Seasonal variability of the Equatorial Undercurrent at 10°W as inferred from recent in situ observations

Eighteen cross-equatorial shipboard current profiling sections along 10 degrees W with conductivity-temperature-depth measurements taken between 1997 and 2007 are used to analyze the mean meridional structure and the seasonal variability of the Equatorial Undercurrent (EUC) at 10 degrees W. Our analysis suggests a seasonal cycle for the EUC transport at 10 degrees W, with a well-defined annual harmonic and some indication of a semiannual component, with a first maximum in January and a second stronger maximum from June to September. The mean EUC transport at 10 degrees W is estimated to be 12.1 Sv and, compared to earlier estimates farther in the west, at 35 degrees W (20.9 Sv) and 26 degrees W (13.8 Sv). The eastward flow transport exhibits a strong variability at 10 degrees W (with a total range of transports from 7.1 Sv to more than 31.7 Sv). The seasonal amplitude of the eastward flow variability is +/- 8.9 Sv, from a minimum of 8.2 Sv in November to 25.9 Sv in August. The eastward flows within the thermocline are divided in two parts: a permanent part within the sigma(theta) = 24.5-26.5 isopycnal layer, with a semiannual cycle, known as the EUC, and a nonpermanent part in the deep thermocline (beneath sigma(theta) = 26.5 isopycnal), associated with a strong eastward transport (up to 15 Sv) during the boreal summer, that is not observed during the rest of the year. The current at the equator in the deep thermocline is even westward during boreal fall. The disappearance of the salinity core of the EUC during the boreal summer, associated with the upwelling of the hydrological structure at 10 degrees W, reveals that the saline subtropical waters carried by the EUC within the thermocline no longer flow into the Gulf of Guinea during the boreal summer. Our data also show the presence of the South Equatorial Undercurrent (SEUC) at 10 degrees W in the Gulf of Guinea with a strong latitudinal and depth variability throughout the year. The mean SEUC at 10 degrees W is centered near 5 degrees S and is farther south than observed at 35 degrees W and 26 degrees W, suggesting its poleward shift from west to east.