Oxygen tongues and zonal currents in the equatorial Atlantic

Equatorial zonal currents and associated oxygen distributions are studied using shipboard hydrographic data, trajectories from isopycnic floats drifting at about 300 m depth, and velocity time series from the upper 1100 m obtained at two equatorial moorings located at 35 degrees W and 23 degrees W. Mean profiles of zonal velocity measured by moored acoustic Doppler current profilers yielded a westward flowing Equatorial Intermediate Current (EIC) below the Equatorial Undercurrent (EUC) at both locations. The EIC consists of two westward current cores at about 250 and 450 m. The upper core of the EIC deepens by about 30 m from 23 degrees W, where it has a mean velocity of 6 +/- 2 cm s(-1,) to 35 degrees W, where the mean is 5 +/- 3 cm s(-1). The lower core of the EIC is about twice as strong with 12 +/- 1 cm s(-1) at 23 degrees W and 9 +/- 2 cm s(-1) at 35 degrees W. The flow below the EUC is characterized by substantial interannual variability. From May to December 2005 a strong, zonally coherent eastward jet occurred at 300 to 350 m depth, found to be an expression of shallow stacked jets superimposed on the mean EIC. Shipboard hydrographic observations in June-July 2006 revealed the existence of a high-oxygen tongue that can be traced from 35 degrees W to 10 degrees W in the depth range of the eastward jet prevailing during the preceding year. On the basis of an advection-diffusion balance, it is suggested that the oxygen decrease from 35 degrees W to 10 degrees W within the oxygen tongue is mainly balanced by lateral eddy diffusivity and oxygen consumption, with diapycnal turbulent diffusivity playing only a minor role.