Effects of stratification and mesoscale eddies on Kuroshio path variation south of Japan

Abstract

Bimodality of the Kuroshio current path south of Japan is investigated, focusing on the effects of stratification and mesoscale eddies. For this purpose, wind-driven numerical experiments are executed in barotropic and two-layered ocean models. Stratification has two effects on the path selection of the Kuroshio south of Japan. First, it makes an alongshore path stable at intermediate wind stress strength τ 0 by arresting an eddy southeast of Kyushu. This enables an alongshore path to appear in the entire experimental range of τ 0 . Second, the upper limit of τ 0 which allows a meandering path decreases from 4.75 × 10 - 1 N m - 2 ( 76 × 10 6 m 3 s - 1 in the Sverdrup transport at the Tokara Strait) to 2.50 × 10 - 1 N m - 2 ( 40 × 10 6 m 3 s - 1 ) as Δ ρ / ρ 0 increases from 2.0 × 10 - 3 to 4.0 × 10 - 3 . While an anticyclonic eddy imposed upstream (southeast of Kyushu) can cause the transition from an alongshore to a meandering path, it occurs most easily when τ 0 = 1.5 – 2.0 × 10 - 1 N m - 2 ( 24 – 32 × 10 6 m 3 s - 1 ). The transition from a meandering to an alongshore path requires an eddy imposed downstream (east of the meandering segment) which suppresses redevelopment of the meandering segment and breaks the balance between the advective and beta effects. Applicability of the results to previously observed path variations is discussed.