OCEANOGRAPHY 



61 



Surface and deep drogues placed in the channel on a 

 flooding tide reversed on the ebbing tide and returned to 

 near their original position. It was estimated that the net 

 volume transport of the Deep Entrance is approximately 

 zero over a tidal cycle. 



The South Channel (Fig. 2) has a nearly continuous 

 outflow. During flood tide, surface water drifts westward 

 across the channel; on ebb tide, the surface current turns 

 from westward, to southwest, to south. The surface water 

 in the South Channel tends to move westward as wind 

 drift, while water below a depth of 5 m moves southwest 

 to south, depending on the tide condition. The current 

 speeds range from 8 to 30 cm s^'. Based on 20 drogue 

 measurements and dye releases over a complete tidal 

 cycle, the average outflow was estimated to be 6.9 X 10 

 m'^ per tidal cycle; this represents 75% of the total lagoon 

 surface current volume transport and 105% of the 

 estimated water flowing inward over the windward reef 

 flats (Table 2, and material presented later). 



The Southwest Passage is a shallow break in the lee- 

 ward reef, yet it has a reversing current similar to the 

 Deep Entrance (Fig. 2). The calculated volume exchange 



between ocean and lagoon is approximately 0.8 X 10* m^ 

 per tidal cycle. Because the currents are reversing (see pre- 

 vious discussion), the net outflow through these channels is 

 small in terms of the water budget. The calculation of 

 volume transport of water over the entire leeward reef 

 includes this net outflow through this passage. 



Lagoon Currents 



Currents of the central lagoon may be characterized by 

 a surface current, a mid depth current, and a deep current. 

 The currents are distinguishable by their characteristic 

 speed and direction The water column in Enewetak 

 Lagoon is nearly isohaline and isothermal; salinity ranges 

 0.20 °/oo at most (average near 34.4 °/oo), and tempera- 

 ture varies by no more than 0.5°C (annual range, 27°C to 

 29°C). 



The surface current is wind-driven. The general surface 

 drift is southwesterly, or downwind (Fig. 4). The spatial 

 and temporal variations in the current directions are 

 considerable. In the central lagoon, drogues move south, 

 west, and north, appearing to respond to the wind direc- 



TABLE 2 

 Water Budget; Estimates of Mean and Range 



Current 



Transport (range) 



10* m^ per 12.4 hour 



(+ is to lagoon; 



— is from lagoon) 



Bases for calculation 



Comments on current 



Windward cross-reef 



Leeward cross-reef 



Deep Entrance 



South Channel 



Southwest Passage 



Surface 



Mid-depth 



Deep 



-6.6 ( + 2 2 to +198) 



-0.4 (0 to -0.8) 



Net = (-1.0 to +1.0) 

 (3.0 X 10* m^ transport 

 each way) 



-6.9 (-4 5 to -8.5) 



Net = (-0.2 to -0.2) 

 (0.8 X 10* m^ transport 

 each way) 



9.2 (3 to 30) 



8.6 (unknown) 



2.2 (unknown) 



0.56 m s ' m ' reef front 

 27,000 m open reef front 



0.05 ms"' 



0.4 m (half tide range) 



47,000 m open reef front 



40 ms^' 



34,000 m^ (cross-sectional area) 



0.15 (0.07 to 0.23) m s"' 

 145,000 m^ (cross-sectional 



area) (2/2)"* (conversion to 



normal direction) 



0.40 ms"' 



9,000 m^ (cross-sectional area) 



0.06 m s~' 



10 m X 34,600 m (maximum cross- 

 sectional area at 5 m depth) 



0.03 m s"' 



20 m X 32,000 m (maximum cross- 

 sectional area at 20 m depth) 



0.01 ms"' 



18 m X 28,000 m (maximum cross- 

 sectional area at 39 m) 



June 21-29, 1971 

 continuous inflow. 



Variable flow. None 

 to fast channel 

 currents. 



Reversing. Typical 



tidal currents to 0.80 m s" 



Pulsing, continuous 

 outflow. 



Reversing. Typical 

 tidal currents. 



Variable. Functional 

 to wind speed 



Variable. Functional 

 to wind speed. 



Variable. Functional to 

 windward cross-reef inout 



