OCEANOGRAPHIC CLIMATE OF HAWAIIAN ISLANDS REGION 



375 



temperature or below tlie sea surface. It is use- 

 ful, then, to define a verticiil tenipcratiu-e gradient 

 which separates the mixed surface lajer from 

 the thermodine. A gradient of 3.6° C. per 100 

 in'. (1° F. per 50 ft.), which apparently is of sig- 

 nificance in connection with underwater sound 

 properties,^ has been used by the Scripps In- 

 stitution of Oceanography. By examining tem- 

 perature- and salinity-depth curves it was found 

 that this gradient was too large and that the 

 apparent depth of the surface layer in many 

 cases was well within the haloclme. Lumby 

 (1956) took as the criterion for the homogeneous 

 layer the depth of the isothermal layer as judged 

 by the absence of temperature gradients in ex- 

 cess of 1° C. per 100 m. Upon examination, it 

 was found that the apparent depth of the mixed 

 layer, using Lumby 's criterion, was in good agree- 

 ment with the depth of mixed layer as determined 

 from temperature and salinity depth curves. 



To summarize, it was shown that the change 

 from neutral stability in the mixed surface layer 

 to high stability in the pycnocline was abrupt 

 and that the surface separating the two layers 

 was one of dATiamic significance. For this rea- 

 son, the eflective depth through which energy 

 changes due to processes at the sea sm-face were 

 distributed was assumed to be the mixed surface 

 layer. 



Further examination of figure 1 showed that 

 the vertical density distribution is primarily a 

 function of the vertical temperature distribution. 

 The depth of the mixed layer could therefore be 

 determmed from temperature deptli cm'ves by 

 using the first knee below the surface (excluding 

 the diurnal thermodine) and Lumby's criterion 

 that the isothermal layer be judged by the ab- 

 sence of temperature gradients in excess of 1° C. 

 per 100 m. 



B. DISTRIBUTION IN THE HAWAIIAN ISLANDS 

 REGION 



The definition of the mixed surface layer and 

 an understandmg of the vertical density and 

 temperature relationships enable us to use bathy- 

 thermograph observations in the Hawaiian region 

 to determine the areal distribution of the depth 

 of mixed layer. Thus, the pertuient data were 

 taken from several thousand ba thy thermograms 

 and assembled into monthly charts showmg the 



' Porsonal communication, Mrs. Margaret Robinson, Scripps Institution 

 of Oceanography. 



619237 C^-^2 2 



distribution of the depth of the mixed layer 

 (appendix B, chart I). The origmal data, their 

 treatment, and manner of construction of the 

 charts is described in appendix A. 



Here, it should be mentioned that those charts 

 have been based on data which were collected 

 for diverse purposes at um-elated times. Informa- 

 tion to be gained from them, therefore, is limited 

 and attention will be focused on gross features 

 in the distribution of the depth of mixed layer 

 rather than details of individual contours. Even 

 though many undulations in contours are believed 

 to be due to shortcomings in the data, they have 

 not been smoothed unless warranted by some 

 evidence. One can say, however, although un- 

 dulations in depth of the mixed layer contours 

 may not reflect an actual situation, they do point 

 to the fact that the ocean is not smooth and 

 changes do not occur at regular time intervals. 

 In that respect, then, they reflect reality. 



A prominent feature in the January and Feb- 

 ruary charts (chart I) is a trough in which the 

 depth of the niLxed la.yer is greater than in sur- 

 rounding areas extending east to west between 

 15° and 20° N. Its depth is generally greater 

 than 250 feet (76 m.), exceeding 400 feet (122 m.) 

 west of 165° W. Another trough, centered about 

 165° W., extends from 30° X. southward and then 

 eastward between the high islands and 25° N., 

 where its depth is greater than 350 feet (107 m.). 

 To the north of this feature is a shallow area, 

 with a mixed layer depth of less than 200 feet 

 (61 m.). The two troughs are separated by a 

 ridge of less than 250 feet (76 m.) and a mininmm 

 depth of less than 150 feet (46 m.) in the north- 

 west portion of the area. 



South of the main east-west trough, between 

 10° and 15° N. and east of 175° W. (January), 

 the depth of mixed layer is as shallow as 150 

 feet (46 m.) or less. In February, the shallow 

 area is reduced in size by the deepening of the 

 mixed layer west of 167° W. and along the south- 

 ern boundary of the region. Also of interest 

 here is the formation of a shallow "dome" at 

 15° N. and 163° W. 



Charts for the months of March, April, and 

 May, indicate a period of change. In March, 

 although the Januarv-February features are still 

 evident, a change in the depth distribution of the 

 mixed layer is beginning to take place. The small 

 dome of February, at 15° N. and 163° W., has 



