Drift ice - Ryuhyo

Japanese: 流氷 - りゅうひょう

Sea ice that is drifting due to the influence of wind and ocean currents. In English, it is generally called drift ice, but the term pack ice is also used for areas where the density of ice chunks is 7/10 or more. Shapes and distribution vary. In contrast, sea ice that is attached to the shore is called fast ice. Drift ice moves at a speed roughly 1/40th the wind speed, but there are considerable differences depending on the shape and density of the ice; the more undulating and scattered the ice surface is, the faster it moves.

In the Sea of Okhotsk, freezing begins on the northern coast around the end of October on average, and as the ice thickens it spreads over a wider area. It is carried south by seasonal winds and ocean currents, and appears as drift ice along the coast of Hokkaido in mid-January. The drift ice gradually spreads and reaches its peak in early March, when about 80% of the Sea of Okhotsk is covered in drift ice. In years when the drift ice is strong, it flows out into the Pacific Ocean through the Nemuro Strait and the straits of the Kuril Islands, reaching the coast of Kushiro, and sometimes even reaching the coast of Hidaka, around Cape Erimo. It can also reach the Sea of Japan through the Soya Strait, and extend to Rishiri and Rebun Islands. By mid-March, the temperature rise becomes noticeable, the ice floes break up and melt, and the ice area shrinks in the opposite direction to the arrival season. Along the coast of Hokkaido, ice generally disappears by early April, and in the northern part of the Sea of Okhotsk, it disappears by mid-June. However, the speed of change in the ice area and the amount of ice vary from year to year. The area around Hokkaido is the lowest latitude in the world where full-scale drift ice can be seen.

The largest maritime accident caused by drifting ice in Japan was a mass shipwreck that occurred on March 17, 1970 in Hitokappu Bay, Etorofu Island (drift ice came in, eight ships sank and ran aground, and 30 people were killed or missing).

[Masaomi Akagawa]

"White Okhotsk - Records of the Sea of Drift Ice" by Kikuchi Keiichi (1973, Soei Publishing)" ▽ "Nature of Hokkaido 7: Drift Ice" by Tabata Tadashi (1978, Hokkaido Shimbun Press)" ▽ "The World of Drift Ice" by Kikuchi Keiichi and Yamazaki Takeshi (1982, Iwasaki Shoten)" ▽ "Animals Brought by Drift Ice" by Nizuma Akio, Ishii Eiji and the NHK Kushiro Broadcasting Station Reporting Team (1987, Japan Broadcasting Publishing Association)" ▽ "Tales of the Okhotsk Drift Ice" by Kikuchi Keiichi (1987, Kyodo Bunka Publishing)" ▽ "The Sea of Okhotsk and Drift Ice" edited by Aota Masaaki (1989, Northern Regions International Symposium "The Sea of Okhotsk and Drift Ice" Executive Committee)" ▽ "Study on the Relationship between Drift Ice in the Sea of Okhotsk and Human Life" edited and published by the Okhotsk Drift Ice Research Group (1989)" ▽ Nakamura Keizo, "Towns Where Drift Ice Comes" (1992, Kokon Shoin) Aota Masaaki, "White Sea, Frozen Sea - The Mysteries of the Sea of Okhotsk" (1993, Tokai University Press) Otsuka Hisashi, Fukuda Masami, Ishiki Kenkichi, Sakai Akira, Sakuma Toshio, et al. eds., "Japan's Nature: Regional Volume 1: Hokkaido" (1994, Iwanami Shoten) Hydrology and Water Resources Society Editorial and Publication Committee eds., "Hydrology and Water Resources in Snowy and Cold Regions" (1998, Shinzansha Psytech, University Press) Kikuchi Keiichi, "Documentary: The Drift Ice Comes!" (2000, Kyodo Bunka Publishing) Kikuchi Keiichi, "Okhotsk Ice Cape Travelogue - The Sea of Drift Ice and 58 Lighthouses" (2001, Kyodo Bunka Publishing)

[Reference items] | Sea of Okhotsk | Sea ice | Hokkaido

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

風や海流などの影響で漂流している海氷。英語ではドリフトアイスdrift iceと総称するが、氷塊の密集度が10分の7以上の流氷域には、パックアイスpack iceも用いられる。形や分布はさまざまである。これに対し、海岸に固着している海氷は定着氷とよばれる。風による流氷の動きは、おおむね風速の40分の1の速さであるが、氷の形や密集状態によってかなりの違いがあり、氷の表面に起伏が多く、またばらついた流氷ほど速く動く。

　オホーツク海では、平均的には10月末ころ北部沿岸で結氷が始まり、厚さを増しながら結氷域を広げる。これらは季節風や海流によって南方に運ばれ、1月なかばには流氷となって北海道沿岸に現れる。流氷域はしだいに広がって3月上旬に最盛期となり、オホーツク海の約80％が流氷に覆われる。流氷の勢力の強い年には、根室海峡や千島列島の海峡を通って太平洋側に流れ出て釧路(くしろ)沖に、ときには襟裳(えりも)岬を回って日高沿岸にも及ぶことがある。また、宗谷海峡から日本海に出て、利尻(りしり)、礼文(れぶん)島に延びることがある。3月なかばになると気温の上昇も目だち、氷盤は分裂、融解して、氷域は到来期と逆方向に縮小し、北海道沿岸では、だいたい4月上旬にはみられなくなり、オホーツク海北部も6月なかばには消滅する。しかし、氷域の変化の速さや氷量は年によって異なる。北海道周辺は本格的な流氷がみられる世界でもっとも緯度の低い海域である。

　流氷による海難事故としては、日本では1970年（昭和45）3月17日の択捉(えとろふ)島、単冠(ひとかっぷ)湾における集団遭難（流氷襲来、8隻沈没座礁、30人死亡・行方不明）が最大である。

［赤川正臣］

『菊地慶一著『白いオホーツク――流氷の海の記録』（1973・創映出版）』▽『田畑忠司著『北海道の自然7　流氷』（1978・北海道新聞社）』▽『菊地慶一・山崎猛著『流氷の世界』（1982・岩崎書店）』▽『新妻昭夫・石井英二・NHK釧路放送局取材班著『流氷が連れてきた動物たち』（1987・日本放送出版協会）』▽『菊地慶一著『オホーツク流氷物語』（1987・共同文化社）』▽『青田昌秋編『オホーツク海と流氷』（1989・北方圏国際シンポジウム「オホーツク海と流氷」実行委員会）』▽『オホーツク流氷研究会編・刊『オホーツク海の流氷と人間生活とのかかわりに関する研究』（1989）』▽『中村圭三著『流氷の来る街』（1992・古今書院）』▽『青田昌秋著『白い海、凍る海――オホーツク海のふしぎ』（1993・東海大学出版会）』▽『小疇尚・福田正己・石城謙吉・酒井昭・佐久間敏雄ほか編『日本の自然　地域編1　北海道』（1994・岩波書店）』▽『水文・水資源学会編集出版委員会編『積雪寒冷地の水文・水資源』（1998・信山社サイテック、大学図書発売）』▽『菊地慶一著『ドキュメント流氷くる！』（2000・共同文化社）』▽『菊地慶一著『オホーツク氷岬紀行――流氷の海と58の灯台』（2001・共同文化社）』