Oyashio Current

A typical cold current in the North Pacific Ocean, also known as the Kuril Current. It is a western boundary current that is part of the Subpolar Gyre, which flows counterclockwise around the northern part of the North Pacific Ocean, and corresponds to the Labrador Current, which flows south in the western part of the Labrador Sea in the North Atlantic Ocean, and the Falkland Current, which flows north off the coast of Argentina in the South Atlantic Ocean.

It originates in the Bering Sea, flows southwest off the coast of the Kamchatka Peninsula and the Kuril Islands, then turns east from the east coast of Hokkaido and joins the Subarctic Current. During this time, it also takes in cold water that flows into the Pacific Ocean from the southern part of the Sea of ​​Okhotsk. From near Hokkaido, a part of it flows south east of Honshu, and then splits into a coastal branch that flows south along the Honshu coast, and an offshore branch that extends southwest around 145 degrees east longitude. The surface current speed of the Oyashio is about 1 knot (about 1.8 kilometers per hour) in most ocean areas. The southern limit of the coastal branch varies greatly from year to year, and its average location is near the border between Iwate and Miyagi prefectures, but it can flow as far south as Inubosaki in Chiba prefecture. Some of the coastal branches flow south through the intermediate layer of the mixed water area between the Oyashio and Kuroshio Currents, mix with the intermediate water of the Kuroshio Current in the Kuroshio Extension region, and form the North Pacific Intermediate Water, which is widely distributed in the intermediate layer of the subtropical North Pacific.

The sea surface temperature in the Oyashio region off the coast of Hokkaido varies greatly depending on the season, from around 1°C to nearly 20°C, but at depths of around 100 to 150 metres, intermediate cold water below 2°C can be found throughout the year.

The waters of the Oyashio system are low in salinity, with a salinity of less than 33.5 psu (psu stands for practical salinity unit), and rich in nutrients necessary for the growth of plankton and seaweed, giving the water a light green color. The name Oyashio was given to the waters because it produces many useful marine products, and the basin is one of the world's leading fishing grounds.

[Kouichi Nagasaka and Koichi Ishikawa]

"Hydrochemistry of the Kuroshio Current and the Oyashio Current" by Kawai Hideo (included in "Basic Marine Science Lectures 2," 1972, Tokai University Press)" ▽ "The Story of Ocean Currents" by Hidaka Koji (1983, Tsukiji Shoten)" ▽ "Japan's Nature 7: The Seas Surrounding the Japanese Archipelago" by Horikoshi Masuoki, Nagata Yutaka, and Sato Takuhiro (1987, Iwanami Shoten)" ▽ "Japan's Nature 3: The Seas of Japan," edited by Hoshino Michihira and Kubota Tadashi (1987, Heibonsha)" ▽ "Understanding the Marine Environment and Mechanisms for Maintaining Feed Organism Production in the Oyashio Waters," edited and published by the Secretariat of the Agriculture, Forestry and Fisheries Technology Council, Ministry of Agriculture, Forestry and Fisheries (1991)" ▽ "An Introduction to Ocean Physical Sciences," 4th edition, by Sekine Yoshihiko (2003, Seizando Shoten)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

千島海流ともよばれる北太平洋の代表的な寒流。北太平洋の北部を巡る反時計回りの亜寒帯環流の一部をなす西岸境界流で、北大西洋のラブラドル海の西部を南下するラブラドル海流や、南大西洋のアルゼンチン沖を北上するフォークランド海流に対応する。

　ベーリング海に端を発し、カムチャツカ半島、千島列島の沖を南西に流れ、北海道東岸より東に向きを転じ、亜寒帯海流に接続する。この間、オホーツク海の南部から太平洋に流出する冷水も取り込んでいる。また、北海道付近から、一部が本州東方を南下し、本州沿岸を南下する沿岸寄りの分枝と、東経145度付近を南西に延びる沖合い分枝に分かれる。親潮の海面流速はほとんどの海域で1ノット（時速約1.8キロメートル）程度である。沿岸寄りの分枝の南限位置は年により大きく変わり、平均位置は岩手県と宮城県の県境付近だが、千葉県の犬吠埼(いぬぼうさき)付近まで南下することがある。沿岸寄りの分枝の一部は、親潮と黒潮の間の混合水域の中層を南下し、黒潮続流域で黒潮の中層の水と混合し、北太平洋亜熱帯域の中層に広く分布する北太平洋中層水を形成している。

　北海道近海の親潮域の海面水温は1℃前後から20℃近くまで季節により大きく変化するが、深さ100～150メートル付近には年間を通じて2℃以下の中冷水がみられる。

　また親潮系の水は塩分33.5psu（psuはpractical salinity unitの略、実用塩分単位）以下と低塩分でプランクトンや海藻の生育に必要な栄養塩に富み、うす緑に近い水色をしている。多くの有用水産物を生み出すため親潮の名がつけられ、流域は世界有数の漁場となっている。

［長坂昂一・石川孝一］

『川合英夫著「黒潮と親潮の海況学」（『海洋科学基礎講座2』所収・1972・東海大学出版会）』▽『日高孝次著『海流の話』（1983・築地書館）』▽『堀越増興・永田豊・佐藤任弘著『日本の自然7　日本列島をめぐる海』（1987・岩波書店）』▽『星野通平・久保田正編著『日本の自然3　日本の海』（1987・平凡社）』▽『農林水産省農林水産技術会議事務局編・刊『親潮水域における海洋環境と餌料生物生産維持機構の解明』（1991）』▽『関根義彦著『海洋物理学概論』4訂版（2003・成山堂書店）』

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