KOKO HEAD, OAHU, SEA-SURFACE TEMPERATURES AND 



SALINITIES, 1956-73, AND CHRISTMAS ISLAND 



SEA-SURFACE TEMPERATURES, 1954-73 



GUNTER R. SECKEL 1 AND MARIAN Y. Y. YONG 2 



ABSTRACT 



Sea-surface temperatures and salinities have been collected twice weekly at Koko Head, Oahu, 

 Hawaii, since 1956; and at Christmas Island in the central equatorial Pacific, sea-surface temperatures 

 have been collected daily since 1954. In 1971, Seckel and Yong used harmonic analysis as a curve- 

 fitting method to bring these observations, 1 yr at a time, through 1969, into a form useful for 

 descriptive and numerical applications. In this paper the analyses are updated through 1973 and 

 the method is used to describe the entire data series. 



The data series have been separated into several scales of variability: long-term variability 

 (periodicities larger than 1 yr), short-term variability (12-mo and shorter periodicities), average 

 annual cycle (the 12-, 6-, 4-, and 3-mo periods), and the residual variability that characterizes 

 individual years (the short-term variability with the annual cycle removed). In contrast to the 

 Koko Head temperature where the annual cycle predominates, the interannual variability pre- 

 dominates, at times obscuring the annual cycle, in the Koko Head salinity and Christmas Island 

 temperature. The interannual change of the Koko Head salinity can be about three times, and that 

 of the Christmas Island temperature can be about four times the average annual variability. In the 

 average annual temperature and salinity cycles at Koko Head the amplitudes of the 6-, 4-, and 

 3-mo periods are small in relation to the 12-mo period. In the average annual temperature cycle 

 at Christmas Island, however, the amplitude of the 6-mo period is almost one-half that of the 12-mo 

 period. The residual variations exhibit changing amplitudes and periodicities at intervals of more 

 than 1 yr that resemble amplitude and frequency modulations. 



Speculations are made about processes that contribute to the temperature and salinity variations. 

 It appears that in addition to the heat exchange across the sea surface, advection contributes 

 materially to the observed changes at Koko Head and Christmas Island. 



Harmonic coefficients resulting from the analyses are listed in the appendices to facilitate repro- 

 duction of the data presented. 



In an earlier paper, Seckel and Yong (1971) used 

 harmonic analysis as a curve-fitting method, 

 bringing rapidly into usable form regularly sam- 

 pled sea-surface temperatures and salinities. 

 Analyses were made of sea-surface temperature 

 and salinity obtained once or twice weekly from 

 1956 to 1969 at Koko Head, Oahu (lat. 21°16'N, 

 long. 157°41'W,), and of sea-surface temperature 

 obtained daily from 1954 to 1969 at Christmas 

 Island (lat. 1°51'N, long. 157°23'W). The tempera- 

 ture and salinity variations for each year were 

 then specified by sets of harmonic coefficients 

 and phase angles. Values calculated at 15-day 

 intervals from the resulting annual functions 



'Southwest Fisheries Center Pacific Environmental Group, 

 National Marine Fisheries Service, NOAA, c/o Fleet Numerical 

 Weather Central, Monterey, CA 93940. 



2 Southwest Fisheries Center Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 3830, Honolulu, 

 HI 96812. 



were used in long-term analyses of the entire data 

 records. These analyses showed that interyear 

 differences in the Koko Head salinity and Christ- 

 mas Island temperature were larger than sea- 

 sonal changes. 



The long-term changes in surface properties 

 reflect climatic scale ocean-atmosphere processes 

 and, in turn, affect these processes. The changes 

 in properties and processes affect life in the sea. 

 For example, the Koko Head salinity changes 

 indicate primarily changes in the advection pro- 

 duced by variations in ocean circulation (Seckel 

 1962). It was postulated that changes in circula- 

 tion also affect the concentration and, therefore, 

 the availability of skipjack tuna caught in Hawaii 

 (Seckel 1972). 



The long-term changes in the Christmas Island 

 temperatures are linked with large-scale (at 

 least ocean-wide) ocean-atmosphere processes. 

 Bjerknes (1969) related anomalously high tern- 



Manuscript accepted March 1977. 



FISHERY BULLETIN: VOL. 75, NO. 4, 1977. 



767^ 

 9&? 



