Sources of the Equatorial Undercurrent in the Atlantic in a High-Resolution Ocean Model

Abstract

The ventilation of the Equatorial Undercurrent (EUC) in the Atlantic is investigated using data from a high-resolution ocean model. Overturning streamfunctions, subduction patterns, and pathways are determined from Eulerian and Lagrangian mean transports. The role of high-frequency variability is highlighted. The meridional overturning circulation shows that the EUC is mainly ventilated from the south. This is seen because transports induced by high-frequency variability compensate for tropical cells that are associated with downwelling at 5° poleward of the equator and upwelling at the equator. The impact of high-frequency variability is large, especially to the north of the equator. Lagrangian trajectory analysis shows that the main subduction sites that ventilate the EUC are located along the South Equatorial Current: one region in the southwest and one in the central subtropical South Atlantic. From these sites water masses transfer toward the western boundary. Following the boundary they finally enter the EUC. A small portion of water in the EUC originates from a region along the North Equatorial Current (NEC). These water masses follow mainly a pathway through the interior of the basin toward the EUC. Seasonal variations in the mixed layer depth lead to spreading subduction sites. High-frequency transport variations act to reduce subduction in the western Tropics and shift the subduction region along the NEC toward the northeast.