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The Oyashio Current (also named 'Oya Siwo') is a cold subarctic ocean current that flows south and circulates counterclockwise in the western North Pacific Ocean. It collides with the Kuroshio Current off the eastern shore of Japan to form the North Pacific Current (or Drift).

The waters of the Oyashio Current originate in the Arctic Ocean and flow southward via the Bering Sea. The current has an important impact on the climate of the Russian Far East, mainly in Kamchatka and Chukotka, where the northern limit of tree growth is moved up to ten degrees south of the latitude it can reach in inland Siberia.

The waters of the Oyashio Current form probably the richest fishery in the world owing to the extremely high nutrient content of the cold water and the very high tides (up to ten metres) in some areas - which further enhances the availability of nutrients. However, the Oyashio Current also causes Vladivostok to be the most equatorward port to seasonally freeze and require icebreaking ships to remain open in winter. Nonetheless, this has relatively little effect on the fish yield through the Sea of Okhotsk because the large tides mean freezing does not occur so easily.

Another important feature of the Oyashio Current is that during glacial periods, when lower sea level causes the formation of the Bering land bridge, the current cannot flow and in the regions the Oyashio affects today, the level of cooling with the onset of glacial conditions (after an interglacial) is much less than in other areas of the Earth at similar latitudes. This allowed Tōhoku and Hokkaidō, which were the only areas of East Asia that receive enough snowfall to potentially form glaciers, to remain unglaciated except at high elevations during periods when Europe and North America were largely glaciated. This lack of glaciation explains why, despite its present climate being much colder than most of Europe, East Asia has retained 96 percent of Pliocene tree genera, whereas Europe has retained only 27 percent.