240 



I CHAP. 10 



remarkably high stability 1 (a thermocline or sprungschicht). The density 

 gradient is usually greatest immediately below the mixed layer and Reid (1948a) 

 has shown that a density vs. depth curve in the zone of the thermocline can be 

 approximated by an exponential function. The thickness of the well-mixed 

 surface layer varies from 1 to 150 m and in both the Atlantic and Pacific this 

 mixed layer becomes thicker toward the west. The characteristics of the mixed 

 layer and the thermocline in the equatorial Atlantic have been described in 

 great detail by Defant (1936). 



A. Stability 



The intensity of the density gradient is not the same throughout the tropical 

 regions. The maximum values are usually found in the vicinity of the counter- 

 current ; hence, there is on the average greater stability north of the equator 



5°S 



5°N 



15° 



20° 



Fig. 4. Maximum "stability" in the Pacific as a function of latitude. Each value is averaged 

 over 50 -m depth. The difference between these values and true stability is less than 

 1%. In other words true stability also varies between 1-10 x 10~ 7 /cm as shown in 

 this figure. (After Knauss, 1961.) 



than south of it. Furthermore the stability appears to be greater in the eastern 

 than the western halves of both the Atlantic and Pacific, although the difference 

 is more marked in the Pacific (Fig. 4) ; perhaps because in the equatorial 

 regions, the Pacific is three times the width of the Atlantic. In both the Atlantic 

 and the Pacific there is a marked stability minimum at the equator (for the 

 Atlantic, see in particular, Defant, 1936, pi. 40). In the Pacific, this minimum 

 has been associated with the Cromwell Current, and it is likely that a similar 

 current is associated with the minimum in the Atlantic. The maximum stability 

 in the Pacific appears to be centered along the north edge of the countercurrent 

 (Fig. 4). Defant's analysis of the Meteor data does not show this feature. This 



1 Stability is defined as U^\ - l^\ 

 refers to the adiabatic density gradient. 



p, where p is density and the subscript A 



