all North Pacific 

 Water and at 4°N it is all Pacific Equatorial Water. The 

 Insert In the diagram shows the depth of transition zones 

 between the two water masses. Enewetak Atoll resides In 

 the region where the transition zone is above 200 m and 

 is only 50 m thick (Barnes et al., 1948). 



Dynamic topography near Enewetak has never been 

 measured. However, data collected during the Operation 

 Crossroads project and by the Japanese indicate dynamic 

 topography to be complex near Bikini, with the presence 

 of eddies northwest and northeast of the atoll (Barnes et 

 al., 1948). Rather permanent eddies probably exist near 

 Enewetak because they do for other islands (Hamner and 

 Hauri, 1981). The complexity of dynamic heights suggests 

 that currents near the atolls may vary in both sp)eed and 

 direction. 



WAVES AND TIDES 



Waves formed by the northeast trade winds break on 

 the northern and eastern reef perimeters of the atoll. This 

 constant pounding of the fore-reef shapes the spurs and 

 grooves on the windward side. For Bikini Atoll, Munk and 

 Sargent (1949) used wind data to calculate wave direction, 

 wave height, and wave energy. The spur and groove sys- 

 tems on the windward side of Bikini dissipate 95% of the 

 calculated wave energy as frictlonal heat and channel 5% 

 of the energy upward to maintain a head of water on the 

 reef flat. The head of water establishes water flow from 

 the ocean to lagoon across the windward reef flats. Waves 

 within the lagoon are generated by local wind patterns and 

 have little Influence In shaping the reef structures, but they 

 do Influence sand transport. 



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