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Figure 4. — Continued. 



The 30-Day Temperature Changes 



The lower panels of the figures in Appendix 1 show 

 contours of temperature change in the upper 200 m dur- 

 ing 30-day periods (of a 360-day yr). The maximum rate 

 of warming was 2°C/mo in June at the surface near 2,600 

 km, which is in the Transition Zone. The maximum rate 

 of cooling was just over 1.5°C/mo during November and 

 December in the same area. The rate of cooling was 

 smaller because the cooling takes place over a depth of at 

 least 50 m whereas the warming is confined to a shal- 

 lower layer of about 25 m. There was very little 

 temperature change at any depth throughout the section 

 from mid-March to mid-April, but there was no cor- 

 responding period in the fall. 



In the fall period the downward mixing of heat into the 

 upper thermocline as the surface cools is evident over 

 most of the route in the temperature changes (Appendix 

 1) and in the vertical profiles (Fig. 4a-g). Beginning in 

 August a subsurface maximum of warming appeared just 

 above 50 m throughout most of the section. The level of 

 maximum warming moved downward during the fall 

 reaching 100 m in December. During this time the sur- 

 face was cooling and a strong gradient of temperature 

 change developed between the surface cooling and the 



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subsurface warming. The maximum subsurface warm- 

 ing decreased as its depth increased with time, and sub- 

 surface warming essentially disappeared by February. 



The patterns of temperature change in the California 

 Current region differ from those over most of the section. 

 For example, from September through November and at 

 distances of 3,200 to 3,400 km along the route, cooling ex- 

 tended downward from the surface to 200 m, at least. 

 This created a break in the pattern of the warming max- 

 imum at 50 m, which existed over the rest of the section. 

 A secondary center of cooling below 100 m occurred at 

 2,700 to 2,900 km on the route. These changes were as- 

 sociated with the development of a wave pattern in the 

 isotherms along the permanent thermocline. The centers 

 of cooling were associated with a steepening of the slope 

 of the isotherms in the corresponding vertical sections, 

 whereas in between these centers the isotherms flatten 

 out. The steepening and flattening indicate a splitting of 

 the broad flow of the California Current into filaments of 

 stronger and weaker flow, respectively. The cooling pat- 

 tern propagated westward along the section at a speed of 

 about 100 km/mo (3.8 cm/s). 



There was a counterpart center of warming which ap- 

 peared in mid-December in the California Current region 

 (around 3,400 km and 90 m) and which could be followed 



