92 



COLIN 



TEMPERATURE C. 

 25° 30° 30° 



30 



30 



Fig. 1 Tempcrature/dcpth profiles from four dives by the 

 DSRV Makali'i at Enewetalt Atoll, summer 1981. Locations of 

 profiles were as follows: a, Biken, west side of the atoll; 

 b. east side of wide channel; c. Bokandretok, just north of 

 Enewetak Island; and d. Runlt Island. Dual tracks represent 

 an ascent and descent profile. 



has never been observed to move northward. It is only 

 near passes into the lagoon that tidal currents potentially 

 cause reversal of current direction on the ocean slope of 

 the atoll. On the leeward (west) side of the atoll, currents 

 are variable, and an eddy pattern often seems to exist. At 

 the West Spit, the extreme northwest tip of the atoll, the 

 currents from north and south seem to converge. 



Currents in the lagoon and passes are considerably dif- 

 ferent and are covered in detail by Atkinson et al. (1981) 

 and in Chapter 5 of this volume. In the lagoon the surface 

 current is generally a wind-driven westerly flow with mid- 

 depth return flow to the east. Most water enters the 

 lagoon over the windward reefs and passes out the wide 

 (south) channel. The deep channel has strong tidal flow but 

 little or no net input into the lagoon. Water residence 

 times have a mean of about 30 days but can vary between 

 a few to more than 130 days. The residence time of water 

 in the northern p)ortion of the lagoon is greater than the 

 mean. 



The nutrient-poor oceanic water eastward of Enewetak 

 is clear, with visibility normally exceeding 50 m. Such 

 water visibility is typical of windward ocean reefs, but visi- 

 bility changes when water enters the lagoon over the reef 

 flat. Increased production and suspended particulates 

 reduce the visibility of lagoon waters to 10 to 25 m and 

 occasionally less. Aerial photographs of Enewetak have 

 features on the ocean side reefs visible to depths of about 



40 m, although this is limited to a maximum of about 15 

 to 18 m in lagoon waters. In the northern lagoon, particu- 

 larly near the islands between Engebi and Bokoluo, two 

 factors may contribute to visibilities at less than 10 m. 

 First, lagoon water residence times in this area are near 

 the maximum, hence high densities of phytoplankton and 

 zooplankton can develop in this water. Second, the pres- 

 ence of fine, easily suspended particulates produced as a 

 result of nuclear tests and cratering in this area may 

 greatly reduce water visibility. Phytoplankton blooms, man- 

 ifested both as "brown water" and large, thick windrows of 

 extremely dense phytoplankton, have been observed on 

 several occasions in the northwestern lagoon. Visibility in 

 such waters is consequently extremely limited. 



Trade wind conditions with steady 10 to 20 knot 

 winds from the east to northeast prevail throughout most 

 of the year at Enewetak. During the summer, trade winds 

 are usually lighter than during the winter, and they can 

 cease for periods of several days. The normal trade winds 

 produce oceanic waves about 1 to 2 m high which hit the 

 windward reef of the atoll. Within the lagoon, the margin 

 on the windward side is relatively calm, being protected by 

 islands and interisland reefs. At high tide, however, much 

 more wave action comes across the reef flat from ocean to 

 lagoon, making conditions choppier on the lagoon margin. 

 The waves which do cross the reef are of small height and 

 wave length, making the surface rough for small boats but 

 having little effect below a few meters depth. 



Moving westward across the lagoon, a significant fetch 

 is achieved quickly, and when trade winds prevail, the cen- 

 tral and western areas of the lagoon are far from placid. 

 Waves of 1.5 to 2 m with whitecaps can occur, and the 

 lagoonward edge of the leeward reef can have significant 

 surf breaking on it. Significant wave action can also reach 

 the lagoon sides of the islands west of Engebi and the 

 southwestern islands. 



The ocean side of the leeward reefs and islands is calm 

 under trade wind conditions with the tide level affecting 

 the wave action crossing the reef from lagoon to ocean. In 

 the lee of islands it is extremely calm for an oceanic area. 



Waves in the Marshall Islands as a whole are from the 

 east or northeast, a consequence of persistent trade winds. 

 Waves exceeding 3.5 m high comprise fewer than 2% of 

 waves in the Marshall Islands area (Japan Meteorological 

 Agency [JMA], 1971 to 1978). Waves greater than 3.5 m 

 can occur any time of the year and are generally associ- 

 ated with (1) local storms or typhoons from the east 

 through southwest and (2) more distant northern and 

 southern hemisphere storms. The greatest wave amplitude 

 observed (JMA report) was a long-period 6.5-m swell from 

 the northeast. 



Local conditions can also greatly affect wave action. 

 Where tidal currents lun against the trade winds, steep 

 standing waves develop. The east channel at Enewetak can 

 be treacherous under strong trade winds with the tide 

 dropping sharply. A distinct central tongue of breaking 

 waves extending out the channel to the ocean is visible 

 from the air under these conditions. Similarly, the west 



