Midlatitude ocean-atmosphere interaction in an idealized coupled model

Interannual-to-interdecadal ocean-atmosphere interaction in midlatitudes is studied using an idealized coupled model consisting of high resolution two-layer quasigeostrophic oceanic and atmospheric components with a simple diagnostic oceanic mixed layer. The model solutions exhibit structure and variability, resembling some aspects of the midlatitude climate.

The atmosphere is characterized by a zonally-modulated climatological jet. The oceanic climatology consists of a midlatitude double jet, representing the Gulf Stream and Labrador currents, which are parts of the subtropical and subpolar gyres respectively.

The leading mode of atmospheric low-frequency variability is equivalent barotropic and involves localized intermittent highs and lows of atmospheric pressure situated over the ocean. This pattern does not seem to be affected by oceanic coupling. The atmospheric modes of variability determine the structure of the oceanic variability on both regional (boundary currents, inertial recirculations) and basin scale. The former is characterized by a delayed modulation of the baroclinic time-dependent circulation in response to the slowest components of the atmospheric forcing. The variations in the two oceanic jets are synchronous.

The first basin scale EOF of SST has largely one polarity and is similar in structure to the interdecadal mode found by Kushnir (1994). A warm SST anomaly is accompanied by anomalous atmospheric low pressure and an intensified model Gulf Stream. Preliminary results indicate that in the western part of the ocean, this pattern is forced by the atmospheric pressure anomaly through its influence on the barotropic ocean circulation. The anomalous barotropic currents that result advect mean SST in the mixed layer and produce the SST anomaly described above. Thus, essential to the existence and time-dependence of this mode is a spatial and temporal structure of the atmospheric intrinsic variability.