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Thursday, July 4, 2013

A Negative Feed Back in the Arctic

We have all being focusing on positive feed backs  which enhance  climate change in general and arctic ice melting in particular.  The most commonly quoted positive feed back is the fact that as more of the Arctic ocean is open water, it can absorb much more heat from the sun than when it was ice and snow covered and this will melt more ice.  Give the system a push and it goes further in the direction of the push than you expected.  Now we see a negative feed back.

If you have been following the NSIDC web site with it's graph of sea ice extent, you see that as of today (June 28, 2013) the rate of decrease in Arctic Ice Extent is going down parallel to the average line from 1979 to 2000.  It is about one standard deviation below the line but running parallel to it.  This time last year, the ice extent graph had already dipped sharply downward and was diverging from the average line.  So what is happening.

For the last few weeks there has been a strong low pressure, counter clockwise weather system bouncing back and forth between the shores of the Arctic ocean. Low pressure is, of course, an area of rising air; a storm.  As air rises it reaches the dew point and clouds form.  Clouds, of course reflect heat back into space and keep the land below cool.  Earlier in the year, a great deal of fracturing occured north of Alaska and this may well be the source of the added heat that is powering the  low pressure area.  In addition, the ice is thinner which would allow more heat into the air.  Remember what happened last summer.

The ice extent was falling rapidly when on Aug6, 2012, a hurricane moved into the Arctic.  This sent the graph plummeting even faster.  Why the difference between these two storms.

The present storm is happening with the Arctic ocean almost completely covered with ice.  The wind can not act on lots of open water and build up large waves which break up the remaining ice.  It can't stir the water, bringing up the deep, salty, warmer water from below.  The August storm, with much less ice covering the sea,  did all this. plus flinging ice outwards into the trans-polar drift to be sent out through the Fram Straight.  We may have a mechanism here that will cause some stalling of the rapid year to year decrease in the extent of the ice.

One wonders what is powering this low pressure system.  Classically, the power comes from sensible heat from  the ocean below and latent heat as water vapour condenses.  With the Arctic ocean covered with ice, where would the water vapour be coming from.  Perhaps the ocean is not as covered as the NSIDC web site indicates.  Apparently the satelite that measures ice extent reads any area with more than 15% ice as completely covered.  There could be a lot of open water between the ice flows and still appear to the satellite to be completely covered.

Did you see earlier this spring a time lapse of the ice North of Alaska.  The clockwise weater system was apparently in effect and you see pictures of the ice splitting up leaving lots of water between the flows.  Water, is, of course, around zero degrees while the ice can be much colder.  Cosiderable water vapour could be contributed from a significant area of openings between the ice flows.

Whatever the mechanisms of this phenomenon, we have a low pressure area over the Arctic shading the area from the sun.  This year is going to be fascinating.  Apparently a storm early in the melt season is not equivalent to a storm later on.

Of course, to at least some extent, the effect could be an artifact.  Since any area with more than 15% ice cover is seen by the satellite as being completely covered.  In another blog, I described how a counter clockwise air circulation pattern tends to spread ice out.  It could be that there is much more clear water than the satellite indicates but in small leads between the ice flows rather than in one area as is the case with a clockwise air circulation system that tends to pull the ice together leaving open water outside the location of the ice.  In fact clear water has been observed by the North pole.  The existence of a low pressure, counter clockwise system over the Arctic tends to support this possibility as it is a sign of energy being transferred to the air from the water. 

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