Annual Cycles 



At the surface the annual period is predominant at all 

 locations (Fig. 3a-g). The annual range was smallest 

 (about 4 C C) near Oahu, largest (about 7°C) in the Tran- 

 sition Zone, and again smaller near the California coast 

 (about 5°C). From near Oahu to the California front, the 

 cycles at 50 m diminished in amplitude and the summer 

 maximum lagged that at the surface by 1 to 2 mo. At the 

 low salinity core of the California Current (Fig. 3f) the 

 summer maximum penetrated almost simultaneously 

 from the surface to 200 m. In the inshore area, California 

 Current (Fig. 3g), the temperature range at 50 m was 

 small (about 1°C). Here the minimum and the maximum 

 temperatures lagged those at the surface by about 4 mo, 

 and appear to be related to the occurrence of upwelling 

 and the subsurface countercurrent, respectively. 



Mixed Layers and Thermoclines 



The surface mixed layers reach their maximum depth 

 in winter, mid-February through early April, Appendix 

 1. Depths of the mixed layers were generally at least 100 

 m, except they decreased to 75 m near California. They 

 were deepest, about 150 m, in the central part of the sec- 

 tion (2,000 to 2,200 km) in the neighborhood of the sub- 

 tropical front. 



We consider the permanent thermocline to be the 

 region of the maximum vertical temperature gradient in 

 winter (January through March) ; vertical sections of Ap- 

 pendix 1 and profiles of Figure 4. In the western half of 

 the section it was deeper (200 to 250 m) and warmer (15° 

 to 17 °C) than in the California Current region where it 



lay at depths of 100 to 120 m and had temperatures of 11 ° 

 to 13°C. The seasonal thermoclines are formed by warm- 

 ing in spring and summer (May through September) and 

 are generally confined to the upper 50 to 100 m which are 

 vertically mixed in winter. In the California Current 

 region the seasonal thermocline merged with the perma- 

 nent thermocline into a single feature, whereas in the 

 Transition Zone the two thermoclines were separated in 

 the spring by temperature inversions (next section) and 

 later by a near thermostad (vertically isothermal) layer, 

 Figure 4d. The latter was particularly evident in the 

 summer (July through August) sections by the steeper 

 slope of the 15°-19°C isotherms at depths from 50 to 150 

 m at distances of 2,000 to 3,000 km from Honolulu. In the 

 Eastern North Pacific Central waters from Honolulu to 

 near 1,800 to 2,000 km along the route, a layer of weak 

 vertical temperature gradient occurred between the 

 seasonal and permanent thermoclines. 



Temperature Inversions 



A characteristic feature found by Saur (1974) in the 

 individual profiles in the Transition Zone (between 2,000 

 and 3,000 km from Honolulu) was the occurrence of com- 

 plex vertical thermal structure, especially in the spring 

 months. The thermocline would often be interrupted by 

 isothermal layers and temperature inversions appeared 

 in some profiles. These were attributed to interleaving, 

 by horizontal mixing, of layers of cool, low-salinity water 

 with warmer, higher salinity water of nearly the same 

 density. These features usually are relatively small scale 

 and transient, so that during our computation of means, 

 they were generally smoothed out. However, some 



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Figure 3. — Station position chart and mean temperature cycles at selected depths (meters) for seven typical locations, great circle distances 

 from offshore reference point (lat. 21°12'N, long. 157°42'W) near Honolulu, and geographic coordinates: a. Near Oahu: 185 km (100 n.mi.i: 

 lat. 22 C 10'N, long. 156°15'W. b. Eastern North Pacific Central Water: 1,390 km (750 n.mi.); lat. 28°17 r N, long. 146°20'W. c. Subtropical front: 

 2,130 km (1,150 n.mi.); lat. 31°39'N, long. 139°42'W. d. Transition Zone: 2,500 km (1,350 n.mi); lat. 33U2N. long. 136°12'W. e. California 

 front: 2,870 km (1,550 n.mi.); lat. 34°39'N. long. 132°35'W. f. Low salinity core. California Current: 3,430 km (1,850 n.mi.); lat. 36°36'N, long. 

 126°55'W. g. Inshore region, California Current: 3,615 km (1,950 n.mi.); lat. 37°12'N, long. 124-59'W. 



