Analytical prototypes for ocean-atmosphere interaction at midlatitudes. Part II: Mechanisms for coupled gyre modes

A simple midlatitude coupled model for idealized ocean basins is used to in vestigate processes of ocean-atmosphere interaction and its role in interde cadal climate variability at midlatitudes. The ocean model consists of a li nearized quasigeostrophic upper ocean layer and a sea surface temperature ( SST) equation for an embedded surface mixed layer. The atmospheric response to the ocean is through wind stress and heat flux feedbacks associated wit h SST. Eigenvalue analysis of both coupled and uncoupled models presented h ere complements previous work on the stochastically forced system. Comparis on of the eigenspectrum of coupled and uncoupled cases shows that coupling creates an oscillatory interdecadal mode whose properties are distinct from any other mode in the system. This mode exists whether the atmospheric fee dbacks are weak or strong, and is stable even in the strong feedback case. The weak decay rate makes it possible fbr me mode to be maintained by atmos pheric stochastic forcing. Analytic approximations to the dispersion relati on show, how the spatial structure of the atmospheric feedback tends to sel ect a large-scale spatial pattern for this eigenmode. The oscillation invol ves westward Rossby wave propagation in the ocean with the atmosphere carry ing information back eastward into the interior of the basin in response to SST anomalies produced by advection. ST modes are also found, which purely decay in most cases due to both local and nonlocal negative heat flux feed backs. A case with large positive heat flux feedback can produce a purely g rowing SST mode but does not greatly impact the interdecadal mode.