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Thursday, August 22, 2019

Arctic Storms

The Arctic, occasionally has really big storms.  They are called polar lows.  They can be as strong as hurricanes.

How do we measure the strength of a storm.
In the open Atlantic ocean, where they are called Hurricanes, one measurement is the pressure in the eye of the storm.  The lower the pressure, the greater the strength of the storm.   Wind speed is also a measure of the storm but let's leave that aside for now.

Storm categories

Storm rating      Mbar in eye     mmHg in eye     Wind Speed Kts
     1                    980                  28.94                  64 - 82
     2                    979 - 965         28.5 - 28.91        83 - 95
     3                    964 - 945         27.91 - 28.47      96 -113
     4                    944 - 920         27.17 - 27.88      114 - 135
     5                 <919 -="" 136="" 157="" 919="" br="" nbsp="">

What powers Hurricanes.
It is mainly the effect of Latent Heat of Vaporization.  If you evaporate water which is already at 100 degrees C, it takes 540calories per gram of water to change the water to water vapor.  To put that into context, it is enough heat to raise that gram of water from zero degrees to 100 degrees five times  and then from zero to 40 degrees as well.  That is a lot of heat.

More important for our discussion, the same amount of heat is released when the water vapor condenses back into liquid water.

How warm must the surface water be to power a hurricane
The critical surface temperature of the water to power a hurricane is about 25 degrees C.  At that temperature, enough water vapor is transferred to the air that if some little disturbance causes the air to rise and cool and begin the condensation process, the heat released keeps the air warmer than the surrounding air.  The rising air sucks  more moist air across the warm sea, to be pulled upward where it cools and condenses out the water and you are well on the way to a thunder storm or a hurricane.

Despite the fact that the H2O is now heavy water rather than light vapor, the heat released into the air is enough to keep the air surging upwards like a hot air balloon.

One more factor is needed.  Coriolis. Air at the equator is traveling  at about 1000mph.  At the pole it is not moving at all, merely turning on the spot once per day.  Think of a body of air moving northward from the equator.  If it travels a mile northward, the rotational speed of the land under it hardly changes.  It would move to the right but very slightly.  Think now of a body of air a mile south of the North Pole traveling toward the pole.  It has a velocity but the pole has none.  It will veer sharply to the right.  But there is another effect at play here.

Just like a skater that pulls in her arms while she is spinning, a body of air gets an added spin by moving it's mass closer to the center of rotation.

Coriolis is hardly felt near the equator but is more and more effective, the closer you get to the poles.  Near the poles, you are moving almost a km nearer to the center of rotation for every km you travel over land.  At the equator, a km over land hardly moves you toward (or away) from the center of rotation at all.

Incidentally, do the exercise in your head of a body moving southward.  You will find that in the Northern Hemisphere, something moving southward is also veered to the right.

In the open Atlantic ocean, all the power for the hurricane is from the suck in the Middle caused, as mentioned, by the release of heat from condensing water vapor.

So how do we have such strong storms over the Arctic Ocean???
Some of the Arctic storms are as strong as  Hurricanes.  For instance, in Aug 2012 there was a Polar Low over the Arctic  Ocean that, if it had occurred in the Hurricane zone further south would have qualified as a high 2 or a low category 3 Hurricane.  (due to the low pressure in the eye which was 966 millibars).

According to the scientists this is because of the high pressure over the ice remaining over parts of the Arctic Ocean.  The ocean water on the surface is no where near the 25 or so degrees that is necessary to power a hurricane so something else must be at play.  it is apparently the 'push' from the high pressure area over the ice.  The pressure gradient caused by the suck in the middle is augmented by the push from the air over the ice.

Apparently polar lows tend to follow the line between open water and the edge of the ice.

So let's speculate a bit.
What will happen over the years as the Arctic Ocean is more and more open earlier and earlier and collects more and more heat.  Let's say we have reached the situation in which the Arctic ocean is ice free and remains that way for the final month of the summer.  The sun becomes weaker and weaker and the land cools off rapidly.  Only the top couple of feet of the land warms and cools so it warms and cools  rapidly.  In the fall, the sea has absorbed huge amounts of heat to depth.  Say, now, we have a wee storm that coats the land with white.

