Equatorially trapped waves and wave-induced mixing in the mid-depth Atlantic Ocean

A wave-induced mixing and transport mechanism in the mid-depth equatorial Atlantic Ocean is proposed and examined using a hierarchy of models ranging from relatively simple analytical models to a comprehensive eddy-resolving OGCM. First, the kinematic stirring study using linear solutions suggests that a superposition of a few simple equatorial waves can lead to strong Lagrangian stirring and transport along the equator. In particular, a combination of an annual long Rossby wave and a high-frequency Yanai wave appears to be most effective and relevant in producing strong stirring in the interior region. An inverted, fully nonlinear reduced gravity model is then used to investigate the chaotic stirring properties by including dynamical effects. Finally, the output of a continuously stratified, eddy-resolving OGCM simulation is used to validate the chaotic stirring hypothesis. The conclusions of both reduced gravity model and OGCM studies support the results of the kinematic analyses. By evaluating the finite-time Lyapunov exponents, two regions where chaotic stirring is most active are identified.; Meanwhile, the variability of equatorial circulation in the mid-depth Atlantic Ocean is explained in terms of vertically-propagating, equatorially trapped waves. In particular, four types of waves are identified. The first type is the annual Rossby wave which is forced by surface wind stress directly or by the fluctuation of the thermocline. It is found that low-order modes (l = 1) initiated at the surface are unlikely to reach the deep ocean directly due to the narrow basin: most of the energy is trapped near the western boundary of upper ocean. On the other hand, higher-modes (l = 2 or l = 3) can reach the central basin in the mid-depth without encountering the western boundary. The second is the semiannual Rossby wave which contributes significant energy in the mid-depth due to its steep energy propagating paths. The third is a group of 40-60 day short Rossby waves found near the western boundary to the north of the equator, which are related to the instability associated with the NBC retroflection. The fourth is a 30-day Yanai wave which is attributed to the shear instability between the SEC and NECC.