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

[ACG36-P01] Stability Condition of a Flow Constrained by Topography and Its Application to the Large Meander Path of the Kuroshio

*Yuki Tanaka¹ (1.Faculty of Marine Science and Technology, Fukui Prefectural University)

Keywords:Stability Condition, Kuroshio Large Meander, Koshu Seamount, Shikoku Basin

A general condition for the stability of a two-layer quasi-geostrophic flow constrained by bottom and coastal topography is derived and applied to the Kuroshio paths south of Japan. First, using a conserved quantity called pseudoenergy that is proportional to the square of the disturbance amplitude, we derive a sufficient condition for stability for the simplest steady background field in which the potential vorticity and the stream function are proportional to each other. The condition enables us to judge the stability of various background fields by explicitly taking into account the limitation imposed on the scale of the disturbance by the domain size. We then perform an idealized numerical simulation with a realistic situation in mind where the Kuroshio flows over Koshu Seamount and is confined within the Shikoku Basin (a basin bounded by Kyushu Island to the west and the Izu-Ogasawara Ridge to the east). It is shown that a flow similar to the straight path of the Kuroshio becomes unstable while passing over the seamount, developing into a flow similar to the large meander path of the Kuroshio. The application of the theoretically obtained stability condition shows that the stability of the two flow states can be successfully distinguished, suggesting that the Shikoku Basin stabilizes the large meander path of the Kuroshio while Koshu Seamount fixes the position of the crest and trough of the meander. This result may provide a reasonable explanation for the recently revealed path variability of the Kuroshio south of Japan.