[A-CG36] 海洋と大気の波動・渦・循環の力学

[ACG36-06] Absolute instabilities in the spatially developing Kuroshio Extension

*X. San Liang^1,2,3、Jianyu Hu^2,3 (1.Fudan University, Shanghai, China、2.Xiamen University, Xiamen, China、3.State Key Laboratory of Marine Environmental Science, Xiamen, China)

キーワード：Kuroshio Extension, Absolute instability, Convective instability, Intrinsic oscillation, Intraseasonal variability

Satellite observations have long revealed to us a spatially growing Kuroshio Extension (KEx), but its underlying dynamics is yet to be studied. With a normal mode model of absolute/convective instability, it is found that the mean zonal jet is unstable at all the sections in the downstream region (east of 154^oE). In each of the resulting complex dispersion relation diagrams there lies a single saddle point associated with a positive temporal growth rate; that is to say, the mean jet is absolutely unstable, implying that KEx favors self-sustained oscillations. By calculation the absolute instability wave has a period increasing from about 27 days to 72 days, and a slightly decreasing wavelength from 360 km to 250 km, as longitude increases from 154 ^oE to 174 ^oE, agreeing with those inferred from the wavelet power spectra and Hovmöller diagram of the satellite observations. As KEx travels downstream, the associated eigen-structure of the perturbation velocity changes from a surface trapped mode to a mode with components maximized in the vertical interior. This study shows that at least a portion of the KEx intraseasonal variability is of intrinsic origin, and may be predictable with the absolute/convective instability theory.