The influence of the large scale circulation on an eastern boundary current

The wind-driven gyre circulation in the ocean interior varies across large temporal and spatial scales, while the current along the eastern boundary is concentrated in a narrow jet with smaller temporal and spatial scales. These boundary currents are often hydrodynamically unstable and generate mesoscale and sub-mesoscale variability. In this study, we investigate the influence of the large scale circulation on an unstable eastern boundary current. One example is the influence of the Pacific subtropical gyre on the California current system. We study the problem using both a linear stability analysis and a nonlinear numerical model in a barotropic and quasi-geostrophic framework. The large scale circulation and the boundary current are specified in the linear analysis and are generated by an Ekman forcing in the numerical model. The linear stability analysis shows that to the lowest order the eastward (westward) flow of the large scale circulation stabilizes (destabilizes) the boundary current. Additionally, the meridional flow contributed by the large scale circulation accelerates or decelerates the originally parallel boundary current and modifies the stability of the current through the Doppler effect. Unstable perturbations which can be represented by normal modes for a parallel current then develop streamwise spatial structures. In the nonlinear numerical simulations, the streamwise nonuniformity of the boundary current influenced by the large scale circulation is clearly shown in the eddy kinetic energy. The location of the maximum eddy kinetic energy depends on the relative strength of the large scale circulation and the boundary current. The meridionally nonuniform eddy activities are important in offshore tracer transport. The nonlinear numerical simulation is forced by a wind curl field which generates a southward eastern boundary current and a large scale circulation with double gyres (white contours). The mean eddy kinetic energy of the boundary current (in color) exhibits a meridionally varying structure showing the influence of the large scale circulation on the boundary current stability.