OCEANOGRAPHIC CLIMATE OF HAWAIIAN ISLANDS REGION 



383 



temperatures. In the vicinity of (he islands, 

 however, low and higli salinities occur ai)out three 

 months later and at a time of mininmm and 

 maximum depth of the mixed layer. One also 

 notes that the low salinities in the southeast 

 portion of the region occur in the fall, approxi- 

 mately where the depth of the mixed layer distri- 

 bution suggests a different oceanographic climate. 

 Finally, Midway Island is located in the 

 vicinity of the salinity maxunum, so that the 

 sharp decline of about 0.35 °/oo during July may 

 be due to either a soutliward or a northward 

 displacement of the high salinity cell. On the 

 basis of chart III and figure 8 however, keeping 

 in mind the scarcity of observations, the following 

 postulate is favored: during the spring, the high 

 salinity boundary moves northward east of 165°W. 

 and northwestward west of this meridian, reaching 

 Midway Island in July. 



4. THE GEOPOTENTIAL TOPOGRAPHY 



A fourth feature of importance in climatology 

 is the geopotential topography, which is analogous 

 to the pressure distribution in meteorology in that 

 it provides a measure of the geostrophic currents. 

 Again, data are inadecjuate to construct monthly 

 charts. There have been no oceanographic ex- 

 peditions to survey, as a primary objective, the 

 regidn under investigation here. Dynamic height 

 data have therefore been obtained chiefly from 

 the Bureau of Commercial Fisheries Biological 

 Laboratorj' (Honolulu) cruises in transit to other 

 regions and from isolated expeditions passing 

 through the area. Since potential gradients, 

 rather than absolute values of dynamic height, 

 are of importance in estimating geostrophic cur- 

 rents, cojubining isolated observations at different 

 times of the year and from different years may 

 result in charts materially different from the true 

 flow pattern. Nevertheless, on the basis of 

 availability and compatibilit^y, data for the months 



of June through October and those for the months 

 of December through April were combined into 

 a summer and winter chart (appendix B, chart 

 IV) to enable the reader to estimate the gross 

 distribution of geopotentials. 



Tlie summer ciiart indicates that the subtropical 

 convergence is approximately at 24° N., rather 

 than at 31° N. as indicated by Schott (1935). 

 It also shows that, as compared to the flow in the 

 southeast portion of the region and that just 

 south of the convergence between 158° and 

 170° W., there is a broad band of weak westerly 

 flow to the southwest of the islands. Comparing 

 the summer chart with the April to August 

 salinity chart (chart III), one notes that the 

 featm'eless geopotential distribution to the south- 

 west of the islands corresponds with an equally 

 featureless salinity distribution, both bounded by 

 higher gradients to the northwest and southeast. 

 It should be remembered that the combined data 

 of tlie salinity chart are 2 months out-of-phase 

 with those of the geopotential chart. 



During the winter months, the subtropical 

 convergence, as indicated in chart IV, (December 

 to April) again lies to the south of the latitude 

 indicated by Schott. In general, the area north 

 of 20° N. appears to be one of very little net 

 flow. In the southern portion of the region there 

 is a large counterclockwise vortex centered at 

 approximately 14° N. between 160° and 170° W. 

 This feature was first apparent in data from Hugh 

 M. Smith cruise 2, February 1950 (Cromwell, 

 1951, fig. 3) and occupies approximately the same 

 area as the dome in the depth of the mixed layer 

 during February (chart I). 



Thus, although the geopotential topography as 

 presented here should be regarded with caution, 

 compatibility of special features, both with the 

 salinity and the depth of mixed layer distributions, 

 lends credibility to these charts. 



PART II. PHYSICAL PROCESSES AND THEIR RELATION TO THE DISTRIBUTION 



OF SURFACE VARIABLES 



In the first section of this atlas, patterns of 

 distribution and regular periodic changes in the 

 fields of temperature, salinity, deptli of mixed 

 surface layer, and geopotential were described. 

 These features are associated to a varying degree 

 of complexity with physical processes. 



619237 O— 62 3 



The importance of one or another process is 

 usually not intuitively apparent. For example, 

 the temperature of the water obviously depends 

 on the heat exclumge across the sea surface and 

 the (leptli througli which this is distributed, the 

 manner in wliich heat is transported by currents, 



