toward the south, there is a component of flow to the west, and 

 when it slopes downward toward the south, the component of 

 flow is to the east. The steeper the slope, the faster the cur- 

 rent. Thus (in fig. 3), the westward flow between lat. 19° and 

 20° N. is faster than that between lat. 14° and 18° N.. which in 

 turn, is faster than the flow between lat. 10° and 20° N. in fig- 

 ure 2. When the thermocline slope is small, the flow direction 

 is indeterminate. The convention for the direction of flow 

 shown in these figure applies to the Northern Hemisphere. 



THE MERIDIONAL SALINITY DISTRIBUTION 



The definition of water masses and their distribution in the 

 trade wind zone which was given above is principally based 

 on the salinity distribution. Thus, lateral or vertical changes 

 in the position of salinity isopleths (lines of equal salinity) also 

 may indicate changes in the lateral distribution and thickness 

 of water masses. This relation holds particularly for those 

 isopleths that are associated with water mass boundaries. 



Consider, for example, the salinity sections tor May and 

 October 1964 at long. 148° W. At the sea surface one finds the 

 boundary of the North Pacific Central Water in May at lat. 22° 

 N. and in October at lat. 18' N. The North Pacific Equatorial 

 Water is not present during May but is found at 17° N. in Octo- 

 ber. Displacement distances of the subsurface boundary of the 

 North I'acilic Central Water (at or just below 100-ni. depth) 

 are similar. This boundary is at lat. 12° N. in May and at lat. 

 17° N. in October. 



The thickness ol the North Pacific Central Watei- and North 

 Pacific Intermediate Water masses is greater in the northern 

 portion of the sections than in the southern portion. In the 

 northern portion the North Pacific Central Water extends Ironi 



NORTH LATITUDE 



Figure 3. --Schematic presentation of the 

 meridional thermocline structure and east or 

 west direction of the ocean current in the 

 trade wind zone based on the temperature 

 section at long. 157° W. , March 1964. 



