RESTRICTED 



10 



ing southward and the salinity decreasing. Thus the water enter- 

 ing the lagoon from the north is about 0.1°C colder and 0. 30/00 

 more saline than that v/hich enters the southern passes. 



(b) Superia;posed on this basic difference is a reef effect. ' 

 During the short period of its passage over the reef, the water 

 is subjected to an intensification of the surface processes of 

 heating, cooling, and evaporation which in deeper water v/ould 



be distributed dov/:iward by vertical iuixing. The salinity of the 

 water coining over the reef is constantly increased by evapora- 

 tion. AssuLiing what appears to be a reasonable value for eva- 

 poration of U.5 cm. per day, i:he salinity will be increased 0.01 

 to 0.03°/oo, depending on the width of the reef and the strength 

 of the current. The greatest flov/ over the reefs is on the nor- 

 thern side of the lagoon. Therefore the effect of the reefs is 

 to inake the north-south gradient in salinity slightly stronger 

 in the lagoon than it is in the oceanic water outside. The 

 effect of the reefs on temperature appears to be inportant only 

 locally. The water is heated one to two degrees as' it cones 

 over the reef during the day and is cooled at night. The ten- ■ 

 perature is therefore iiore variable than it is elsev/here in the 

 lagoon, but the net effect on lagoon tenperature appears to be 

 negligible. Because of the effect of surf, and possibly by the 

 photosynthesis of reef algae during the daytime, the oxygen con- 

 tent of the water is higher near the reef than in the main body 

 of the lagoon. These variations are used in a later section to 

 analyze diffusion rates. 



(c) Vihat has been said of surface exchanges over the reef 

 applies to a lesser extent to the lagoon as a whole. In the 

 open ocean, the effects of surface heating and evaporation are 

 readily distributed through a mixed layer 300 to 400 feet deep. 

 In the lagoon, with an average depth of I75 fee..,, these effects 

 are more pronounced. It is estiiiated that evaporation will in- 

 crease the salinity of the lagoon an average aiaount of 0v-01°/oo 

 in three days. 



The distribution of salinity in the lagoon, shov/n in J^'igures 

 9 to 12, is initially dependent on these three factors but is 

 modified by the existing current pattern. In general, the evi- 

 dence gained from examination of the salinities corroborates the 

 current data previously shown. The current observations and the 

 additional evidence derived from study of the salinities are com- 

 bined to produce the cxorrent patterns shown in i'igures 13 and I4, 

 v/hich although somewhat idealized appear to be logical. 



The direction of the currents in the north and northwestern 

 part of the lagoon Indicates that 'it is an area of relatively 

 closed circulation. The high salinity of the area is addition- 

 al evidence. The presence of water with ^ salinity about O.io/oo 

 higher than any incoming water is indicative that some of it re- 

 mains in the same general area a minimum time of 30 days. 



