Anomalous southward intrusions of the Oyashio east of Japan: 2. Two‐layer numerical model

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<jats:p>Anomalous southward intrusions of the Oyashio along the Japanese Coast in the northwestern Pacific occur from winter to late spring, with cold sea surface temperatures exerting various influences on the temperature in Japan and on aquaculture. A two‐layer numerical model is employed to examine the interannual response of the ocean to the observed wind stress change over 1961–1987, with special reference to the occurrence of the anomalous southward Oyashio intrusion. Since 1971, anomalous southward Oyashio intrusions were observed in winter of 1974, 1981, 1984 and 1986, and the significant southward shift of the Oyashio is well simulated in these years. Because the observed southward latitude of no wind stress curl in the North Pacific and the Sverdrup transport show clear simultaneous correlation with the calculated southernmost latitude of the subarctic circulation, the anomalous southward intrusion of the Oyashio is found to be essentially due to the barotropic response of the ocean to the change in the wind stress characterized by the southward development of the enhanced Aleutian low. The model also shows that a baroclinic response from the eastern boundary is mainly confined to a region east of 140°W, and an uplift of the interface in a central area of the subarctic circulation is also formed by the divergence of the surface Ekman layer in winter of the anomalous southward Oyashio intrusion. The calculated upper layer thickness south of the subarctic circulation has a clear positive simultaneous correlation with the observed southernmost latitude of the Oyashio defined by the 5°C isotherm at a depth of 100 m, which has been used as an index. However, the observed southernmost latitude of the Oyashio follows the observed latitude of no wind stress curl and the Sverdrup transport by ∼2–4 months, while the calculated southernmost latitude of the subarctic circulation shows simultaneous correlation with no wind stress curl and the Sverdrup transport. From this, it is pointed out that the observed southernmost latitude of the Oyashio defined by the 5°C isotherm at a depth of 100 m lags by ∼2–4 months the barotropic response of the ocean which appears in the surface pressure fields and upper layer velocity.</jats:p>

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