The Equatorial Undercurrent (EUC) is a quasi-permanent feature of the zonal equatorial circulation in both the Atlantic and Pacific oceans.  Its main role in both oceans is to supply thermocline waters from the shallow subduction zones in the subtropics to the main upwelling zones in the central and eastern part of the equatorial basins. It may also partly supply the off-equatorial upwelling domes near the eastern boundary.  While in the Pacific Ocean the EUC is rather well-described from over a decade of intensive shipboard and time-series observations in the TOGA and TAO/TRITON programs, the EUC in the Atlantic remains relatively poorly sampled and neither its mean structure across the basin or its seasonal-to-interannual variability is understood.  Observations are very sparse in the eastern part of the basin where the EUC decays and exhibits strong variability in its eastward penetration. In the Pacific the variability of the EUC is closely linked to sea surface temperature variations in the eastern cold tongue region on both seasonal and interannual (El-Nino) time scales, and a similar behavior is expected in the Atlantic in association with the seasonal cycle and the analogous “Atlantic Nino” phenomenon.

The objectives of this project are to: (i) provide a thorough description of the mean state and variability of the EUC at in the central and eastern Atlantic including its seasonal intensity and eastward penetration of into the Gulf of Guinea; (ii) to investigate the forced response of the EUC across the basin to seasonal and interannual wind stress variability over the tropical Atlantic and (iii) to improve understanding of processes influencing sea surface temperature (SST) variability in the eastern tropical Atlantic and associated climate variability in the tropical Atlantic region.

This program will observe the EUC in the eastern Atlantic through new moored current meter (ADCP) observations at 10ºW and 0ºE, for a three-year period from September 2007 to September 2010.  The measurements coincide with an array of ADCP moorings that will be deployed on the equator at 23ºW in the central Atlantic by German (IFM-GEOMAR) investigators, and with repeated shipboard CTD/ADCP surveys of the EUC in the eastern equatorial Atlantic and Gulf of Guinea by French (IFREMER/IRD) investigators.  The resulting data set will be analyzed together with participating French and German colleagues and will lead to a vastly improved understanding of the EUC in the Atlantic. This program contributes to the goals of TACE (the “Tropical Atlantic Climate Experiment”), a 2007-2012 collaborative international CLIVAR program designed to improve climate predictions for the tropical Atlantic region, with a particular focus on the eastern equatorial Atlantic.

Figure 3. Mooring designs for the ADCP moorings to be deployed at 10°W (Type-I) and at 0°E (Type-II).