Numerical study on the Oyashio water pathways in the Kuroshio-Oyashio confluence

In this paper, results of a high-resolution regional model of the Kuroshio-Oyashio confluence, where the mixed water region (MWR) forms off the northeastern coast of Japan, are discussed. The model simulates major characteristics of the Kuroshio and the Oyashio system well, such as the separation of the Kuroshio Extension from the Japanese coast and southward intrusion of the Oyashio. Further, potential temperature and salinity structures in the intermediate layer σ_β = 27.0 resemble those obtained from historical data. Upon the success of this simulation, the authors focus on the diagnosis of the Oyashio water pathways intruding into the subtropics. It is found that the pathways of the Oyashio water form in the vicinity of the Japanese coast, where warm core rings and the Oyashio intrusion are active. These pathways are shown to be primarily eddy driven. Of particular interest is the water that originates in the Sea of Okhotsk, characterized by low potential vorticity (PV). Impacts of the Okhotsk water are identified by conducting an experiment in which the exchange of waters between the Pacific Ocean and the Sea of Okhotsk is blocked. The impacts are striking. If the exchange were blocked, the pathways would not form in the MWR. Instead, a strong cyclonic recirculation, caused by separation of the Kuroshio from the Japanese coast, dominates the MWR and advects warm and salty Kuroshio water northwestward, letting it occupy the entire MWR. It is found that the low-PV flux from the subpolar region tends to reduce the cyclonic circulation in the MWR. As a result, a southward intrusion of the Oyashio is induced. Concurrently, this intrusion blocks the northward advection of the Kuroshio water, maintaining the Oyashio water pathways in the MWR.