Bifurcation and upwelling of the equatorial undercurrent west of the galápagos archipelago

Abstract

The Equatorial Undercurrent (EUC) encounters the Galápagos Archipelago on the equator as it flows eastward across the Pacific. The impact of the GalápagosArchipelago on the EUC in the eastern equatorial Pacific remains largely unknown. In this study, the path of the EUC as it reaches the Galápagos Archipelago is measured directly using high-resolution observations obtained by autonomous underwater gliders. Gliderswere deployed along three lines that define a closed region with the GalápagosArchipelago as the eastern boundary and 93°W from 2°S to 2°N as the western boundary. Twelve transects were simultaneously occupied along the three lines during 52 days in April-May 2016. Analysis of individual glider transects and average sections along each line show that the EUC splits around the Galápagos Archipelago. Velocity normal to the transects is used to estimate net horizontal volume transport into the volume. Downward integration of the net horizontal transport profile provides an estimate of the time- and areal-averaged vertical velocity profile over the 52-day time period. Local maxima in vertical velocity occur at depths of 25 and 280 m with magnitudes of (1.7 ± 0.6) x 10-5 ms-1 and (8.0 ± 1.6) x 10-5 ms-1, respectively. Volume transport as a function of salinity indicates that water crossing 93°W south (north) of 0.4°S tends to flow around the south (north) side of the Galápagos Archipelago. Comparisons are made between previous observational and modeling studies with differences attributed to effects of the strong 2015/16 El Niño event, the annual cycle of local winds, and varying longitudes between studies of the equatorial Pacific.