SECT. 3J EQUATORIAL CURRENT SYSTEMS 247 



Using the above criteria as indicators of the current, it can be shown that, 

 although the Cromwell Current appears to be best developed in the central 

 eastern Pacific, it can at times be traced as far west as 150°-160°E. On the 

 basis of the Meteor work (Defant, 1936; Wattenberg, 1936) a similar current 

 can be traced along the equator in the Atlantic. 1 The observations from the 

 Indian Ocean are not yet sufficient to indicate from the distribution of proper- 

 ties that a similar current exists there. The mechanisms which control this 

 current are not yet clear; however, most attempts to date require a zonal 

 pressure gradient commensurate with an upward slope of the sea surface to 

 the west (Fofonoffand Montgomery, 1955; see also several articles in Deep-Sea 

 Res., 6, no. 4, 1960). As noted before, the sea surface slopes up to the west in 

 the Atlantic and Pacific, but slopes up to the east in the Indian Ocean (Figs. 5 

 and 6), which would indicate, assuming these theories are correct, that there 

 should not be a similar current in the Indian Ocean. 



B. On the Accuracy of the Geostrophic Equation in Equatorial Waters 



Because the Coriolis force approaches zero at the equator, it has long been 

 of interest to oceanographers to know how close to the equator the geostrophic 

 equation can be used. The core of the Cromwell Current, at a depth of 100 m, 

 appears to be in approximate geostrophic balance even to within half a 

 degree of the equator. More recently a comparison of direct current measure- 

 ments with hydrographic observations has shown at least partial geostrophic 

 balance in the region below the thermocline in the Pacific North Equatorial 

 Countercurrent (Knauss, 1961). In the Cromwell Current speeds were in 

 the range of 50-150 cm/sec; in the deep countercurrent speeds ranged from 

 5-15 cm /sec. In both cases the pressure and Coriolis forces which were balanced 

 had values of 1-2 x 10~ 4 dyne/g. In terms of dynamic heights, this gradient is 

 equivalent to a change of one to two dynamic centimeters per 100 km. Because 

 of short period changes in the density structure caused by internal waves, 

 small scale advection or other reasons, it is difficult in practice to measure 

 gradients so small, except by averaging a large number of observations. 



C. Seasonal Variations in the Pacific North Equatorial Countercurrent 



Since the horizontal pressure gradients in the equatorial region are primarily 

 determined by the temperature distribution, the geostrophic currents can be 

 inferred from the slope of the thermocline. For an observer facing downstream, 

 the thermocline would slope up to the left in the Northern Hemisphere and up 

 to the right in the Southern Hemisphere ; the steeper the slope the stronger 

 the current. As an example, the position of the countercurrent, as shown in 

 Fig. 7, is well defined by the temperature structure alone as being between 

 5°-10°N. The westward-flowing North and South Equatorial Currents are to 

 the north and south of the countercurrent. 



1 The equatorial undercurrent in the Atlantic has now been observed (Metcalf, Voorhis 

 and Stalcup, 1962). 



