A transequatorial Atlantic oceanographic section in July 1963 compared with other Atlantic and Pacific sections

A line of oceanographic stations from 15°S to 5°N, taken aboard the Argo in July 1963 in the central Atlantic, has been compared with other Atlantic and Pacific transequatorial sections. In the Atlantic the two subsurface tongues of high salinity that extend equatorward from the tropical high-salinity cells are approximately symmetrical about the equator and are separated by a high-salinity core at the equator. In the Pacific the two tongues are not symmetrical about the equator: the boundary appears to be at about 6 to 8°N. The presence of an equatorial high-salinity core in the Pacific has not been established and seems unlikely. At greater depths the Intermediate Waters that extend northward from the Antarctic show different salinity values at the salinity minimum beneath the surface at the equator: in the Pacific the minimum value is about 34.55‰ and in the Atlantic about 34.50‰. This is remarkable since the Atlantic is in general the more saline. The presence of the Equatorial Undercurrent in the Atlantic is indicated on the Argo section by weakening of the thermocline, the high-salinity core, deep penetration of high oxygen concentration, and, within the limits of the method, geostrophic flow. The value of salinity at the core, the shape of the core, and the density distribution are markedly different from a section 278 km to the west made in April, 1961: these differences suggest that the Undercurrent was wider in July 1963. Between 5 and 12.5°S the section clearly indicates eastward geostrophic flow at a maximum speed of about 9 cm/sec near 200-m depth. Such other data as are available do not preclude that this is a regular (perhaps seasonal) feature, and perhaps evidence of a South Equatorial Countercurrent in the Atlantic Ocean.