The Role of Air–Sea Interaction in Controlling the Optimal Perturbations of Low-Frequency Tropical Coupled Ocean–Atmosphere Modes

In this paper the structure and dynamics of the optimal perturbations of tropical low-frequency coupled oceanatmosphere oscillations relevant to El Nin o-Southern Oscillation (ENSO) are explored. These optimal perturbations yield information about potential precursors for ENSO events, and about the fundamental dynamical processes that may control perturbation growth and limit the predictability of interannual variability. The present study uses a hierarchy of hybrid coupled models. Each model is configured for the tropical Pacific Ocean and shares a common ocean general circulation model. Three different atmospheric models are used: a statistical model, a dynamical model, and a combination of a dynamical model and boundary layer model. Each coupled model possesses a coupled ocean-atmosphere eigenmode oscillation with a period of the order of several years. The properties of these various eigenmodes and their corresponding adjoint eigenmodes are explored.