OCEANOGRAPHIC CLIMATE OF HAWAIIAN ISLANDS REGION 



393 



(E-P) at 25°-30° N., respectively. An excess 

 of precipitation over evaporation occuis south of 

 15° N. between March anil Deceniljcr, reaching 

 farthest north during September and October. 

 E.xcess precipitation over evaporation also occurs 

 north of 29° N. in February. 



The seasonal variation of (E—P), together with 

 the variation of evaporation as plotted in figure 

 20 for 20° X. 160° W., illustrates the manner in 

 which precipitation modifies the evaporation. 

 Tlie precipitation is approximately 1.2 cm. mos.~' 

 in July and 7.2 cm. mos.~' in December and 

 January. 



Meridional profiles of (E—P) and evaporation 

 at 160° W., illustrating the modifying effect of 



18 



10 



E-P 



X 



/ \ 





I I I I 



J_J L 



MN MAR MAY JUL SEP NOV JAN MAR 



Figure 20.— Seasonal variation at 20° X., 160° W., in the 

 rate of evaporation, precipitation, evaporation minus 

 precipitation and equivalent salinity change per month 

 (upper panel). 



precipitation, are shown in figure 21. In April, 

 at the time of niaxinmm evaporation at 20° X., 

 the minimum precipitation of 4.8 cm. mos.~' occurs 

 at 22° and 23° X. It increases northward to 10 cm. 

 mos.~' at 30° X. and southward to 16 cm. mos.~' at 

 10° N. In August precipitation is essentially 

 lacking l)etween 21° and 24° X., but it increases 

 northward and southward to 6 cm. nios."' at 30° X^. 

 and 16.4 cm. mos.~' at 10° X. Finally, in De- 

 cember, the time of maximum (E—P) at 15° N., 

 minimum precipitation of 7 cm. mos."' has shifted 

 southward along the meridian to 16° X., with 

 12.4 cm. mos."' and 10.6 cm. mos.~' at 30° and 

 10° X"^., respectively. 



In order to illustrate the effect of evaporation 

 minus precipitation upon the salinity in terms of 

 change of salinity per month, equation 7 states 

 that (E—P) must be divided by the depth of the 

 mixed layer and multipHed by the salinity. The 

 upper panel in figures 20 and 21 shows the change 

 of salinity due to (E—P) in terms of parts per 

 thousand of salt per month. Thus, at 20° X^. 

 160° W., figure 20, the evaporation minus precipi- 

 tation would increase the salinity- by 0.02 %o 

 mos.~' in December and January, 0.09%o mos.~' 

 in May and July, and in June by 0.19%o mos.~' 

 because of the shallow depth of mixed layer. 



In figure 21 the April maximmn change of 

 sahnity of 0.09°/oo mos."' occurs at 23° X. (maxi- 

 mum (E—P) occurs at 20° N.). During August 

 the maximum change of 0.1%© mos.~' occurs at 22° 

 N., and in December the maximum of 0.04°/oo 

 mos.~i occurs at 13° X. During December 

 the change of salinity due to evaporation minus 

 precipitation is positive along the entire section 

 shown, whereas during April and August it is 

 negative south of 13° and 15° X^., respectively. 



The seasonal changes of salinity at 13°, 16°, 21°, 

 and 26° X^. (fig. 9) can now be examined in the 

 hght of the evaporation minus precipitation. At 

 each of these latitudes except 13° X., (E—P) is 

 positive throughout the year, so that dechning 

 salinities cannot be explained bj* excess precipi- 

 tation, but must be associated with movement of 

 the surface water. 



At 21° X. the salinity is dechning and reaches a 

 minimum when the monthly change of sahnity 

 due to (E—P) is rising and reaches a max-imum. 

 This phenomenon can be explained by the north- 

 ward movement of the high salinity gradient or 

 boundary described on page 382 (fig. 10). 



