Unstable Frontal Waves along the Kuroshio Extension with Low-Potential Vorticity Intermediate Oyashio Water

A linear stability analysis was conducted for a three-layer primitive equation model including viscosity with a basic state, which modeled the stratification and velocity fields with the vertical and horizontal variations across the Kuroshio Extension. An unstable wave with a wavelength of 220 km and a phase speed of 0.24 m s−1 propagating in the downstream direction was found to grow the fastest. Characteristics of this unstable baroclinic wave were similar to those of waves observed along the Kuroshio Extension. The growth rate of the fastest-growing waves became greater with an increase of the cross-stream difference of the potential vorticity (PV) in the intermediate layer. For a cross-frontal stratification structure without the PV gradient in the intermediate layer, which is similar to that in the Gulf Stream, the wavelength of the fastest-growing unstable wave changed to 390 km and the unstable wave had a much different structure. Thus, the unstable frontal waves observed along the Kuroshio Extension occur only for the cases when low-PV Oyashio water exists on the northern side of the main stream in the intermediate layer. The unstable frontal waves revealed in the present study greatly contribute to the formation of a clear salinity minimum in the Kuroshio Extension.