WATER MASSES AND CURRENTS OF THE OCEANS 1 83 



This picture shows a gyral in the South Atlantic Ocean and demonstrates 

 the complicated character of the currents near the Equator, where a 

 countercurrent toward the east, the Equatorial Countercurrent, is 

 imbedded between the equatorial currents of the two hemispheres. 



Evidently the countercurrent is related to the distribution of mass, as 

 is demonstrated by the slope of the discontinuity surface. In a profile 

 from south to north the discontinuity surface rises toward the Equator, 

 reaches a maximum elevation at the Equator, and drops toward a 

 minimum in 2° to 3°N latitude at the northern boundary of the South 

 Equatorial Current. Between 2° to 3°N and 6° to 8° N the discontinuity 

 surface rises toward the north, reaching a second and more pronounced 

 maximum in the latter latitude. This is the region of the countercurrent, 

 and to the north of the second maximum, where the surface slopes down- 

 ward again, the North Equatorial Current is found. This distribution of 

 mass was first recognized by means of the Carnegie observations in the 

 Pacific (fig. 52, p. 194). 



The dynamics of the countercurrent has recently been discussed by 

 Montgomery, who writes: 



The trade winds, by continually exerting a westward stress on the sea 

 surface, produce a westward ascent of sea level along the Equator. This 

 slope amounts to about 4 cm per thousand km, and the accompanying pres- 

 sure gradient extends down to about 150 m in the Atlantic Ocean. The 

 Equatorial Countercurrents are found in the doldrums and apparently 

 result simply as a downslope flow in this zone where the winds maintaining 

 the slope are absent. 



The ascent of sea level from east to west is due to a greater accumula- 

 tion of light surface water along the coasts of South America. Fig. 49 

 shows that such accumulation exists, because the thickness of the homo- 

 geneous surface layer above the discontinuity surface increases from less 

 than 40 m in the east to about 140 m in the west. Above a depth of 150 

 m the isosteric surfaces, therefore, slope downward from east to west, but 

 below 150 m they are practically horizontal. Accordingly, Montgomery 

 and Palmen find that at the Equator the dynamic height of the sea 

 surface, referred to the 1000-decibar surface, is 14 dyn/cm greater in the 

 west than in the east, but that the 150-decibar surface is parallel to the 

 1000-decibar surface. The countercurrent is therefore a very shallow 

 current that is confined to the surface layer above the discontinuity. 



A smft current that is embedded between water masses moving in the 

 opposite direction must be subject to a considerable retardation because 

 of friction. A certain amount of energy is therefore needed for maintain- 

 ing such a current, and this energy, according to Montgomery and 

 Palmen, is derived from the trade winds, which maintain the slope of the 

 sea surface. Montgomery and Palmen assume that the retarding friction 

 may be due to lateral mixing with the westward-flowing equatorial 



