Mixing induced by the Atlantic equatorial wave activity in an eddy-resolving OGCM

The variability of equatorial circulation in the middepth Atlantic Ocean from an eddy-resolving ocean general circulation model (OGCM) was analyzed. Four types of equatorially trapped waves were identified which dominate the variability of the circulation: an annual Rossby wave, a semiannual Rossby wave, a group of 40- to 60-day short Rossby waves, and a group of 30-day Yanai waves. In particular, the 40- to 60-day short Rossby waves were found near the western boundary to the north of the equator, which may be related to nonlinear instability processes associated with the north Brazil current (NBC) retroflection. The wave-induced stirring and transport along the equator was investigated by calculating the trajectories of a large ensemble of particles in the middepth equatorial Atlantic Ocean from the eddy-resolving, model-simulated velocity field. The distribution of finite-time Lyapunov exponents derived from the simulated velocity at a depth of similar to 2000 m bears a close resemblence to the observed CFC tongue, suggesting that the wave-induced chaotic stirring and transport may contribute to the formation of the observed CFC tongue in the middepth equatorial Atlantic Ocean along the equator.