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

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  • 1 Faculty of Fisheries, Kagoshima University, Kagoshima, Japan
  • 2 Frontier Research Center for Global Change, Japan Agency for Marine–Earth Science and Technology, Yokohama, Japan
  • 3 Earth Simulator Center, Japan Agency for Marine–Earth Science and Technology, Yokohama, Japan
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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.

Corresponding author address: Hirohiko Nakamura, Faculty of Fisheries, Kagoshima University, 4-50-20, Shimoarata, Kagoshima 890-0056, Japan. Email: nakamura@fish.kagoshima-u.ac.jp

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.

Corresponding author address: Hirohiko Nakamura, Faculty of Fisheries, Kagoshima University, 4-50-20, Shimoarata, Kagoshima 890-0056, Japan. Email: nakamura@fish.kagoshima-u.ac.jp

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