Internal waves

Internal waves occur at discontinuities in seawater below the ocean surface or in layers with varying densities, and are not noticeable on the ocean surface. Ordinary ocean waves also occur at discontinuities between air and seawater, fluids with different densities, and are essentially the same phenomenon. The existence of internal waves was theoretically known in the 18th century, but it was difficult to prove. When Nansen embarked on an Arctic expedition aboard the Fram in 1893, he discovered that ships in the Barents Sea moved significantly slower where there was a thin layer of fresh water on the surface of the seawater, and observed that internal waves were occurring at the boundary between the seawater and the fresh water. Ekman reproduced this phenomenon in a water tank experiment, and confirmed that part of the ship's propulsive energy was spent generating internal waves, which reduced the ship's speed. He named this phenomenon "dead water." The same phenomenon was also known in Japan as "ghost ship."

It was not until the 1930s that internal waves could be precisely measured. However, it was not possible to directly measure the vertical movement of the boundary surface like with surface waves, and so internal waves are observed indirectly by taking advantage of the property of ocean water getting colder as it gets deeper, and measuring the time change in the temperature of the lower layer water due to the vertical movement of the ocean water, and the time change in the speed of the ocean water flow due to internal wave motion.

The period of internal waves in the open ocean is well known as internal tides when it is close to the tides, but it also varies widely from the Brundt-Weissela period determined by the vertical distribution of seawater density (7 to 8 minutes when fresh water sits on top of the seawater, usually 20 to 30 minutes) to the inertial period determined by latitude (12 hours ÷ sin latitude, around 20 hours near Japan, 70 to 80 hours at low latitudes). When the inertial period is close to the tidal period, it resonates, and the amplitude can reach 100 meters at deep places.

Internal waves change the temperature distribution in the ocean, which in turn changes the distribution of the speed of sound in water, which is greatly affected by water temperature, and thus have a significant effect on the distance and range that sound waves can travel. Therefore, internal waves have a serious effect on acoustic communication between submersibles or between a submersible and a mother ship, as well as on the detection of submarines by acoustic waves.

Similarly to surface waves, when internal waves surge against shallow shorelines, they eventually become internal breaking waves and reach the surface. Since internal waves are originally the movement of cold ocean water at the bottom, when internal breaking waves occur, the water temperature on the coast drops significantly, affecting both fishing and swimming.

[Tadashi Yasui]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

海面下の海水の密度不連続面または密度変化のある層内におこり、海面では目だたない波。普通の海の波も、空気と海水という密度の異なる流体の不連続面におこり、本質的には同じ現象なので、内部波の存在は理論的には18世紀にわかっていたが、なかなか実証されなかった。1893年にナンセンがフラム号で北極探検に赴いた際、バレンツ海で海水の表層に清水の薄い層のある所は船脚が著しく遅くなる現象を発見し、このときに海水と清水との境界部に内部波がおこっていることを観測した。エクマンはこの現象を水槽実験で再現し、船の推進エネルギーの一部が内部波をおこすことに費やされるため速力が減少することを確かめ、この現象に「死水(しにみず)」と名づけた。同じ現象は日本でも「船幽霊(ふなゆうれい)」として知られていた。

　内部波を精密に測定できるようになったのは1930年代に入ってからである。それでも、表面波のように境界面の上下動を直接測ることはできず、海水の温度が深くなるにしたがって冷たくなる性質を利用し、海水の上下動に伴う下層水温の時間変化や、内部波動に伴う海水の流速の時間変化を測定して、間接的に内部波を観測している。

　外洋における内部波の周期は、潮汐(ちょうせき)に近い周期のものが、内部潮汐としてよく知られているが、海水密度の鉛直分布で決まるブルント‐バイセラ周期（海水の上に清水がのっているような場合は7～8分、普通は20～30分）から緯度による慣性周期（12時間÷sin緯度　日本付近で20時間前後、低緯度で70～80時間）のものまで広くおこっている。慣性周期が潮汐周期に近いときには共振して、深いところでは振幅が100メートルにもなることがある。

　内部波は、海中の温度分布を変化させるから、水温に大きな影響を受ける水中音速の分布も変化させ、音波の到達距離や到達範囲に大きな影響を及ぼす。したがって、潜水船どうし、あるいは潜水船と母船との間の音波通信や、音波による潜水艦の探知などに及ぼす内部波の影響は深刻である。

　また、表面の波と同じように内部波が遠浅の岸に押し寄せると、最後には内部くだけ波となって表面まで現れる。内部波はもともと下層の冷たい海水の運動であるから、内部くだけ波がおこると海岸の水温が著しく冷たくなり、漁業にも海水浴にも影響する。

［安井　正］

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