OCEANOGRAPHIC CLIMATE OF HAWAIIAN ISLANDS REGION 



407 



30- 



20* 



15' 



10* 



Figure 28. — Advection charts: A. Displacement of the 

 June-July 26° C. "intrinsic" isotherm and schematic 

 spring displacement of the 35°/oo isopleth. B. Displace- 

 ment of the October-November 26° C. "intrinsic" 

 isotherm and schematic autumn displacement of the 

 35°/oo and 34°/oo isopleths. 



circulation systems. This can be explained by 

 the fact that since the Hawaii region is neither 

 one of extreme evaporation nor precipitation, the 

 surface salinity is aflfected less by evaporation 

 miims precipitation than the temperature is by the 

 heat exchange. In other words, in the Hawaii 

 region the surface salinity is less "non-conserva- 

 tive" than the temperature. The salinity bound- 

 ary passing seasonally through the islands is 

 therefore not obsciu'ed by the events at the sea 

 surface, which is the case with an "intrinsic" 

 temperature boundary that would exist between 

 the Western North Pacific water and the California 

 Current Extension. Of course, faint traces of a 

 temperature boundary similar to the salinity 

 boundary were noted in the zonal temperature 

 distribution (fig. 6). 



The seasonal variations in the heat advection 

 appear to be closely related to climatic features in 

 the distribution of the surface depth of mixed 

 layer which probably reflect changes in the wind 

 stresses at the sea surface. Advective peaks also 

 coincide with clumges in salinit}* which must be 

 attributed to salt advection. The temperature 

 and salinity gradients, however, differ both in 

 magnitude and direction, so that one would not 

 expect isopleths of salinity to move in the same 

 direction as the isotherms (fig. 28). 



It is now possible to add seasonal, climatic 

 changes to the circulation model which was 

 schematically illustrated in figure 27. During 

 spring and early summer, the California Current 

 system intensifies and as a result the California 

 Current Extension spreads and displaces the 

 North Pacific Central water within the Hawaiian 

 Islands. The peak displacement of this water, 

 as illustrated by the 35°/oo isopleth in figure 

 8, is reached during July and coincides with 

 the June-July heat advection peak. In late sum- 

 juer and early autumn, as the California Current 

 system weakens, the transition type of water of 

 the California Current Extension retreats and is 

 again replaced by the higher salinity North Pa- 

 cific Central type of water. This movement of 

 the boundary between the two types of water is 

 associated with the October-November head ad- 

 vection peak. Concurrently with the weakening 

 of the Cahfornia Current system. North Pacific 

 Equatorial water spreads into the southeast por- 

 tion of the survey region, as reflected both in the 

 sharp salinity decline and the major October- 

 November heat advection peak for that area. 



During this period the surface temperatm-e 

 charts also show a counterclockwise rotation of 

 the tongue-shaped lower temperature water south 

 of the islands from the July position to the No- 

 vember position (chart II). In addition, there is 

 an appearent southward displacement of the depth 

 of the mixed layer boundary in the western por- 

 tion of the region. Those features can be inter- 

 preted to mean that, as the CaUfornia Current 

 Extension weakens, its recirculation into the 

 Countcrcurrent shifts from west of 175° W. to 

 between 160° and 170° W. 



In the Hawaii region the February to April 

 period appears to be one of relaxation. In the 

 northern half, the parallel structure of isotherms 

 probably reflects negligible net current motion, as 



