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Monday, April 8, 2013

The Beaufort Gyre

The Beaufort Gyre is a clockwise rotating ocean current situated in the Arctic Ocean  north of Alaska (on the Bearing Strait side). This gyre has some interesting connections with climate change.  Let's go back a few steps and trace the cause/effect chain.

In times when the Arctic Ocean is largely covered with ice, most of the radiant energy from the sun is reflected back out into space.  The air above the Arctic also radiates energy into space*.   The air becomes cold, dense and sinks.  When it approaches the surface of the earth it spreads out southward across the Tundra.

*Any material which is above 00 Kelvin {minus 273 centigrade} radiates energy

Coriolis gives the south flowing air a bend to the right so we have North East winds (flowing toward the south west).  In other words this results in a clockwise rotating weather system.  The wind pushes on the water and ice and causes the clockwise rotating Beaufort Gyre.  Now something interesting happens.

You would expect that with a rotating gyre, water would be flung outward resulting in a slightly lower water level in the middle.  Think of those rotating mercury filled dishes which form perfect mirrors for studying the stars.  However, something peculiar happens with the Gyre.  The middle has been observed to  be a little higher than the surrounding water*.  Coriolis is once more the answer.

* This probably explains the accumulation of Sargasso weed and plastic in the middle of the world's other oceanic gyres.

The water which is being pushed clockwise around the gyre is also bent to the right.  In a clockwise rotating current, 'right' is into the centre.  Since the surface water of the Arctic Ocean is fresher than the warmer saltier deeper water, this bulge is of fresher water.  The fresh water comes from rivers and melting ice and the halocline is about 200m deep on average.  The Beaufort Gyre Exploration Project reports that the amount of fresh water stored on the surface of the Arctic Ocean is equal to a number of years flow of rivers emptying into the Arctic ocean.  However, the Beauford Gyre can reverse direction.  As reported by the BGEP, the cycle is from 4 to 8 years in each direction.

All it takes is a low pressure area over the Arctic with rising air causing surface air to be sucked in from the surrounding area.  With a bend to the right, this will result in a counter clockwise rotating weather system and if it is strong enough and persists for long enough, it will result in a counter clockwise rotating Beauford gyre.  Such a situation is likely to happen with increased frequency as the Arctic Ocean becomes more and more ice free and open water absorbs the radiant energy that used to be reflected back into space.  This will cause an ocean which is warmer than the surrounding land and lead to rising air and off-shore winds.

With a counter clockwise rotating body of water in the Northern Hemisphere, a bend to the right is away from the center.  This layer of fresher surface water along with any floating ice trapped in the gyre, will be moved away from the centre of the gyre, resulting in lower water in the center.  This may be exacerbated by the "liquid mirror" effect.  If strong enough, this should result in upwelling of the deep, warmer, saltier water which underlies the surface water.  The normal (historical) clockwise rotating Beaufort Gyre keeps fresher surface water and floating ice in the Arctic and helps the production of multi year ice by keeping the ice in the Arctic Ocean.  Fresher water also freezes more easily.  A counter clockwise Beaufort Gyre would push surface water and ice into the trans-polar drift and out of the Arctic.

A related effect will be the freshening of the North Atlantic surface water and if great enough could shut down or weaken the Gulf Steam*. When would we be likely to see a reversed Beaufort Gyre?

* As odd as it seems, this could result in a year or two with winter temperatures on the West coast of Europe similar to those on the East Coast of North America at the same latitude.

It could be that it already has happened.  If you look at the October 2012 edition of the NSIDC report, they already report rising air over the Arctic ocean with winds from the south.  It makes sense that fall would be when we would first see this situation.  The land is cooling rapidly as the sun leaves the north while the sea has an accumulation of heat from the summer.  Offshore (southern) winds,  with Coriolis, will  cause a  counter clockwise rotating weather system which in turn will reverse the direction of the Gyre.  As mentioned, such a system will also fling floating ice and surface fresher water outward, possibly to be caught by the Transpolar Current and expelled from the Arctic through Fram Strait.  Whether this 2012 reversal of the air circulation was strong enough or lasted for long enough to reverse the Beaufort Gyre is unclear.

There is enough heat in the deeper Atlantic water to melt all the surface ice about three times over.  This looks to be another of these much anticipated tipping points. Warmed surface water gives rise to a counter clockwise Beauford gyre which in turn expels surface fresher water from the Arctic and brings up deep salty warmer water to melt more ice which leads to more warming as more of the surface of the ocean becomes open water.

 A thinner surface layer of fresher water is also easier to disrupt  than the present thicker layer.  The disappearance of the Arctic ice  would then accelerates far faster than would be expected from considerations of climate sensitivity or thermodynamics.

There is another little wrinkle in this scenario.  If you look at wave dynamics, the circle of rotation of a particle in a wave, halves for every ninth of a wave length you go down into the water*.  So the mixing effect of short waves on layers of water decreases rather quickly.  The effect of longer waves penetrates deeper than short waves.  With a shallower layer of fresher water and the huge waves caused by hurricanes, mixing can be much more effective*.  It seems likely that at some point as the fresher layer thins and storms increase, we will see a relatively sudden (over a few years) warming and increase in salinity of Arctic surface waters*.

*As the ice thins, much larger, longer waves can be anticipated.

The internal waves between layers of water of different density is something else again.  The height of a wave is inversely proportional to the density difference between the layers.   The density difference between adjacent layers of water is small so the waves are large.  They also break as they reach shallow water just as water/air waves do, mixing the layers.

*Suppose you have a wave which is 18m from peak to peak and the wave is 1m high.  On the surface, a particle is describing a 1m circle as the wave goes by.  At a depth of two meters (1/9th of a wave length), a particle is describing a 50cm circle.  By the time you get to 10 meters, the circle of rotation is only 3cm.  The higher and longer the wave, the deeper the effect is felt.  The mixing due to such waves is particularly strong when the waves move into shallow water and break.  The break is not only on the surface between water and air but also between layers of water of different densities.  Wave height is inversely proportional to the density difference between the two media in contact where the wave occurs.

Of course we then get to the possibility of really large waves; of land slumps under water.  As methane evolves out of the bottom of the Arctic ocean, the Clathrate which is at present holding the sediment together melts.  A small tremor can then induce a land slide, causing localized but rater intense tsunamis.  As these bounce back and forth across the Arctic, some severe mixing is likely

Quite exciting if you are in to horror movies.

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