Quasigeostrophic dynamics of the western boundary current

A local model is used to investigate the dynamics of the western boundary c urrent in a midlatitude, wind-driven gyre. This current is important for th e gyre as a whole, and its local instability is correlated with structural changes of the separated eastward jet and the interior gyres. In particular , the eastward jet can be disrupted and broadened in the regime with a stro ng, local instability in the western boundary current. Such a regime occurs with a no-slip lateral boundary condition. Alternatively, in the absence o f local instability, the eastward jet is narrow and penetrates farther in t he basin interior. This behavior is typical with free-slip boundary conditi on. Both the linear stability and nonlinear time-dependent behavior of the west ern boundary current are analyzed for a wide range of parameters. The curre nt loses stability at moderate Reynolds numbers, and the stability threshol d strongly depends upon the vertical stratification profile. The nonlinear time-dependent flow contains well-defined mesoscale eddies with adjacent me anders. The finite amplitude dynamics is fundamentally different in the no- slip and free-slip situations, because the free-slip boundary substantially stabilizes the flow. It is shown that fluctuations in nonlinear regime are rather different from the linearly unstable modes. Multiple stable equilib ria are also found.