The phenomenon in which lower seawater rises up. There are five main types: (1) Coastal upwelling In the Northern Hemisphere, when the wind blows to the left of the coast, the wind-driven current carries the surface water downwind to the right, that is, offshore. The lower layer of seawater then upwells to replenish it. In the Southern Hemisphere, the relationship between the wind direction and the wind-driven current is opposite to that in the Northern Hemisphere, so coastal upwelling occurs when the wind blows to the right of the coast. (2) Equatorial upwelling Easterly trade winds blow along the equator and its neighboring areas to the north and south of it. Wind currents caused by easterly winds carry surface seawater northward north of the equator and southward south of the equator. As a result, lower seawater rises to make up for the shortage of seawater at the equator and its neighboring areas. The speed of upward movement for both coastal and equatorial upwelling depends on the strength of the wind, but roughly speaking it is 0.1 to 1 meter per day. (3) Upwelling caused by wind-driven currents in places far from the coast or the equator The strength of ocean winds varies from place to place, so the amount of surface seawater carried by wind-driven currents also varies from place to place. As a result, surface seawater accumulates (converges) in some places, and disperses (diverges) in others. In the divergence area, the lower seawater rises up. The speed of this depends on the strength of the wind and the latitude, but is slower than coastal and equatorial upwelling, at about 1 centimeter per day. (4) Deep upwelling The three types (1) to (3) above are upwellings from depths of several tens of meters or 200 to 300 meters, but deep upwelling generally occurs from depths of more than 1,000 meters. Places where surface seawater sinks close to the seafloor are limited to the northern North Atlantic and the Antarctic Ocean (especially the Weddell Sea). The sunken seawater spreads throughout the deep oceans of the world and also moves upward, until it rises to the surface again after hundreds or thousands of years. The speed of the upward movement can be calculated by dividing the volume of sinking seawater by the total area of the oceans. This is about one centimeter per day. (5) Small-scale local upwelling Upwelling can occur when the seafloor protrudes and forms a mountain. Attempts have been made to create small seamounts and induce artificial upwelling to increase biological productivity. Upwelling can also occur on the lee side of islands. [Kenzo Takano] [Reference items] | | | |©Shogakukan "> Schematic diagram of coastal upwelling (Northern Hemisphere) ©Shogakukan "> Ocean areas where upwelling occurs Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend |
下層の海水が湧き上がってくる現象。大別して次の5種がある。 (1)沿岸湧昇 北半球では、風が海岸を左に見て吹くと、吹送流によって表層の海水は風下に向かって右、つまり沖合に運ばれる。そのあとを補充する形で下層の海水が湧き上がる。南半球では、風向きと吹送流の向きの関係が北半球とは逆になるので、風が海岸を右に見て吹くときに湧昇が起きる。 (2)赤道湧昇 赤道とその南北近傍では東寄りの貿易風が吹く。東風による吹送流によって表層海水は赤道の北側では北向きに、赤道の南側では南向きに運ばれる。その結果、赤道とその近傍での海水不足を補う形で下層の海水が湧き上がる。沿岸湧昇も赤道湧昇も上向きの速さは、風の強さなどによるが、いちおうの目安は1日に0.1~1メートルである。 (3)海岸や赤道から遠い場所での吹送流による湧昇 洋上の風の強さは場所によって異なるので、吹送流によって運ばれる表層海水量も場所によって変わる。その結果、ある場所には表層海水が集積(収束)し、別の場所では周りに散ってゆく(発散する)。発散域では下層の海水が湧き上がる。その速さは、風の強さと緯度によるが、沿岸湧昇や赤道湧昇よりも遅くて、1日に1センチメートルくらいである。 (4)深層の湧昇 前記(1)~(3)の3種は数十メートルないし200~300メートルの深さからの湧昇であるが、深層の湧昇は、だいたい1000メートルよりも深いところから起きる。表層海水が海底近くまで沈み込む所は、北大西洋北部と南極海(とくにウェッデル海)などに限られている。沈降した海水は世界中の深海に広がりながら上向きにも動いて、数百年、数千年ののちに再び表層に浮かび上がる。上向きの速さの目安は、沈降する海水量を全海洋の面積で割れば得られる。1日に1センチメートルくらいである。 (5)小規模の局所湧昇 海底が突起して山となっていると湧昇が起きることがある。小さな海山をつくり、人工湧昇を起こして生物生産性を高めることは試みられている。また、島の風下側でも湧昇が起きることがある。 [高野健三] [参照項目] | | | |©Shogakukan"> 沿岸湧昇の発生模式図(北半球) ©Shogakukan"> 湧昇がおこる海域 出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例 |
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