Measurements of the equatorial undercurrent in the eastern Pacific

Abstract

During the Piquero Expidition (June 26–August 4, 1969) direct measurements of the Pacific Equatorial Undercurrent were made at four longitudes between 115.5° and 93.5°W. Eastward volume transports of the Undercurrent were 19 × 10 6 m 3 sec −1 at 115.5°W and 8 × 10 6 m 3 sec −1 at 93.5°W. Transports of the South Equatorial Current and the Undercurrent were comparable to those measured on Swansong (September–November 1961). Comparison of the Swansong and Piquero measurements with those of Dolphin (April–May 1958) and Eastropac (April 1968) suggests that the Undercurrent transport is less when the southeast trade winds and the South Equatorial Current are strongest. Measurements east of 118°W on all expeditions show higher velocities to the south of the equator than the north. This asymmetry may be due to the southerly component of wind at the equator in the eastern Pacific. Beneath the Undercurrent there was a secondary maximum of east flow at the base of the thermostad; speeds ranged between 12 and 34 cm sec −1 and the thermosteric anomaly at the maximum was near 160 cl ton −1. At 115.5°W there was a divergence in meridional transport in the surface layer (upper 28 m). The magnitude of the integrated poleward transport was approximately equal to the poleward Ekman transport given by the zonal wind stress component. In order to satisfy continuity of mass upwelling of water into the surface layer was required. The computed vertical velocity component is 8 × 10 −3 cm sec −1. Current measurementsat the depth of the Undercurrent did not indicate equator-ward flow towards the equator. This is in contrast to the Swansong results. Slopes of the pressure surfaces relative to 500 db indicate that there was no mean zonal pressure force east of 115.5°W. Computation of the longitudinal distribution of maximum east current of the Undercurrent with a simple model indicates that frictional dissipation may be important west of the Galápagos Islands. Low-salinity and high-salinity cores were found north and south of the equator at 115.5°W. The high-salinity core was present west of the Galápagos Islands but the low-salinity core was absent. The high-salinity core was located at the depth of maximum Väisälä frequency. Computation of the coefficient of vertical eddy diffusion of salt gave a value of 4 × 10 −2 cm 2 sec −1.