THE MERIDIONAL TEMPERATURE DISTRIBUTION 



18° 20° 



NORTH LATITUDE 



Figure 2 .--Schematic presentation of the 

 meridional thermocline structure in the 

 trade wind zone based on the temperature 

 section at long. 148° W., February 1964. 

 W indicates a geostrophic flow setting west- 

 ward . 



In the trade wind zone of the North Pacific, the meridional 

 temperature distribution is basically defined by the charac- 

 teristic thermocline structure. The thermocline structure is 

 illustrated schematically in figure 2 by the 15° and 20° C. iso- 

 therms, which lie within the thermocline. Isotherms slope up- 

 ward towards lat. 10° N. where the minimum depth occurs in 

 this schematic example at lat. 10° N. Maximum depths of iso- 

 therms below 20° C. are north of lat. 20° N. In addition to the 

 upward sloping of the isotherms, the vertical distance between 

 isotherms becomes smaller toward lat. 10° N. 



Thus, as one progresses southward from lat. 20° to 10° N., 

 at a depth of 150 m. for example, the temperature decreases. 

 This meridional temperature gradient at 150 m. is not obvious 

 from the surface temperature, which increases toward the 

 lower latitude. Also, the temperature decrease with increas- 

 ing depth (the vertical temperature gradient) is greater at lat. 

 10° N. than at 20° N. 



This information can be usefully applied when longlining for 

 fish. Assume, for example, a fish which prefers a range of 15° 

 to 18° C. In fishing at lat. 20° N., fishermen must place the 

 hooks at greater depth than when fishing at 10° N. Also at lat. 

 20° N. the depth range within which hooks must be placed is 

 larger than at 10° N. where 15° and 18° C. water may be found 

 in depths which are less than 20 m. apart. 



The thermocline structure, which is evident from the tem- 

 perature sections, can also be used to estimate qualitatively 

 the east-west component of the ocean current. This use is 

 demonstrated in figure 3, showing schematically the thermo- 

 cline fluctuations at long. 157° W., in March 1964 and the re- 

 lated current directions. When the thermocline slopes upward 



