376 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



spread sufficiently to interrupt the main east-west 

 trough between 158° and 163° W. In general, 

 the depth of the mixed layer is decreasing through- 

 out most of the survey area, except along the 

 southern boundary, east of 162° W., where it is 

 increasing. Durmg April and May, the depth of 

 the mixed layer continues to decrease throughout 

 the survey area, except in the southern portion 

 where it is increasing. 



In June, a new distribution pattern of the depth 

 of the mixed layer has been established. A trough 

 is now located between 10° N. and 13° N., west 

 of 156° W., where the depth reaches approxunately 

 300 feet (91 m.). East of 156° W. the trough, 

 now 200 to 250 feet (61-76 m.) deep, turns north- 

 eastward. Over much of the area north of 20° N., 

 the mixed layer is less than 100 feet (30 m.) deep. 



The distribution of the depth of mixed layer 

 during July and August does not differ materially 

 from that durmg June. It shows little of interest 

 north of 20° N., where the depth has increased 

 but is still less than 150 feet (46 m.). The trough 

 in the southern portion of the survey region re- 

 mained between 10° and 15° N., except that it 

 now curves northeastward between 160° and 165° 

 W. with depths in excess of 300 feet (91 m.). 



In the southeast portion of the region the depth 

 has been steadily decreasing from about 200 feet 

 (61 m.) in June to less than 100 feet (30 m.) in 

 August. 



The September depth of mixed layer chart 

 shows that the trough has started, as it were, a 

 northward movement and is now centered about 

 15° N., west of 160° W. with depths between 250 

 (76 m.) and 300 feet (91 m.). 



In October, the trough has essentially resumed 

 its winter (January and February) position. The 

 depth north of 20° N. is increasing and the begin- 

 nings of the typical January and February struc- 

 ture are visible. This trend continues during 

 November and December, and throughout this 

 period the depth of mbced layer is shallow in the 

 southern portion of the region. 



The primary feature, then, of the distribution 

 of the depth of the mixed layer in the Hawaiian 

 region is a trough and seasonal differences in its 

 location. Maximum development of this feature 

 occurs in January, when the depth of the trougli 

 may exceed 400 feet (122 m.) and extends from 

 east to west between 15° and 20° N. The June 

 distribution typifies the other extreme, when it 



is found in its southernmost position. At this 

 time the depth is also at a minimum in the north- 

 ern portion of the region. It is interesting to note 

 that the converse to the gradual northward motion 

 of the mixed layer trough is not true during the 

 March-April-May transition period. During that 

 tune the irregular depth distribution suggests 

 disintegration of a pattern rather than a south- 

 ward movement. 



These features can also be illustrated by showing 

 meridional profiles of the depth of mixed layer. 

 The January and June profiles representing the 

 mean depth of the mixed layer between 153° and 

 161° W. are given in figure 2A and those for 168° 

 to 176° W. in figure 2B. The January profiles 

 are similar. They show ridges in the vicinity of 

 12°, 20°, and 26° N. and troughs at 17°-18° N. and 

 23° N. The western profile is generally deeper 

 than the eastern profile, except for the trough at 

 23° N. Both figures convey the impression of 

 standing waves in the southern portion of the 

 region, with nodes between 14° to 15° N. in the 

 eastern portion and 12° to 13° N. in the western 

 portion. 



Although January and June exhibit extremes 

 in the winter and summer distribution patterns, 

 the depth of mixed layer during those months is 

 not necessarily at the extreme in all portions of 

 the survey region. To illustrate this, the maxi- 



20- 25' 



NORTH LATITUDE 



Figure 2.— Meridional profile of the depth of mixed layer 

 in January and June, 10° to 30° N. Panel A, 153°- 

 161° W.; panel B, 168°-176° W. 



