openings. Cnly two of these openings, Kauai Channel between Oahu and Kauai, and 

 Alenuihaha Channel between Hawaii and Maui, extend belovv the 1,000-m. depth that 

 has been assumed as the level of no motion in the computation of dynamic heights. 

 The third, Kaiwi Channel between Molokai and Oahu, has a sill depth of only 614 m. 

 (336 fathoms). The fourth, between Molokai and Maui, is almost negligible; it is 

 split by Lanai Island and has a sill depth of only 80 m. (44 fathoms) and a width 

 of only 8 miles. 



Sverdrup (1946, p. 723) states that "a clockwise rotating gyral is pre- 

 sent in the eastern North Pacific with its center to the northeast of the Hawaiiein 

 Islands. It is probable that the location of this gyral changes with the seasons 

 and shifts from year to year, so that occasionally the gyral may lie entirely to 

 the northeast of the Hawaiian Islands, whereas in other circumstances the Hawaiian 

 Islands may lie inside the gyral." This implies that the flow incident to the is- 

 lands is either toward the west or slightly northwest. It may be further assumed 

 that it is continually fluctuating, since the Eastern North Pacific High, which 

 provides the driving force, varies constantly in intensity and position. 



Further evidence of the seasonal variation in the incident currents and 

 the variations in the incident currents which might be expected during a single 

 month is shown by the charts of mean wind and mean currents (summarized in tables 1 

 and 2) for the area east of Hawaii. The average monthly current directions, which 

 were taken from the Pilot Charts, illustrate the annual cycle. The most northerly 

 average direction, 310°T, occurs in January, when the greatest percentage, 42 per- 

 cent, of easterly winds was reported. The most westerly direction (260 T) occurs 

 in September. This is also the period when the Eastern North Pacific High starts 

 to decline in intensity, and this could account for the relatively weak flow of 

 cruise 12. The greatest variation of both winds and currents occurs during the 

 November to March period, when the Eastern North Pacific High is least developed. 

 It is during this period that low pressure cells, such as those that were present 

 during cruises 1 and 12, move into the island area bringing the " kona "2/ storms 

 (Simpson 1952) . 



When the nature of the island profile and the westerly flow are consid- 

 ered, the quasi-permanent features can be ascribed to (1) a stream flow impinging 

 upon a lamina at a shallow angle, (2) flow around a single barrier (Long 1952), 

 (3) a jet or wake stream discharged into a fluid at rest (Rossby 1936), or (4) a 

 combination of (2) and (3). 



Although there was some flow through the channels, the middle group of 

 islands (Cahu-Maui) seemed very much like a solid barrier, so that the circulation 

 to the east resembled in many respects a "stream flow on a lamina" (Lamb 1932). 

 In such a case, the flow is split by the barrier; the point of separation and the 

 amount going in either direction are dependent upon the angle of impact. The in- 

 cident flow of both cruises 10 and 12 was from an easterly direction, so that the 

 major part of the flow was deflected to the right or northwest and only a small 

 part to the left or southeast. The easterly flow southeast of station 32 during 

 cruise 10 was the result of the segment of the incident current that was deflected 

 to the left. 



The angle of deflection or degree of anti-cyclonic motion imparted' to the 

 segment of the incident current that was deflected to the right controlled the flow 

 through Kauai Channel during cruises 10 and 12. During cruise 12, when the geo- 

 strophic currents north of Oahu had a strong northerly component, the flow was 

 northerly through the channel. During cruise 10, when the angle of deflection was 

 small at the surface and decreased still further with depth, the flow was across 

 the channel above the 200-decibar surface, where a slight westerly flow was in- 

 dicated. 



2/ Kona . a Polynesian adjective meaning "leeward," is used locally to describe 

 periods during which the usually persistent tradewinds are replaced by southerly 

 winds and rain squalls. Each year the drier leeward sides of the Hawaiian Islands 

 receive more than half of their rainfall from two or three of these storms. 



16 



