SECT. 3] EQUATORIAL CURRENT SYSTEMS 245 



at the surface, such as those constructed for the Atlantic by Defant (1941), for 

 the Pacific by Reid (1961) and for the Indian Ocean by Lacombe (1951), in- 

 dicate good first-order agreement with the charts of currents based primarily on 

 ship-drift data published by the maritime services of the United States, Great 

 Britain, Germany and the Netherlands. Likewise, there is agreement between 

 the geostrophic currents and the flow pattern as deduced by the distribution 

 of properties using such methods as isentropic analysis (Montgomery, 1938). 



The analysis of dynamic height observations show that there is a marked 

 current shear at the thermocline, and that the flow beneath the thermocline is 

 slow (Fig. 7). Direct observations in the North Equatorial Countercurrent also 

 indicate a marked current shear in the region of the thermocline. In addition, 

 these observations show the current maximum to be somewhat below the 

 surface as in Fig. 7 (Knauss, 1961). In general this section (Fig. 7) agrees 

 with the mean current chart (Fig. 2) in the position of the North Equatorial 

 Countercurrent, in the fact that the highest speeds associated with the North 

 and South Equatorial Currents are to be found at their southern and northern 

 edges respectively, and in the fact that the South Equatorial Current has a 

 higher speed than the North Equatorial Current. The South Equatorial Counter- 

 current, described by Reid (1959), can clearly be seen (albeit weakly) between 

 10° and 14°S. The Cromwell Current is indicated between 2°S and 2°N. 



A. Cromwell Current 



This sub-surface, eastward flow along the equator (first described by Crom- 

 well, Montgomery and Stroup, 1954) is a major feature of the ocean circulation. 

 Recent measurements (Knauss, 1960) have shown that it is a fast, thin current. 

 As defined by the 25 cm/sec contour, it is 300 km wide and about two-tenths of 

 a kilometer thick; at its core the speed is 100-150 cm/sec. It is symmetrical 

 about the equator. The transport of this current is remarkably high, about 

 40 x 10 6 m 3 /sec. It is very likely the largest current in the equatorial Pacific. 



The Cromwell Current is associated with the thermocline. As the depth of 

 the mixed layer shoals toward the east, so does the depth of the core (Fig. 9). 

 Hydrographic measurements suggest that the flow is in geostrophic equilibrium 

 to within half a degree of the equator. The water in the region of the equator 

 appears to be better mixed vertically than the water on either side of it. In 

 the vicinity of the current, there is a marked reduction in the gradients of 

 properties associated with the thermocline. The temperature gradient is 

 weaker. Water with high oxygen content is mixed down from the surface to 

 depths of at least 300 m and near-surface values are lower than those on either 

 side. Similarly, water low in phosphate appears to be mixed downward through 

 the thermocline. The sharp tongue of high salinity water, which moves up from 

 the south along the thermocline, is dissipated at the equator (Figs. 7 and 10). 



The major features of the Cromwell Current (including its general shape, the 

 depth of the core and the upward slope of the core to the east) are consistent 

 with a geostrophic flow in response to the horizontal pressure gradient which 

 would result from mixing across the thermocline at the equator (Knauss. 1960). 



