A numerical study on the generation of the small meander of the Kuroshio off southern Kyûshû

AbstractThe Kuroshio south of Japan shows bimodal path fluctuations between the large meander (LM) path and the nonlarge meander (NLM) path. The transition from the NLM path to the LM path is triggered by a small meander generated off southwestern Japan. The small meander first propagates eastward (downstream) along the Kuroshio and then rapidly amplifies over Koshu Seamount, located about 200 km south of Japan, leading to the formation of the LM path of the Kuroshio. Although Koshu Seamount is essential for the rapid amplification of the small meander, the underlying physical mechanism is not fully understood. In this study, the role of Koshu Seamount is revisited using a two-layer quasi-geostrophic model that takes into account the effects of bottom topography. Numerical experiments show that the transition from the NLM path to the LM path can be successfully reproduced only when bottom topography mimicking Koshu Seamount is incorporated. In this case, the upper-layer meander trough is rapidly amplified together with a lower-layer anticyclone by baroclinic instability during their passage over the northern slope of Koshu Seamount. A linear stability analysis shows that baroclinic instability over a seamount is caused by resonant coupling between the upper-layer Rossby wave in the eastward background flow and the lower-layer seamount-trapped wave during their eastward propagation over the northern slope of the seamount. The spatial scale and structure of this baroclinically unstable mode are close to those of the numerically reproduced small meander in its early amplification stage over the seamount.

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Seasonality of the Kuroshio Path Destabilization Phenomenon in the Okinawa Trough: A Numerical Study of Its Mechanism

Journal of Physical Oceanography ◽

10.1175/2009jpo4156.1 ◽

2010 ◽

Vol 40 (3) ◽

pp. 530-550 ◽

Cited By ~ 10

Author(s):

Hirohiko Nakamura ◽

Masami Nonaka ◽

Hideharu Sasaki

Keyword(s):

General Circulation ◽

Velocity Structure ◽

Numerical Study ◽

Okinawa Trough ◽

Circulation Model ◽

Necessary Condition ◽

Kuroshio Path ◽

The Okinawa Trough ◽

Hypothetical Mechanism ◽

The Kuroshio

Abstract Previous observations have indicated that the Kuroshio’s path in the northern Okinawa Trough of the East China Sea is destabilized and accompanied by meanders with periods of 1–3 months during the winter–spring period. The present study investigates the mechanism responsible for this recurrent seasonally fixed phenomenon. A hypothetical mechanism is constructed based on both a simple wind-driven Ekman-pumping model, acting within the Kuroshio, and a bifurcation model of the Kuroshio path states in the northern Okinawa Trough, established in the previous study. A high-resolution ocean general circulation model is used to examine the hypothetical mechanism. The numerical model reveals the following mechanism: the wintertime northerly wind prevailing over the Okinawa Trough blows against the Kuroshio, generating Ekman divergence, and hence upwelling within the inshore side of the Kuroshio from the sum of the earth’s rotation and the geostrophic current shear. A necessary condition for this upwelling is probably given by the exponential velocity structure of the surface Kuroshio on the inshore side of the current. This kind of wintertime upwelling acts to make the mean Kuroshio path separate from the continental slope in the northern Okinawa Trough, so that baroclinic instability destabilizes the Kuroshio path, as shown by the bifurcation model of Kuroshio path states.

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