Temporal variability of the wind-driven quasi-geostrophic double gyre ocean circulation: basic bifurcation diagrams

In this paper we explore how temporal variability arises through successive bifurcations of the symmetric wind-driven double gyre ocean circulation, as lateral friction is decreased in 1.5-layer and 2-layer quasi-geostrophic models. In the 1.5-layer model, the symmetric double gyre flow becomes unstable to symmetry breaking stationary instabilities, leading to asymmetric flows. These asymmetric solutions subsequently destabilize to two sets of barotropic oscillatory modes, introducing an intermonthly and an interannual time scale, respectively. In the 2-layer model, the symmetric double gyre solution becomes unstable to baroclinic oscillatory instabilities, giving rise to near annual temporal variability. The asymmetric solutions which appear through symmetry breaking at smaller lateral friction, are in this case all unstable.