Abstract
Abstract 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 th...
Highlights
Analyzing sea surface height in regions of the midlatitude western boundary currents, Kelly et al (1996) found that the dominant mode of variation of the surface currents was a change in the structure of the recirculation regions
Another possibility as suggested by Qiu (2000) is that the variability in the elongation/contraction of the recirculation cells is a manifestation of the intrinsic variability of the recirculation gyre dynamics and occurs independently of changes in the wind stress
In the previous section we have shown the existence of multiple equilibria whose main difference is the degree of elongation of the recirculation cells
Summary
Analyzing sea surface height in regions of the midlatitude western boundary currents, Kelly et al (1996) found that the dominant mode of variation of the surface currents was a change in the structure of the recirculation regions. McCalpin and Haidvogel (1996) used a reduced gravity quasigeostrophic model forced by a steady wind stress to study the intrinsic variability of the double-gyre wind-driven ocean circulation They found that the low-frequency variability of the model was associated with irregular transitions between different regimes that could be characterized as having either recirculation cells that were elongated or contracted. There are some important similarities between the structure of the elongated and contracted recirculation cells observed in the recirculation systems of the Gulf Stream and Kuroshio Extension systems and the model’s multiple equilibria This allows us to study the global balances of energy and vorticity of the different flow states in the simple context of a QG model without the complications of time-dependence.
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