Vertical Sections of Semimonthly Mean Temperature 



on the San Francisco-Honolulu Route: From 



Expendable Bathythermograph Observations, 



June 1966-December 1974 



J. F. T. SAUR, ' L. E. EBER, 2 D. R. McLAIN, 3 and C. E. DORMAN 4 



ABSTRACT 



Frequently repeated sections of expendable bathythermograph observations between San Fran- 

 cisco and Honolulu, taken by merchant vessels during the period June 1966 through December 1974, 

 were analyzed to obtain mean seasonal cycles. Results are depicted in a set of semimonthly vertical 

 sections of mean temperatures to 500 m and in a set of corresponding sections of 30-day mean 

 temperature changes to 200 m. In addition, seasonal cycles at selected depths are included along with 

 mean monthly vertical profiles for seven typical locations along the route. 



The analyses reveal geographic and temporal facets of the mean thermal structure, including: 1) 

 depth of the surface mixed layers in winter, 2) growth and decay of the seasonal thermocline, 3) 

 decrease in depth of the permanent thermocline from Oahu to the California coast, 4) a region of 

 temperature inversions or very weak vertical temperature gradients that develops between 50 and 100 

 m during the spring in the Transition Zone, and 5) the location and movement of warming and cooling 

 regions during the year. 



Vertical mixing appears to be the dominant process along most of the route for transmitting the 

 annual surface warming and cooling cycle downwards to depths of 100 to 150 m. However, advective 

 processes are active in the California Current. 



Tables of semimonthly mean temperatures are given in an Appendix. 



INTRODUCTION 



Vertical sections of mean subsurface temperatures 

 from the surface to 500 m, presented here, were derived 

 from a time-series of sections of expendable bathy- 

 thermograph (XBT) observations made from June 1966 

 through December 1974 by merchant ships between San 

 Francisco, Calif., and Honolulu, Hawaii (Fig. 1). The 

 observational program was developed by Saur and the 

 data collected under the direction of the National 

 Marine Fisheries Service (NMFS). With technical as- 

 sistance from the Fleet Numerical Weather Central 

 (FNWC), XBT systems were placed on merchant ships 

 and observations were made routinely by the ship's 

 mates. Saur and Stevens (1972) described the XBT 

 system, observational procedures, and early projects for 

 obtaining observations from cooperating ships. 



Collection of subsurface temperature observations on 

 the San Francisco-Honolulu route began when the first 

 production models of the XBT system became avail- 

 able. The work started as a 1- to 2-yr feasibility and 



'Southwest Fisheries Center La Jolla Laboratory, National Marine 

 Fisheries Service, NOAA, La Jolla, Calif.; present address: Scripps In- 

 stitution of Oceanography, La Jolla, CA 92093. 



2 Southwest Fisheries Center La Jolla Laboratory, National Marine 

 Fisheries Service, NOAA, La Jolla, CA 92038. 



'Pacific Environmental Group, National Marine Fisheries Service, 

 NOAA, Monterey, CA 93940. 



'Department of Geological Sciences, San Diego State University, San 

 Diego. CA 92182. 



development project on the use of the system aboard 

 merchant vessels. It was then continued as an ocean 

 monitoring project, and is now a part of a coordinated 

 program among FNWC, NMFS, and NORPAX (North 

 Pacific Experiment) programs to obtain XBT observa- 

 tions in the Pacific. The data are now routinely collected 



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Q> Howaiion Islands 



Figure 1.— Three great circle routes between Honolulu 

 and U.S. west coast ports, on which frequent XBT obser- 

 vations have been made by cooperating merchant ships, 

 and a schematic representation of the three upper ocean 

 regimes in the area. Mean subsurface temperatures 

 reported here are for the San Francisco-Honolulu route 

 for which the longest time series — starting in June 1966 — 

 exists. 



