378 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Figure 4. — Seasonal variation of the depth of the mixed 

 layer at 12°, 18°, 20°, and 22° N., 153°-16r W., and at 

 27° N., 175° W. 



pared for the Hawaii region, using the same 

 bathy thermograms mentioned in the previous 

 section. Treatment of these data, and the manner 

 of construction of the charts is discussed in ap- 

 pendix A. 



The introductory comments of the previous 

 section deahng with the distribution of the depth 

 of the mixed surface layer also apply here. At- 

 tention will therefore again be focused on gross 

 features in the distribution of the surface temper- 

 ature rather than the details of individual con- 

 tours. For example, if the irregular isotherm 

 pattern to the southeast of the island chain in 

 winter (chart II) were to be smoothed, the average 

 temperature and its distribution would correspond 

 well with the charts of Schott (1935) and those of 

 the U.S. Hydrographic Office. It is beUeved, 

 however, that these irregularities, persisting for 



several months, reflect large-scale mixing proc- 

 esses or eddies which may be real in nature. 



Scanning the temperature charts one notes a 

 parallel isotherm structure north of the island 

 chain between November and April, with a lati- 

 tudinal temperature gradient in the order of 1° to 

 1 .5° F. (0.6° to 0.8° C.) per degree of latitude. In 

 May, the beginnings of a change in the tempera- 

 ture structure become apparent, followed bj' the 

 breakdown of the parallel isotherm pattern in 

 June. The irregular temperature structure per- 

 sists in the northern section of the region through- 

 out July, August, and September, until, in October, 

 isotherms become parallel again. 



To the south of the island chain, the tempera- 

 ture structure is irregular during the winter and 

 spring months. The temperature pattern from 

 December through March, between 160° and 165° 

 W., is suggestive of large-scale mixing processes 

 or eddies. 



The July, August, and September distributions 

 show tongue-shaped areas of low temperature 

 protruding westward in the southern portion of 

 the region. Then, during October and November, 

 it appears as if these tongues, between 155° and 

 165° W., were given a counterclockwise, 90° rota- 

 tion, followed by disintegration into the irregular 

 winter pattern. 



The charts also show that minimum and maxi- 

 mum temperatures generally occur in March and 

 September. In some areas, the minimum may 

 occur in February and the max-imum in either 

 August or October. For these exceptions, how- 

 ever, the temperatures differ little from those in 

 March and September. The March and Septem- 

 ber mean temperature profiles in figures 5A and B 

 for the meridional strips between 153° to 161° W. 

 and 168° to 176° W. therefore represent those for 

 minimum and maximum temperature conditions. 



During March, the temperature in the eastern 

 section of the survey region (fig. 5A) decreases 

 from about 78° F. (25.6° C.) at 10° N. to 75° F. 

 (23.9° C.) at 17° N., at a rate of a httle more 

 than 0.4° F. (0.2° C.) per degree of latitude. 

 After a slight increase of temperature just to the 

 south of the Hawaiian Islands (also see chart II, 

 March), the temperature decreases again 

 northward to about 67° F. (19.4° C.) at 29° N., at 

 an average rate of 1° F. (0.6° C.) per degree of 

 latitude. 



In the western section (fig. 5B), the March 



