Eastward extension jet driven by vorticity anomaly at the western boundary: characteristics and the effects of southward background flow

In numerical models that adopt a no-slip boundary condition, a jet initially separates from the shore and flows eastward south of the inter-gyre boundary. This premature separation is caused by positive vorticity created by friction on the western boundary, which is paired with negative vorticity created by the conservation of the potential vorticity advected from the south. In this study, we propose a new model that includes vortex pair forcing in the western boundary region, where a vorticity structure causes separation and extension of the western boundary current (WBC) jet. Our model can separately treat flow that is locally driven by large-scale wind and the WBC extension jet driven by a vortex pair. We confirmed that our model could successfully replicate an extension jet prematurely separated from the western boundary for a wind-driven, two-layer quasi-geostrophic model. The dominant physical structure of the jet in our vortex pair forcing model differed in the upstream and downstream regions. In the upstream region, the intrusion of vortex pairs due to forcing is the dominant factor, while in the downstream region, the effect of eddies begins to manifest. We also examined the effects of a large-scale wind-driven background flow on the jet. When the background flow only contains an eastward component, the jet extends eastward. When the background flow contains a meridional component, this strongly affects the extended jet and can terminate the eastward jet structure.