The oceanic circulation of the tropical atlantic, and its variability, as observed during Gate

The principal components of the surface current system in the tropical Atlantic are the westward South Equatorial Current south of 3°N, the eastward Equatorial Countercurrent between 3°N and 10°N and the westward North Equatorial Current north of 10°N. The subsurface, eastward Equatorial Undercurrent is confined to about 1 1/2° latitude and is centered on the equator. These mean currents are subject to fluctuations over a spectrum of frequencies. Very high-frequency, turbulent fluctuations are of major importance in the mixed layers at the ocean surface and below the core of the Undercurrent. One-dimensional models cannot simulate these mixed layers because they are strongly influenced by the divergence of large-scale currents. Fluctuations with periods less than the inertial period correspond to inertia-gravity waves but their spectral properties, near 8° N, are unusual in two respects:. iat periods between 1/2 hr and 10 min there are very energetic oscillations associated with a thermodine-trapped internal mode;iiamplitudes of inertial waves below the thermodine are correlated with the intensity of surface winds. Near 5°N the inertial peak in the spectrum disappears and equator ward of 5 latitude equatorially trapped waves dominate the spectrum at periods longer than 10 days. There are hints that wave-like fluctuations do not have a universal spectrum in low latitudes. Particularly energetic equatorial oscillations observed during GATE included the following: 3-to 5-day equatorially trapped inertia-gravity waves which were forced by atmospheric disturbances with the same period; 16-day meanders of the Equatorial Undercurrent which may be related to atmospheric fluctuations with the same period; 30-day, 1000 km waves which appear to be due to instabilities of the surface currents. Superimposed on these oscillations is a trend that is part of the seasonal cycle: for example, the zonal pressure gradient in the equatorial plane increased throughout GATE, practically in phase with the intensification of the tradewinds. This information sheds light on the seasonal upwelling in the Gulf of Guinea which is not correlated with changes in the local winds.