Total Pageviews

Saturday, September 5, 2015

The Arctic Hurricane

The following is a hypothesis of why we have hurricanes in the Arctic and what lies in the future.  It is written as if it is well established fact.  It is not.  The reason I write it this way is that it is simply too annoying to read something that full of might be's and could be's.  Remember too, that while much of it is well established physics, you only need one broken link in  a chain of argument to wreck the whole.  With that in mind, here goes.

Does it ever strike you as odd that we have hurricanes in the Arctic.  The one in 2012 was right on the border between a category 2 and 3; quite a powerful beast.  That was also the year of the greatest ice melt we have yet seen (up to 2015*).

* 2016 could well be a different story all together with the massive El Nino we are experiencing.  We will see. (written Jan, 2016)

In the tropics the surface water must be about 25C or more to create a hurricane. This is because, in the tropics, all the pressure gradient needed to create a hurricane is supplied by the "suck" in the middle.  Warm humid air rises, reaches the dew point, and the water vapor begins to condense, releasing latent heat of vaporization*.  This keeps the air buoyant and powers it upward.  Air is sucked along the surface of the ocean from all around to be sucked up and continue the process.  Only with the sea above about 25 degrees C is there enough water vapor in the air to sustain this process.

* It takes 2260 kj (kilojoules) of heat to vaporize one kg (kilogram) of water.  When water vapour condenses, it gives out this heat.

In the Arctic the surface water never gets this warm (so far) so how do we  achieve the pressure gradient necessary to create a hurricane.  We need the suck in the middle to be augmented somehow by a "blow" from outside.  This is created, especially in the fall when the land begins to rapidly cool down, cooling the air above it.  The ever more open Arctic ocean has accumulated heat in the top tens of meters while the land rapidly cools off. And in the Arctic, the land is all around the sea.  We have a push from almost all sides from this cooling, sinking air which reaches the ground and flows toward an ocean that  each year is accumulating more and more heat.

This is further exacerbated by Coriolis.  In the Northern hemisphere, Coriolis causes anything that is moving, to veer to the right.  Since Coriolis is a function of how rapidly something moves closer to the center of rotation (the earth's axis) it is more powerful in the Arctic than in the tropics.  Near the equator, moving a km hardly changes your distance from the earth's axis.  Near the North Pole, a km of motion across the sea moves you nearly a km toward or away from the center.  However, just like water going down a plug hole, once the rotation starts it is powered by the gradient between the outside and the center.

So what can we expect.  The Arctic ocean, as more of it becomes open ocean, is absorbing more and more heat from the sun.  Snow over top of ice is said to reflect about 90% of the incoming radiation back into space while open water absorbs about 90% of the radiation.  Don't take the exact figures too seriously.  There are a number of whichever's involved but the basic principle holds.  In addition, at present, melting ice helps to keep the water cold as it absorbs latent heat of crystallization.*  We are seeing another one of these positive feed back mechanisms.  As the ice melts, it sets up the situation to melt even more ice.

* To melt a kg of ice takes 334kj.  Water, when it freezes gives out the same amount of heat per kg.  This will become important below.  Incidentally, to get a feel for how much heat this is, it is approximately enough heat to raise the same mass of water by 80 degrees.

So as the ocean becomes more an more ice free, we can expect more hurricanes, especially in the fall.  And we might, in the future, even see them following Sept 15 when the freeze up starts again.  Remember it is the gradient that creates hurricanes.  If we have got to the point where most of the Arctic is ice free then a lot of latent heat is released as the water freezes.  If this freeze doesn't start until the surrounding land is really cold and snow covered, a serious pressure gradient could be created.  Of course, as it freezes, sublimation replaces evaporation which is less vigorous.  If the land has really cooled down and air is falling over the land,  the gradient could still be enough to create a hurricane.  Other effects occur.

In the historical situation with the Arctic Ocean  pretty well covered with ice and snow, the air doesn't get heated from below as it does everywhere else on earth due to the absorption of heat from the sun.  It is an area of falling air which gives rise to a clockwise, high pressure weather system.  This clockwise rotating air pushes the water in the same direction giving rise to the clockwise rotating Beaufort Gyre to the North of Alaska.  Coriolis comes into play again as surface water rotating in this gyre veers toward the center.  Fresh water is pouring into the ocean from surrounding rivers and this floating fresh water (and ice) accumulates in the gyre. The gyre is actually a little higher in the middle than on the outside contrary to what we would expect in a rotating body of liquid.  The fresher water layer is a couple of hundred meters deep and holds an amount of fresh water equivalent to a couple of years  flow from the surrounding rivers. 

Now what happens if we start to have anti-clockwise (storm) patterns over the Arctic.  If sufficiently long and strong, the Beaufort gyre should reverse direction, flinging ice and surface fresher water outwards (Coriolis again) to be expelled through the Bering and Fram straights.  This creates  another positive feed back loop.

The surface water in the Arctic is colder than the deeper water.  It is only kept on the surface because of its lower salinity.  If we start to expel this surface layer, the deeper warmer water comes closer and closer to the surface.  The very storms which expel this fresher water from the arctic, create large waves which mix the layers. 

Surface waves are not the only waves of concern.  Between layers of water of different density there are also waves.  They can be induced by surface waves.  When they reach shallow water, they also break, mixing the layers and decreasing the density difference between the layers.

It would appear that in the coming years we should expect more and more hurricanes occurring in the arctic.  As the melting of the ice progresses, these hurricanes should occur earlier and earlier in the melt season.  It is also possible that they will continue into the beginning of the re-freezing season.  If their effect is to send more ice and fresh water into the Atlantic and Pacific, this will exacerbate the melting.

I wouldn't want to live in a coastal community on the shores of the Arctic Ocean.  I would be moving to higher ground.Increased storms with an increased fetch for the waves to build up will cause severe erosion of the coast.

1 comment:

Aaron said...

I suggest that such a storm event will be the trigger for abrupt loss of sea ice leading to our first substantially ice free Arctic.