and selected vertical sections of the temperature 

 distribution, with individual XBT profiles, have been 

 published regularly in Fishing Information 5 since March 

 1972. 



The ship routes between Honolulu and U.S. west coast 

 ports cross the eastern limb of the major anticyclonic 

 gyre of the North Pacific Ocean. If we confine our atten- 

 tion to the upper ocean, from the surface to a few hun- 

 dred meters, we can identify three oceanic regimes: the 

 California Current and the Eastern North Pacific Cen- 

 tral waters separated by a Transition Zone (Fig. 1). 



The waters in the California Current are mainly cooler, 

 lower salinity waters of subarctic origin that are modified 

 in their slow southeastward movement along the Cali- 

 fornia coast. The Eastern North Pacific Central waters 

 are warmer, higher salinity waters that occupy about the 

 southwestern one-half of the route. 



The Transition Zone is a complex region, not yet fully 

 understood. In our region of interest it is bounded on the 

 south and southwest by the subtropical front (Roden 

 1971, 1975). On the north and northeast it is bounded, re- 

 spectively, by the subarctic front (Dodimead et al. 1963) 

 and some type of southeastward extension of this fea- 

 ture, which LaFond and LaFond (1971) called the Cali- 

 fornia Front. Saur (1974) described criteria for identify- 

 ing these regimes from the XBT profiles, changes in 

 slopes of isotherms in the vertical sections, and accom- 

 panying surface salinity observations. Laurs and Lynn 

 (1977) discussed features of the Transition Zone from 

 oceanographic observations made in June of several dif- 

 ferent years by fishery research vessels. 



Mean temperatures presented here provide a base for 

 study of temperature anomalies (Dorman and Saur 1977, 

 1978) and for further research on the relation of tempera- 

 ture variability to air-sea interaction and the changing 

 environment of marine organisms. 



METHODS 



Observations 



The time-distance distribution of XBT observations 

 for the period June 1966 through December 1974 is shown 

 in Fig. 2. The great circle distance from a reference point 

 near Oahu was used for location. About 90^ of the obser- 

 vations were made by ships on the great circle route. 

 Some departures from the great circle track resulted 

 from storms and the fact that tankers of Chevron Ship- 

 ping Company generally followed a rhumb line (constant 

 heading) course. For these observations taken at loca- 

 tions displaced from the usual route by 100 to 150 km, 

 the use of great circle distance from Oahu tends to 

 minimize temperature errors, because the general orien- 

 tation of isotherms in the upper layers is northwest- 



^Fishing Information is a National Marine Fisheries Service monthly 

 publication, containing fisher.' advisory information and environmental 

 charts for the equatorial and North Pacific Ocean. It is compiled and dis- 

 tributed by the Southwest Fisheries Center, National Marine Fisheries 

 Service. NOAA. P.O. Box 271. La Jolla. CA 92038. 



1966 



1967 



1968 



1969 



1970 



1971 





. -:■:-: ; 





1972 



1973 



1974 



Figure 2. — The time-distance distribution of XBT observations on or 

 near the San Francisco-Honolulu route from June 1966 through 

 December 1974. Location of an observation is measured by its great 

 circle distance from an offshore reference point (lat. 21°12'N, long. 

 157°42'W) near Honolulu. 



southeast. The San Francisco end of our section is a point 

 on the edge of the continental shelf a short distance 

 south-southwest of the Farallon Islands and 3,800 km 

 (2,050 n.mi.) from the reference point. 



With the exception of the first year and one-half when 

 only four observations per day were scheduled, the XBT 

 observations were taken on a 4-h schedule related to the 

 ship's watch, rather than at prespecified "stations." 

 Thus the location of observations along the route differs 

 from one section to another. Also, the distance between 

 observations depended upon the ship's speed. Of those 

 ships cooperating in the program, normal speeds were 

 either about 16 to 17 kn or about 22 kn, so that the dis- 

 tance between observations was about 120 km (65 n.mi.) 

 or 165 km (90 n.mi.), respectively. The slower ships would 

 generally get 27 to 30 observations per transit and the 

 faster ships about 17 to 20 observations. A few sections 

 with more closely spaced observations for special studies 

 were made when scientific personnel were aboard. 



The frequency of sections reflects the growth and 

 change in character of the project. With the exception of 

 six sections made by oil tankers in the summer of 1970, 

 all of the observations from the beginning of the project 

 in June 1966 through January 1971 were made from one 

 vessel, Californian, a bulk-cargo and container vessel of 

 Matson Navigation Company. This 17-kn ship made a 

 round trip about every 18 to 21 days, generally making 

 observations on one 5-day leg only. During this period 

 several gaps of 4 to 8 wk duration occurred because of 

 ship repair schedules, short labor strikes, and equip- 

 ment failures. 



