Nonlinearly induced low-frequency variability in a midlatitude coupled ocean–atmosphere model of intermediate complexity

Abstract.

The possibility that essentially nonlinear large-scale processes in the coupled midlatitude ocean–atmosphere induce a preference for variability on interdecadal time scales is explored. The intermediate complexity model used for this purpose assumes quasi-geostrophic dynamics in both ocean and atmosphere. The latter are coupled through processes controlling the sea surface temperature within a constant depth surface layer. Linear stability analysis of an idealized background state, characterized by zonal jets both in the atmosphere and ocean and a simplified sea surface temperature profile, reveals that a large-scale mode may destabilize once the coupling strength is large enough. The mode corresponds to a near-stationary barotropic Rossby wave in the atmosphere coupled to a large-scale baroclinic oceanic Rossby wave. Computations of transient flows in the highly nonlinear regime show that nonlinear rectification processes of this coupled mode change the mean state in such a way that it becomes susceptible to high-frequency instabilities which, again through rectification processes, stabilize the coupled mode. The (generic) low-frequency variability which arises by these rectification processes is associated with amplification/weakening and north/south shifting of the zonal jets, very much resembling observed patterns of variability at midlatitudes.