Multiple Equilibria and Low-Frequency Variability of the Wind-Driven Ocean Circulation

The link between low-frequency time-dependent variability and the existence of multiple unstable steady-state solutions in a reduced gravity quasigeostrophic ocean model for the midlatitude wind-driven circulation is investigated. It is shown that a sequence of successive symmetry-breaking pitchfork bifurcations lead to multiple equilibria that differ from each other primarily in the elongation of the recirculation cell, in the amount of meandering present in the intergyre jet, and in a north–south shift in the eastward jet. The elongation of the recirculation cells and the meandering of the jet play compensating roles in the establishment of the global energy and vorticity balance.The solutions also have distinct energy levels, but general agreement between them and the bumps in a histogram of the total energy obtained from a 1200-yr time-dependent simulation is not found. Nevertheless, a substantial fraction of the variance (30%) can be accounted for by four coherent structures that capture the subspace spanned by four vectors that point from the mean state to four selected fixed points. The steady-state solution with the most elongated recirculation cells acts most strongly in steering the trajectory of the time-dependent model in phase space and sets a rough upper bound on the energy of the flow