Two-Layer Model on the Branching of the Kuroshio Southwest of Kyushu, Japan

Atsuhiko Isobe Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Japan

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Abstract

Observations suggest that a separation branch of the Kuroshio appears southwest of Kyushu, Japan. The observed density and surface current fields clearly show a separation branch in autumn. However, the separation branch disappears in winter and spring when the relatively homogeneous structure develops due to the intense surface cooling. A scenario generating a separation branch is examined using simple two-layer model. The northward Kuroshio along the shelf slope turns to the east abruptly, and leaves the shelf slope around Tokara Strait. A hump (local increase of the upper-layer thickness) is made by the lower-layer flow impinging on the bottom slope. This hump extends to the shallow shelf region due to the combined effect of advection and dissipation of the upper-layer thickness. Then the cross-isobath flow, that is, the separation branch, appears around the hump. When the thin upper layer obstructs the advection process in winter and spring, a separation branch disappears. Moreover, the separation branch disappears when a small damping coefficient is used.

Corresponding author address: Dr. Atsuhiko Isobe, Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1, Kasuga-Koen, Kasuga, 816-8580, Japan.

isobe@eest.kyushu-u.ac.jp

Abstract

Observations suggest that a separation branch of the Kuroshio appears southwest of Kyushu, Japan. The observed density and surface current fields clearly show a separation branch in autumn. However, the separation branch disappears in winter and spring when the relatively homogeneous structure develops due to the intense surface cooling. A scenario generating a separation branch is examined using simple two-layer model. The northward Kuroshio along the shelf slope turns to the east abruptly, and leaves the shelf slope around Tokara Strait. A hump (local increase of the upper-layer thickness) is made by the lower-layer flow impinging on the bottom slope. This hump extends to the shallow shelf region due to the combined effect of advection and dissipation of the upper-layer thickness. Then the cross-isobath flow, that is, the separation branch, appears around the hump. When the thin upper layer obstructs the advection process in winter and spring, a separation branch disappears. Moreover, the separation branch disappears when a small damping coefficient is used.

Corresponding author address: Dr. Atsuhiko Isobe, Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1, Kasuga-Koen, Kasuga, 816-8580, Japan.

isobe@eest.kyushu-u.ac.jp

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