Quasigeostrophic dynamics of the western boundary current

A local model is used to investigate the dynamics of the western boundary current in a midlatitude, wind-driven gyre. This current is important for the 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 strong, local instability in the western boundary current. Such a regime occurs with a no-slip lateral boundary condition. Alternatively, in the absence of local instability, the eastward jet is narrow and penetrates farther in the basin interior. This behavior is typical with free-slip boundary condition. Both the linear stability and nonlinear time-dependent behavior of the western boundary current are analyzed for a wide range of parameters. The current loses stability at moderate Reynolds numbers, and the stability threshold strongly depends upon the vertical stratification profile. The nonlinear time-dependent flow contains well-defined mesoscale eddies with adjacent meanders. 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 equilibria are also found.