Coupled ocean-atmosphere dynamics in a simple midlatitude climate model

Midlatitude air-sea interactions are investigated by coupling a stochastica lly forced two-layer quasigeostrophic channel atmosphere to a simple ocean model. The stochastic forcing has a large-scale standing pattern to simulat e the main modes of low-frequency atmospheric variability. When the atmosph ere interacts with an oceanic mixed layer via surface heat exchanges, the w hite noise forcing generates an approximately red noise sea surface tempera ture (SST) response. As the SST adjusts to the air temperature changes at l ow frequency, thus decreasing the heat flux damping, the atmospheric spectr a are slightly reddened, the power enhancement increasing with the zonal sc ale because of atmospheric dynamics. Decadal variability is enhanced by con sidering a first baroclinic oceanic mode that is forced by Ekman pumping an d modulates the SST by entrainment and horizontal advection. The ocean inte rior is bounded at its eastern edge, and a radiation condition is used in t he west. Primarily in wintertime conditions, a positive feedback takes plac e between the atmosphere and the ocean when the atmospheric response to the SST is equivalent barotropic. Then, the ocean interior modulates the SST i n a way that leads to a reinforcement of its forcing by the wind stress, al though the heat flux feedback is negative. The coupled mode propagates slow ly westward with exponentially increasing amplitude, and it is fetch limite d. The atmospheric and SST spectral power increase at all periods longer th an 10 yr when the coupling with the ocean interior occurs by entrainment. W hen it occurs by advection, the power increase is primarily found at near-d ecadal periods, resulting in a slightly oscillatory behavior of the coupled system. Ocean dynamics thus leads to a small, but significant, long-term c limate predictability, up to about 6 yr in advance in the entrainment case.