Low-Frequency Variability in the Equatorial Atlantic

Low-frequency v and u component oscillations in the eastern equatorial Atlantic are addressed using lime series of up to 1.5 years duration from the lower portion of the main thermocline. A distinct frequency separation is observed with characteristic time scales of 1 month for v and 4–8 months for u. The principal vcomponent oscillations correspond to seasonally modulated, linear, equatorially trapped Rossby-gravity waves, apparently generated at the surface in the central equatorial Atlantic by shear instability between the South Equatorial Current and the North Equatorial Countercurrent. Their length and time scales are well defined and they agree with theoretical predictions. The principal u component oscillations correspond to the deep zonal jets observed by recent profiling experiments in the Indian and Pacific Oceans. The vertical scales found here agree with the results from the other oceans; however, insufficient data are available anywhere to ascertain zonal scales. Linear theory would limit our observations to first meridional mode long or short Rossby waves or a Kelvin wave. Conjectures are made based upon ray tracing arguments, but unlike the linear v component oscillations, the u component scales suggest that they may intrinsically be nonlinear features. Theoretical guidance along with additional zonal- and vertical-length scale data are needed