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Kuroshio and recirculation gyres

 

Humio Mitsudera (IGCR, FRSGC, Tokyo, Japan; IPRC, SOEST, University of Hawaii, Honolulu, 96822, USA)

e-mail: humiom@soest.hawaii.edu

 

Abstract

 

The Kuroshio path dynamics is discussed using a high resolution primitive equation model of the Kuroshio and the Oyashio region. The model includes realistic topography, coastal line, and temperature and salinity stratification. The Kuroshio and the Oyashio are driven by inflow/and outflow boundary conditions. It has been shown that the model represents typical features of the Kuroshio flow well.

Here, we discuss a mechanism for the bimodality of the Kuroshio path. Interaction between the Kuroshio and an anti-cyclonic recirculation off the south coast of Japan has been particularly emphasized, since it provides a positive feedback mechanism to the meander formation as follows. When the model Kuroshio takes the meandering path, the potential vorticity of the recirculationis markedly homogenized, accompanying strong transport fluctuations of 50-100 day period. This intensifies the anti-cyclonic recirculation through the 'eddy Sverdrup' balance, which further leads to the enhancement of the Kuroshio meander.

 

1. Introduction

 

The Kuroshio is a western boundary current flowing along western perimeter of the North Pacific basin. It is well-known that the Kuroshio takes multiple paths off the coast of Japan. Kawabe (1985) classified it into three types: Iarge meander path, non-large-meander nearshore path, and non-large-meander off-shore path (Fig. 1). Since the Kuroshio path gives a strong impact on fisheries, maritime safety, ship navigation, local climate and so on, a lot of research from various angles have been conducted. Among them, a hypothesis of non-linear multiple equilibria first discussed by Charney and Frierl (1981) and Masuda (1982) has attracted interests as a large meander mechanism. Motivated by these theories, several numerical experiments has been conducted looking into non-linear nature of the Kuroshio paths, paying attention to the topographic effect of Kyushu and Izu Ridge (e. g. Chao, 1984; Yasuda et al, 1985; Yoon and Yasuda, 1987; Akitomo et al., 1991). Another approach is to discuss the effect of local topography on non-linear waves or eddies, such as Kyushu (White and McCreary, 1976), Kii Peninsula (Yamagata and Umatani, 1989) and Izu Ridge (Mitsudera and Grimshaw, 1994). All these models show multiple paths of the Kuroshio relative to Kuroshio inflow transport or velocity.

 

 

 

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