Equatorial Countercurrent However, one must not overlook possi- 

 bility that a shallow density gradient (fig 7) coupled with strong winds 

 might result in mixing of surface and deeper waters and produce a 

 transient enrichment of surface waters „ Also c horizontal eddies 

 (Cromwell 1951, Mao and Yoshida 1953) may effect a similar condi- 

 tio^ Although deeper waters do not normally reach the surface in 

 this region, the distribution of mass resulting from the zonal currents 

 brings enriched waters into the subsurface euphotic zone. The bio- 

 logical significance of this vertical displacement is not well understood 

 and requires considerable further study D 



The principal features of the vertical distribution of 

 oxygen (fig 10) are the relatively high values at or near the surface 

 (4 - 4 5 ml/1) and the oxygen- minimum layer at intermediate 

 depths,, An oxygen-rich layer is encountered at depths below 500 

 meters (see tabulated station data),, The subsurface layer of higher 

 oxygen concentrations, 5 to 5 7 ml/1, centered at 100 meters 

 between 19° N„ and 14° N 8 may be associated with the intrusion 

 of the northern subtropical water mass, while the inclination of the 

 oxygen isolines in the same latitudes but below 100 meters suggests 

 an association with an intermediate water mass. Comparison at the 

 Equator of figures 7 and 10 suggests the mixing of upwelled waters 

 of relatively low oxygen content with those at the surface. 



Currents ; The dynamic topography is shown on figure 11, 

 the geostrophic current velocities on figure 12. The actual results of 

 the dynamic calculations are plotted in the former in order that the 

 degree of smoothing of the isolines may be observed. The data for 

 the stations at the Equator and at 1°N„ latitude were insufficient for 

 calculations to the 1, 000-decibar surface (see footnote in tabulated 

 data for stations 33 and 34)„ Because of the uncertainty of relative 

 current computation for stations near the Equator, the isovels between 

 3-1/2 N and 3-1/2 S have not been included in figure 12 



In figure 12, only the major features of the velocity dis- 

 tribution as computed by the dynamic calculations are to be considered 

 reliable,, Internal waves undoubtedly exert a strong, but quantitatively 

 unknown influence. 



The most striking feature on both figures is the evidence 

 of the Countercurrent flowing strongly to the east between 5 N„ and 

 10 No „ with the highest velocity of 80 cm, per second near the 

 southern boundary,, This velocity may be somewhat low, as the 

 plotted velocities represent an average between successive stations,. 

 Another easterly flow, but of very small magnitude, is indicated 

 between 17- 1/2° and 19-1/2° N„ The North Equatorial Current exists 

 in a broad band between 10° and 17-1/2° N„ with velocities of 23 

 cm, /sec. or less,, 



8 



