OCEANOGRAPHY 



63 



site the surface flow, at speeds of 2 to 4 cm s The 

 volume transport of this current is approximately 8.6 X 

 10* m^ per tidal cycle (Table 2). 



The deep current flows southward between 30 and 50 

 m. This current is slow, ranging from 0.5 to 1.5 cm s 

 Drogues in this current were followed for up to 10 days; 

 while the cumulative direction and speed were consistent 

 and predictable, a 6- to 12-hour east-west variability 

 ("slosh") was noticed in their movement. This motion was 

 attributed to flow around the lagoon pinnacles and/or tidal 

 pulsing. The volume transport of the deep current is 

 approximately 2.2 X 10* m^ per tidal cycle through a 

 cross section near the middle of the lagoon (Table 2). 



Vertical Current Profiles in Lagoon 



Figures 6a and 6b are photographs of vertically 

 suspended fluorescine dye dispensers. These profiles reveal 

 the spiral current structure of the lagoon water. The pro- 

 files show that the deeper currents are offset to the right 

 (clockwise) from the shallower currents. Figure 7 is a 

 graphic summary of the vertical dye profiles in the lagoon 

 The number at the end of each arrow is the depth in 

 meters of the observation. The arrows have no magnitude 

 because current speeds were not determined. In all stations 

 across the lagoon, from Runit to West Spit (Fig. 7), the 

 current spiraled to the right, forming a substantial east- 

 ward flow which is referred to as the mid-depth current. 

 At two stations, deep scuba dives were made to verify the 

 southern flow of the deep current previously documented 

 with the deep current drogues. 



Figure 8 is constructed from all the deep-drogue mea- 

 surements and selected surface-drogue measurements 

 made during the summer and winter periods. These data 

 points represent end points of current vectors emanating 

 from the origin. The shaded spiral indicates the resulting 

 current structure. There arc not sufficient data to resolve 

 the spiral more accurately. The spiral reveals the basic 

 three-current system: the surface current (0 to 10 m) is 

 southwesterly, the mid-depth current (10 to 30 m) is 

 northeasterly, and the deep current (30 to 50 m) flows 

 southward. 



The vertical current structure, as summarized by 

 Fig. 8, can be altered by the cross-reef currents and tidal 

 currents. The diagonal lines in Fig. 9 delineate the area of 

 the lagoon directly affected by the windward cross-reef 

 currents. At the northern end of the lagoon (region 1 in 

 Fig. 9) these currents follow the contour of the atoll. Along 

 the central part of the windward back-reef (region 2) the 

 current may be going north, west, or south, depending on 

 the tide and surf conditions. Near Enewetak Island (region 

 3) the current also follows the contour of the atoll. The 

 surface current and deep currents in regions 1 and 2 move 

 in the same direction when large volumes of water cascade 

 over the reef. During spring low tide, however, when little 

 water enters the lagoon over the reef, a surface current, 

 mid-depth current, and deep current characteristic of the 

 open lagoon can be observed (Fig. 7). 



The currents directly behind the windward reef are 

 variable in speed, being fastest when large surf drives 

 water into these regions. Figure 10 is a plot on two suc- 

 cessive days, showing the current increase with rising tide. 

 Notice that the second date had higher surf and a slightly 

 higher wind speed than the first date. The two linear 

 regression coefficients are significantly different at the 95% 

 significance level. These data were taken at the site 

 denoted "A" in Fig. 9. 



The cross-hatched areas in Fig. 9 delineate water that 

 experiences reversing current through the Deep Entrance 

 and the Southwest Passage. The area near the 

 northwestern leeward reef, marked by circles in Fig. 9, is 

 an area of convergence. The lagoon surface water cannot 

 escape over the leeward reef, particularly when large surf 

 drives oceanic water over these reefs into the lagoon. 

 Large aggregations of jellyfish have been observed in this 

 region, as well as strong southwesterly flow along the 

 lagoonward margin of the reef. 



WATER BUDGET 



Table 2 is a summary of the volume transports for the 

 important components of the water budget. 



Input 



The water can flow into the lagoon from the windward 

 reef, the Deep Entrance, and the Southwest Passage. The 

 windward cross-reef current transports about twice as 

 much water as the Deep Entrance current. Because the 

 windward cross-reef current never reverses, the volume 

 transport over the windward reef represents net input of 

 water into the lagoon. The Deep Entrance and the 

 Southwest Passage show net transports of approximately 

 zero over each tidal cycle. 



Output 



The water can flow out of the lagoon from the leeward 

 reef, the Deep Entrance, and the Southwest Passage. 

 Because the Deep Entrance and the Southwest Passage 

 have net transports near zero over each tidal cycle, the net 

 inflow from the windward reef must exit as outflow over 

 the leeward reef and out the South Channel. Because the 

 flow over the leeward reef is relatively small (Table 2), 

 most of the water flows south, exiting out of the South 

 Channel. 



The numbers in Table 2 do not sum to zero over a 

 tidal cycle; however, these data were collected during dif- 

 ferent tide stages. Ranges were included in the table to 

 indicate the natural variability of the system. 



CIRCULATION MODEL 



Lagoon circulation can be explained as a response to 

 three sources of energy: (1) the surf on the windward 

 ocean reef, (2) the wind, and (3) the tides. 



