Equatorial undercurrent and deeper flows in the central Pacific

Abstract

In April, 1971, absolute current velocity was measured down to 1000 m at 1°N, 0° and 1°S, along 150°W. Two velocity time series were obtained: a 25-day record from the Equatorial Undercurrent (102 m) at 1°N and a 152-day record from the deep water (3100 m) at 1°S. The profiles show eastward flow at the surface, a strong Equatorial Undercurrent, a secondary maximum of eastward flow at the bottom of the thermostad, and a westward flow below 500 m. The surface current was counter to the trade winds. Although there was a high-salinity core south and a low-salinity tongue north of the equator, usually interpreted as indirect evidence of equatorward flow at the level of the Undercurrent, the flow at 1°N (at 102 m) was poleward for all 25 days. There was an accelerating eastward and northward current at 1°N; the data are consistent with a northward translation of the Undercurrent at a speed of 7 cm s −1. The winds at Christmas Island (2°N, 157°W) and the Undercurrent velocity at 1°N, 150°W, showed evidence of a 3- to 4-day period; it appears that at this time the currents in the thermocline may have been responding directly to the winds. The meter moored in deep water at 1°S showed a relatively steady eastward velocity component with a mean of 4 cm s −1. Below the Undercurrent in the central Pacific at least two reversals are indicated in the east-west current profile.