Modification of the Oyashio Water in the Hokkaido and Tohoku areas

Changes of the Oyashio Water properties are examined along the Oyashio northeast of Honshu, Japan. CTD data from simultaneous observations made by multiple ships in September 1989 are used. The Oyashio Water is modified by layering and vertical mixing with warm, saline waters flowing out from the Okhotsk Sea and the Japan Sea at densities lower than 130 cl/ton (26.7 σδ). At densities greater than 130 cl/ton, the properties of Oyashio Water do not change significantly from the southeast of Iturup Island to the coast of Tohoku district at 39°20′N. The large and abrupt modification of Oyashio Water occurs within 400 km of the coast of Tohoku district, i.e. at the westernmost part of the Oyashio Front at about 39°N, by mixing with Kuroshio Water. By this modification, a salinity minimum is formed at densities around 26.8 6e corresponding to North Pacific Intermediate Water, which is the salinity minimum water broadly located in the subtropical region. The modified Oyashio Water flows northeastward along the Oyashio Front and joins the offshore flow of the Oyashio, which is separated from the main flow of the Oyashio southeast of the southern Kuril Islands carrying the typical western subarctic water.