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Effects of Stratification and Bottom Topography on the Kuroshio Path Variation South of Japan. Part II: Path Transitions in a Multiple Equilibrium Regime

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  • 1 Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, Japan
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Abstract

The effects of stratification and bottom topography on the Kuroshio path transitions in a multiple equilibrium regime due to a short-term velocity increase are examined using a two-layer inflow–outflow model with simplified coastal geometry and bottom topography. For an imposed velocity increase on a straight-path state, the typical transition from a straight to a meandering path occurs with or without bottom topography through the same process as in a barotropic case with flat bottom. Thus, the geometrical effect of Kyushu is essential to this transition or to the formation of a small meander triggering the transition; stratification and bottom topography are somewhat secondary. Nevertheless, under the influence of stratification, a small meander significantly develops south of Kyushu during a decreasing phase of velocity through shoaling of the interface depth and triggers the transition with the amplification rate of velocity Amp of 1.5. The continental slope south of Japan prevents a small meander from developing and then the transition needs Amp of 2.0. With subcritical Amp, the transition to a C-type path occurs in almost all cases via similar eastward progression of a small meander. On the other hand, stratification and bottom topography cause significant differences from a flat-bottom barotropic case when a short-term increase in velocity is imposed on a meandering-path state. Stratification causes cyclonic eddy shedding from the tip of the meandering segment resulting in the reduction of its amplitude, and the formation of a distinct small meander, which progresses eastward to coalesce with the reduced meandering segment. The continental slope along the southern coast of Japan, on the other hand, enhances eddy shedding and suppresses development of a small meander. As a result, the reduced meandering segment finally changes into a C-type path with bottom topography, while an initial meandering path is recovered in a flat-bottom ocean. Similarities to observations are found to support the present results.

Corresponding author address: Dr. Shuhei Masuda, Department of Geophysics, Graduate School of Science, Kyoto University, 606-8502 Kyoto, Japan.

Email: masuda@kugi.kyoto-u.ac.jp

Abstract

The effects of stratification and bottom topography on the Kuroshio path transitions in a multiple equilibrium regime due to a short-term velocity increase are examined using a two-layer inflow–outflow model with simplified coastal geometry and bottom topography. For an imposed velocity increase on a straight-path state, the typical transition from a straight to a meandering path occurs with or without bottom topography through the same process as in a barotropic case with flat bottom. Thus, the geometrical effect of Kyushu is essential to this transition or to the formation of a small meander triggering the transition; stratification and bottom topography are somewhat secondary. Nevertheless, under the influence of stratification, a small meander significantly develops south of Kyushu during a decreasing phase of velocity through shoaling of the interface depth and triggers the transition with the amplification rate of velocity Amp of 1.5. The continental slope south of Japan prevents a small meander from developing and then the transition needs Amp of 2.0. With subcritical Amp, the transition to a C-type path occurs in almost all cases via similar eastward progression of a small meander. On the other hand, stratification and bottom topography cause significant differences from a flat-bottom barotropic case when a short-term increase in velocity is imposed on a meandering-path state. Stratification causes cyclonic eddy shedding from the tip of the meandering segment resulting in the reduction of its amplitude, and the formation of a distinct small meander, which progresses eastward to coalesce with the reduced meandering segment. The continental slope along the southern coast of Japan, on the other hand, enhances eddy shedding and suppresses development of a small meander. As a result, the reduced meandering segment finally changes into a C-type path with bottom topography, while an initial meandering path is recovered in a flat-bottom ocean. Similarities to observations are found to support the present results.

Corresponding author address: Dr. Shuhei Masuda, Department of Geophysics, Graduate School of Science, Kyoto University, 606-8502 Kyoto, Japan.

Email: masuda@kugi.kyoto-u.ac.jp

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