Internal waves

Mar 13, 2015, 7:00AM EST
Internal waves
A common but seldom noticed phenomenon

 Internal waves, also known as solitary waves or solitons, are gravity waves that oscillate within a medium, such as water or air, rather than at the interface between two mediums.  A common example would be a bottle of olive oil and vinegar.  If the bottle is left undisturbed, the olive oil sinks to the bottom and the vinegar “floats” on top.  If the bottle is shaken gently, an internal wave occurs at the interface between the two liquids.  The same phenomenon frequently occurs in nature.  Atmospheric internal waves are sometimes accompanied by wave clouds, and may be referred to as a herringbone sky or a mackerel sky.  If there are no clouds and the atmospheric internal wave has significant magnitude, it may result in what is referred to as clear air turbulence.  Science has revealed that similar internal waves occur commonly in oceanic waters.  These waves are difficult, but not impossible, to detect from a ship because the amplitude of these waves on the water’s surface is usually not great.  At depth, though, the amplitude can be significant – over a thousand feet depending on a variety of factors.  The occurrence of oceanic internal waves depends largely on the extent of the difference between the various layers of water.  The steepness of the gradient between the density of the upper water and the lower water is determined primarily by temperature and salinity.  In places where warm brackish water flows over cold saline water, such as near the Hudson Bight on the US Atlantic coast, internal waves are common.  These internal waves stir up sediments, bringing nutrients into the water column.  They can also transport water long distances up a continental slope, carrying phytoplankton and zooplankton closer to shore.  Orbiting space craft have been able to detect the surface effects of internal waves, using both optical photography and radar.  It turns out that these internal waves are quite common.  They are somewhat dependent upon the seasons and tidal cycles, though, as these factors influence the density gradients that cause internal waves.  One of the features of internal waves is the phenomenon called sticky water or dead water.  This somewhat rare condition tends to be found on the backside or trough of internal waves.  At the surface, the water is calm.  Below the surface, near where a ship’s propeller might be located, is the interface between two layers of water.  The energy created by the rotating propeller can sometimes be dissipated by this interface, with the result that the ship loses its forward momentum.  Such a phenomenon was first scientifically reported by the Norwegian oceanographer Fridtjof Nansen, although mariners had reported it for years. 

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