[A-CG36] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[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)

Keywords: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.