OCEANOGRAPHIC CLIMATE OF HAWAIIAN ISLANDS REGION 



405 



temperature and salinity changes in the surface 

 water types. The position of their boundaries 

 would also be subject to changing wind stresses, so 

 that they would not always coincide with the 

 corresponding water-mass boundaries. 



The water tj-pes mentioned also form centers of 

 the large circulation systems of the North Pacific, 

 since one finds the major ocean currents on their 

 periphery. Thus, the currents are located in the 

 transition zones between the principal water types 

 and one would expect this to be reflected in the 

 changing composition of the water as it flows across 

 the Pacific. For example, a parcel of water start- 

 ing in the Kuroshio would be mixed with water 

 from the Oyashio and then in its passage across 

 the Pacific acquire new chemical characteristics 

 during possibly two or three seasons of winter 

 overturn. Then, this parcel of water would be 

 further modified, first in the California Current by 

 American coastal water and later, in the Cali- 

 fornia Current Extension, by North Pacific Equa- 

 torial water. Finally, southeast of the Hawaiian 

 Islands, one would expect its chemical composi- 

 tion to differ from both its original composition in 

 the Kuroshio and from that of the North Pacific 

 Central Water. 



This, of course, is supposition, since no support- 

 ing measurements of chemical tracers are available. 

 However, the relatively high salinity gradient at 

 35°/oo in the vicinity of the Hawaiian Islands 

 (fig. 10) can be interpreted as the boimdary be- 

 tween the Western North Pacific Water and the 

 transition water called here the California Cui'rent 

 Extension. 



The oceanographic environment of the Hawai- 

 ian Islands, therefore, corresponds with that de- 

 scribed by Sverdrup et al. (1942), except that they 

 based their analysis on a study of water masses, 

 in contrast to the water types considered here. 

 Thus, on the basis of Schott's (1935) temperature 

 charts, the North Pacific Equatorial water type is 

 distinct from the South Pacific Equatorial type. 



Sverdrup et al. also distinguish between the 

 Eastern North Pacific and the Western North 

 Pacific Central water mass. Surface salinity data 

 for the November to February period (chart III) 

 show only a single high salinity cell to extend east- 

 ward across the northern portion of the region and, 

 on the basis of winter data collected on Hugh M. 

 Smith cruise 25 (McGary, 1956), there is no evi- 

 dence of an Eastern North Pacific cell. The 



April to August surface salinities (chart III) show 

 two high salinity cells in the northern portion of 

 the area. This can be explained as a separation 

 of the single, winter cell caused by the deflecting 

 effect of the island chain on a westward setting 

 current. The surface water in tlie two high sa- 

 linity cells is therefore believed to be of the North 

 Pacific Central ty^pe. 



New information to be added to this general 

 picture is concerned with the three tj-pes of 

 boundaries described in part I. First, there was 

 the boundar}' separating areas in which the times 

 of maximum and minimum depths of mixed layer 

 differed (fig. 3). This is also coincided with the 

 node at 15° N. (fig. 2) and the associated, seasonal 

 displacement of the depth of niLxed la^^er trough 

 (chart I). Then there was a boundarj' at about 

 18° N. south of which the seasonal temperature 

 range remained relatively constant and north of 

 which it increased rapidly northward (fig. 5). 

 The relatively high salinity gradient of figure 10, 

 moving seasonally through the Hawaiian Islands, 

 was identified as the third type of boundary 

 separating two types of water. 



In order to interpret these features, a simplified 

 heat budget was formulated in part II. This 

 related the rate of change of surface temperature 

 with the processes of net heat exchange across 

 the sea surface and advection. The meridional 

 profiles of figures 17 and 18 revealed a boundary 

 south of which the net heat exchange across the 

 sea surface was positive throughout the }'ear and 

 north of which it was positive during the summer 

 and negative during the winter. The boundary, 

 located at about 18° N., coincided approximately 

 with the temperature boundarj'. The meridional 

 distribution of the net heat exchange across the 

 sea surface therefore appears to be associated with 

 the seasonal changes in the meridional tempera- 

 ture distribution described in part I. 



On the basis of the net heat exchange (fig. 15), 

 one would also expect ma.ximum and minimum sea 

 surface temperatures to be reached in November 

 and April, respectively. Figure 7 shows that the 

 maximum temperature at 20° N. is reached in 

 September and the minimum in March, illustrating 

 that these times are primarily determined b}' the 

 net heat exchange across the sea surface. How- 

 ever, particularly in autumn, there is an important 

 phase difference attributable to advection. 



