Response of Equatorial Oceans to Periodic Forcing

Oscillating wind with a period P induce variability with the following characteristics in the upper few hundred meters of the equatorial zone (5øN to 5øS) of the ocean. (1) P < 10 days: these winds fluctuate too rapidly to generate strong currents and excite primarily waves. (2) 10 days < P < 50 days: At these periods the winds generate intense equatorial jets in the upper 50 m, but at greater depths the variability has a small amplitude. Nonlinear eastward jets are more intense, are narrower, and are deeper than the corresponding westward jets so that winds with a zero mean value give rise to a mean eastward surface current. If the wind is always westward, then its fluctuating component intensifies the eastward equatorial undercurrent maintained by the mean winds. The surface flow is eastward and convergent when winds that are always westward go through a weak phase. (3) 50 days < P < 150 days: An eastward pressure force exists sufficiently long to generate an intense eastward equatorial undercurrent. Variability has a large amplitude in the surface layers and in the thermocline. Eastward phase propagation associated with Kelvin waves is prominent in the upper ocean because the nonlinear currents impede the Rossby waves. (4) P > 150 days: The amplitude of variability is almost independent of frequency. An equilibrium response which is in phase with the forcing and which corresponds to a succession of steady states is approached asymtotically. These time scales are for a basin 5000 km wide. If the width of the basin exceeds 5000 kin, then the 150 daytime scale increases. In the deep ocean below the thermocline, motion corresponds to propagating waves g6nerated by the divergence of the nonlinear currents in the upper ocean.