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Thursday, September 26, 2019

Arctic Tsunamis

When a piece of the edge of the continental shelf breaks loose and plunges into the abys, localized but quite severe Tsunamis can result.  They are localized because usually the slump is only a few kilometers wide - basically a point source in world geography terms and as the wave spreads out over a greater and greater circumference, the energy decreases in proportion to the distance from the source.

By contrast the waves that travel across oceans and cause huge damage at far distant locations are the ones produced by earthquakes in which one side of a long crack (fault) in the sea bottom suddenly rises or falls. A good example of that was the boxing day earthquake off the coast of Summatra on December 26, 2004        .

That is not to say that these point source earth quakes, caused by a slump at the edge of the continental shelf, do not travel far distances.  They do.  But their energy decreases quickly with distance from the source.  So what does this have to do with the Arctic Ocean.  First a little background.

Most continents are surrounded by a continental shelf.  This is a relatively shallow gently sloping area from the shore to the continental drop off where the continental slope starts.    At the drop off, the gradient increases sharply and the Continental Slope leads down to the abyss.  The continents surrounding the Arctic ocean are no exception and the Continental Shelf off Russia is particularly wide.

Permafrost is frozen earth and extends downward to a depth of from a few meters to kilometers, depending on location.  Under thick ice sheets there is no permafrost.  Under the Antarctic, for instance,the bottom of the ice hovers around zero degrees C and there are streams and lakes of liquid water.  Ice is a  good insulator  and geological heat seeping up from the earth has to transverse kilometers of ice before it reaches the air at many tens of degrees below freezing.

It has been found that the land under the Continental shelves of America and Russia in the Arctic ocean have thick layers of permafrost.  Therefore they were not glaciated during the last continental glaciation when sea level was as much as 120m lower at the maximum extent of the ice.  I know this sounds odd.  I find it so as well but if it was covered by an continental ice sheet there would not be permafrost. Perhaps the ice sheet left the continental shelf early in the melt and the permafrost was created before the ocean inundated the shelf.

There is another type of 'permafrost'.  It occurs when methane is in contact with water under pressure.  With enough pressure, methane ice, known as methane clathrate can form at a temperature of as much as 30 degrees C but in the ocean with the deep ocean temperature at a couple of degrees above freezing, you need a pressure equivalent to about 300m of water. (31 atmospheres). Warm this clathrate or reduce the pressure and the clathrate will begin to break down, turning into gaseous methane and liquid water.

An other wrinkle in this story is as follows.  If methane seeps up from disintegrating organic material or from deep deposits of coal, shale or oil and meets a layer of permafrost, it can form a clathrate at a much shallower depth than would be necessary in the open ocean.  The permafrost acts like the lid on a pressure cooker and as the methane accumulates at the bottom of the permafrost, the pressure rises until it combines with the moisture there to create a methane clathrate.  It is likely that the blow-out features seen on the Arctic sea bottom are the result of the permafrost weakening enough under increased ocean temperature for this high pressure methane to blow holes both below and above the ocean.  Some such holes have also been found on land.

So what does this have to do with future Tsunamis in the Arctic Ocean. 

Sea water in the Arctic freezes each winter and during the summer the melting ice keeps the water cold.  This effect is lessening year by year with ever larger areas of the Arctic ocean now clear of ice for ever longer periods in the summer.  Open water, as has often been stated, absorbs solar energy unlike ice and snow which reflect the sunlight back into space.  The surface layer is about 200m deep and the continental shelf is roughly 100m below the surface. The Continental shelf is bathed in this surface layer of fresher water which is gaining more heat year after year.  Clearly a formula for the melting of both types of permafrost.  But that is not all.

 At present, with the prevailing high pressure area over the Arctic, the air circulation is predominantly clockwise.  This causes a predominantly clockwise circulation in the ocean.  Due to Coriolis, anything moving in the Northern Hemisphere veers to the right.  To-the-right in a clockwise circulating system is toward the center.  Because of this, the Arctic ocean tends to hold on to floating ice and to the floating fresh water from the rivers entering the ocean.

As the ocean warms, we can expect more and stronger storms in the Arctic which, in the Northern Hemisphere are counter clockwise rotating bodies of air.  This will push on the water below inducing a counter clockwise water circulation.  In a counter clockwise rotating body in the Northern Hemisphere, to-the-right is away from the center.  We can expect this floating layer of fresher water to thin as it is sent outward to be expelled through the Fram Straight.

Below this fresher water is the warmer more salty Atlantic water.  If it comes close enough to the surface to bath the Continental Shelves of Russia and America, we can expect the melting of the permafrost and methane clathrate to accelerate.  Besides venting methane into the atmosphere; a powerful greenhouse gas, this will remove the 'glue' that is holding the sediments together.  A recipe for Tusnami causing land slides.

Another effect adds to the potential for melting the undersea permafrost, resulting in eventual Tsunami-causing-slumps.  As there is more open water, winds can induce larger and longer waves in the Arctic due to the greater fetch* and storms can become stronger due to warmer open water contributing humidity to the overlying air.  The circle of rotation of a particle of water as a wave goes by decreased with depth.  The formula says that for every nineth of a wave length, you go down into the ocean, the circle of rotation halves.  Longer waves project their effect much deeper than short waves of the same height.


*Fetch - the distance of open water over which a wind can blow.

So, for instance, for a eighteen meter long wave, one meter in height, if you go down two meters, the circle of rotation of a particle of water will be half a meter.  For a one meter high wave with twice the length (36m) you would hae to go down 4 meters before the circle of rotation would have decreased to half a meter.

If the effect of the surface waves penetrates sufficiently deeply, they induce waves between the surface cold fresher water and the deeper warmer saltier water.  These waves break, just a do surface waves as they reach shallow water.  This will cause mixing of the layers, weakening the density gradient between the layers and facilitating future mixing.  But there is a much more worrying possible result.

Tsunami waves are both very large and very long. After the first Tsunami we can expect to see that there has been major mixing between the two layers in the Arctic ocean.  If it has been sufficient, this should rapidly accelerate the process as warmer saltier Atlantic water bathes the continental shelf, leading to more Tsunamis and greatly accelerated melting of surface ice -- which will further increase the amount of heat absorbed in the surface water from the sun.  The first Tsunami caused by a slump looks to be a severe tipping point.












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