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 investigate processes of ocean– atmosphere interaction and its role in interdecadal climate variability at midlatitudes. The ocean model consists of a linearized 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 with SST. Eigenvalue analysis of both coupled and uncoupled models presented here complements previous work on the stochastically forced system. Comparison 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 feedbacks are weak or strong, and is stable even in the strong feedback case. The weak decay rate makes it possible for the mode to be maintained by atmospheric stochastic forcing. Analytic approximations to the dispersion relation show how the spatial structure of the atmospheric feedback tends to select a large-scale spatial pattern for this eigenmode. The oscillation involves westward Rossby wave propagation in the ocean with the atmosphere carrying information back eastward into the interior of the basin in response to SST anomalies produced by advection. SST modes are also found, which purely decay in most cases due to both local and nonlocal negative heat flux feedbacks. A case with large positive heat flux feedback can produce a purely growing SST mode but does not greatly impact the interdecadal mode.