Coastal Disturbance in Sea Level Propagating along the South Coast of Japan and Its Impact on the Kuroshio

The coastal sea level propagating westward along the south coast of Japan and the impact of the disturbance on the generation of the Kuroshio small meander have been examined. The propagation occurs in sea level variations for periods shorter than 10 days and is remarkable for periods of 4–6 days. Characteristics of the 4–6 day component have been studied using the extended empirical orthogonal function (EEOF). The first and second modes of EEOF are almost in-phase throughout the south coast of Japan. The higher four modes of EEOF are significantly excited when the Kuroshio takes the non-large-meander path, and propagate westward with phase speeds of 2.8 m s⁻¹ (third and fourth modes) and 1.6 m s⁻¹ (fifth and sixth modes) in the Kuroshio region west of Mera in the Boso Peninsula. The analysis shows that more than 70% of the small meanders generate in two months after a significant propagating disturbance reaches south of Kyushu when the velocity of the Kuroshio is high. This effect of coastal disturbance is examined by numerical experiments with a 2.5-layer model in which coastal disturbance is excited by vertical displacement of the upper interface. The result is that offshore displacement of the Kuroshio occurs southeast of Kyushu only in the case of significant upward displacement of the interface under the influence of a high Kuroshio velocity. The significant coastal disturbance, which is associated with upward displacement of the density interface, and a high Kuroshio velocity can therefore be important factors in generating small meanders.