Evidence of an Eastward Equatorial Undercurrent in the Atlantic from Measurements of Current Shear

Abstract

RECENTLY there has been speculation as to the existence of an equatorial countercurrent in the Atlantic Ocean similar to the Cromwell Current1,2 in the Pacific. G. Neumann3 has cited evidence for such a current from early observations (1911) of anomalous drift currents and from a re-analysis of dynamic heights from oceanographical data along the equator. In the first two weeks of March, 1961, during an extensive bathymetry survey of the mid-Atlantic Ridge along the equator, I was able to make a number of measurements of horizontal current shear in the upper 100 m. surface water. The shears obtained at different points over an area extending from 19° W. to 10° W. longitude and from the equator to 3° S. latitude do indicate the presence of an eastward flowing equatorial undercurrent. The measurements were made from the R.V. Chain during the first part of a three-month cruise to study oceanographical problems associated with the Romanche Trench and the equatorial mid-Atlantic Ridge.