The termination of the Equatorial Undercurrent in the eastern Pacific

Abstract

The termination of the Equatorial Undercurrent (EUC) in the eastern tropical Pacific has been studied through analysis of the historical hydrographic station data in the region bounded by 5°N, 10°S, 80°W and 100°W. Three distinctive hydrographic features associated with the EUC are used, along with dynamic topography, to trace the mean path of the EUC and to investigate aspects of its seasonal variation. These features are (1) the 13°C thermostad, (2) the high-salinity core, and (3) the high dissolved oxygen concentration tongue. The thermostad is thickest just south of the equator to the west of the Galapagos Islands, with a decreasing maximum extending southeast to the coast of South America near 7°S. The thermostad is poorly developed in the mean near the equator east of the Galapagos Islands. This pattern appears to be produced by convergence of eastward flow in the EUC and in the Southern Subsurface Countercurrent, as the EUC flows southeastward from the Galapagos. This interpretation is supported by the geostrophic flow calculated from the mean dynamic topography. The isolated high-salinity core is found to the south of the equator and is traced all the way to the coast of South America. The high oxygen tongue associated with the EUC splits at the Galapagos, with the southward projecting branch disappearing rapidly as an identifiable feature. The other branch of the tongue continues around the islands to the north and along the equator to the coast of Ecuador. Some evidence of recirculation of EUC water in the South Equatorial Current is seen to the northwest of the Galapagos. A strong seasonal pulsation of the EUC affects these features in a consistent way, with the pulse occurring first west of the Galapagos, and later along the coast. The thermostad first reaches maximum development, and then a pronounced downwelling of the thermocline occurs rapidly, leading to minimum thermostad extent. Associated wtih this downwelling is an eastward protruding high-salinity tongue near the equator which is later seen as an isolated core along the coast at 4°S and 6°S. Pressure gradients along the equator and the coast inferred from dynamic topography suggest that this pulse is associated with an increase in the flow of the EUC, and ultimately the Peru-Chile Undercurrent. The pressure gradient reversal along the equator occurs prior to the annual wind reversal, suggesting that remote forcing plays a role in the seasonal pulse of the EUC in the eastern Pacific.