Shiozakai (English) front

It refers to the boundary between water masses with different properties. A large mass of seawater with almost uniform physical and chemical properties such as temperature, salinity, and nutrient content is called a water mass. For example, a clear tidal boundary can be seen off the coast of Sanriku in Japan, where the water masses of the Oyashio Current and Kuroshio Current collide. At the tidal boundary, the water temperature, salinity, nutrients, dissolved oxygen, etc. change suddenly, and the water color and transparency also change significantly, so it is often visible to the naked eye. A tidal line is a local convergence zone of surface currents seen in this tidal boundary area. At the tidal boundary, seawater is rich in nutrients (for phytoplankton) and has high biological productivity. In addition, fish from both warm and cold water systems gather in the area, such as off the coast of Sanriku and in the Grand Banks area where the Labrador Current and the Gulf Stream meet, making it a good fishing ground. At the tidal boundary, there are both warm and cold water masses with a large temperature difference, so sea fog occurs frequently and low pressure systems can develop suddenly.

[Masao Hanzawa and Kenzo Takano]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

異なった性質の水塊の境界をいう。水温、塩分、栄養塩量など海水の物理的、化学的性質がほぼ一様なものの大きな塊(かたまり)を水塊という。たとえば親潮系と黒潮系の水塊がぶつかる日本の三陸沖では顕著な潮境がみられる。潮境では、水温、塩分、栄養塩、溶存酸素量などが急変し、水色、透明度も著しく変わるので肉眼でわかることが多い。潮目(しおめ)はこの潮境域にみられる表面の流れの局所的な収束帯である。潮境では海水は（植物プランクトンにとっての）栄養に富み生物生産力が高い。また三陸沖やラブラドル海流とガルフストリームとが相接するグランド・バンクス海域のように寒暖両水系の魚が集まるので好漁場となる。潮境では温度差の大きい寒暖両水塊があるため、海霧が多発し、また低気圧が急激に発達することもある。

［半澤正男・高野健三］

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