Now we have the same situation that created the Aug 2012 storm but much larger.  There is more heat in the water and the land all around the Arctic Ocean is a high pressure area  supplying the push to augment the suck over the ocean.

This looks to me to be the formula for some really large storms.

                Effects of large Arctic storms

There are a number of effects of strong Arctic storms, especially when the ice extent is at low levels and the ice is thin.

When the Arctic is covered with thick ice, the wind can not act on the ocean.  Ice flows can be pushed around a little but with complete ice cover, there is almost no place for them to go.  They can be pushed up into ridges and the leads left behind, freeze over.  This creates thick multi-year ice which is hard to melt.

However, when much of the ocean is ice free and what ice there is, is thin first-year-ice, the storms can not only move the ice around but  waves building up from the wind flowing over open water can smash and fracture this thin ice.

The distance a wind acts on water to create waves is called 'fetch' and the longer the fetch with a given wind speed, the larger and longer the waves

Note that the circle of rotation of a particle of water as a wave goes by, decreases by half for every 9th of a wave length depth.  In other words, the length of a wave greatly increases the depth to which it is felt.

Larger waves crash into land that was once protected by ice and erode it.  This is happening around large parts of the shore line of the Arctic.  Permafrost is exposed which is then  easier to melt.

In addition, surface waves can induce internal waves between the deep salty Atlantic water and the surface, fresher Arctic water.  These waves break when the reach shallow water, mixing the layers.

A really interesting effect is connected to our old friend Coriolis.  The 'normal' weather system over the Arctic ocean is a high pressure area.  This is because the energy of the sun is reflected back into space by the ice and its snow cover so the atmosphere is not heated  from below as is the case in much of the world, the air radiates heat into space resulting in dense falling air.  This high pressure area rotates clockwise as the falling air spreads out heading southward.  And it pushes on the ice and water causing a clockwise rotating gyre, especially just North of Greenland, known as the Beaufort gyre.

Remember that everything moving in the Northern Hemisphere veers to the right and in a clockwise rotating gyre, to-the-right is toward the middle.  This is why the Beaufort Gyre collects and holds the fresh water from the surrounding rivers and the ice that floats on the ocean.

The deeper water is warmer but stays down below because of it's saltiness.  It is basically deep Atlantic water.  The layer of lighter, colder surface water is typically about 200m deep.  So what happens when this gyre is reversed by a persistent, stronger storm.  In a counter clockwise gyre in the Northern Hemisphere, to-the-right is away from the center.  We can expect the surface water and ice to be pushed outwards, to be caught by the trans polar current and expelled through the Fram Straights.  This will bring the Atlantic water closer to the surface.

This may be another tipping point.  As the salty Atlantic water comes closer to the surface, it is more easily mixed with the surface water, especially during hurricane strength polar lows.

Also, the mixing of the layers caused by internal waves will reduce the density gradient between the surface and deeper waters.  Once the Atlantic water reaches the surface, the ice will really take a major 'hit'.  We may once again have trees growing right up to the shores of the Arctic Ocean.

Final note
The process doesn't stop with the beginning of freeze up in the middle of September.  You have all heard of the ice-cube-in-the-drink effect.  The drink stays a or very near to zero degrees until all the ice has melted.  the heat seeping into the glass from it's surroundings is used up in melting the ice cube.  Latent heat from solid to liquid is not as much as from liquid to gas but is still substantial.  For water the liquid-gas latent heat is 540 calories per gram while from solid to liquid it is 80 calories per gram.  The same effect works in reverse

As the water in the Arctic freezes over, it releases 80 calories for every gram of water that freezes.  This, of course, doesn't heat up the overlying air.  It simply "tries" to keep the temperature at zero until there is enough ice covering the ocean to provide insulation from the underlying water.  Even then, as more ice freezes on to the bottom of existing ice, the heat produced has to conduct through the ice into the overlying air.  So why is this important.

Remember that the surrounding land is rapidly cooling down as it radiates heat into space and the sun is not replacing this heat in the daytime. The sun no longer shines in the Arctic in the winter.  So all around the Arctic ocean we have cold land and descending  air or a high pressure area.  This should also be able to create powerful storms at the beginning of the winter freeze-up period. 

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