Instability of the Kuroshio in Luzon Strait: Effects of ridge topography and stratification

The spectral analyses of moored current velocities in the central Luzon Strait reveal northward (i.e., downstream of the Kuroshio) propagation of a frontal wave with a five-day period, with wave amplitude increasing northward. Estimated from both curve fitting and frequency domain Empirical Orthogonal Function methods, the characteristics of five-day variations have wave speeds ranging from 32 to 40 cm s⁻¹, wavelengths ranging from 130 to 150 km, and e-folding time scales for growth ranging from 0.8 to 3 days. An analytical two-layer model used to explore linear stability characteristics indicates that bottom topography (two meridional ridges) is important for the Kuroshio stability characteristics in the Luzon Strait. In the two-layer model with the two ridges, the flow is stabilized for the long-wave mode but destabilized for the short-wave mode (due to increasing vertical shear in the horizontal velocity). The analytical model produces wavelengths and phase speeds for the most unstable mode which is similar to the observation, but the growth rate is underestimated. However, a spectral numerical model applied with a more realistic stratification and velocity structure does obtain faster growth rates comparable to the observations. Parameter sensitivity tests were conducted using the analytical model. The characteristics of the most unstable mode are most sensitive to the surface front location relative to the bottom topography but not sensitive to varying the density difference and thickness of the upper layer.