Friday, April 27, 2012

How Can the Past Winter Pattern Influence Hurricane Season?

Earlier in the year, I explained the drivers of this winter's record setting warmth and lack of snow. At the top of the list are the strongly "positive" behavior of the North Atlantic and Arctic Oscillations. (Read my earlier post which explains the specifics of each oscillation) The effects that each oscillation produces can linger for months even after the numbers positive or negative have stabilized. In most seasons when one or more teleconnection is abnormally strong (as was the case the last 3 winters), the change in air pressure, temperature and ocean temperature, the dynamical imprint on the atmosphere can be much greater.  These imprints seem to trickle down into the large scale wave changes and stochastic processes that drive our weather patterns.

A great example of these changes are seen in the Atlantic Ocean sea surface temperatures. Remember the last few winters consistently cold and snowy pattern?  This was largely driven by record low Arctic and North Atlantic Oscillation levels.  Remember that the North Atlantic Oscillation are caused by changes in pressure of the Icelandic Low and the Azores High. If the Azores High becomes weaker, the Icelandic Low stays north/stronger, the NAO is negative, more frequent cold air outbreaks/more clipper snows.  When the Azores high becomes stronger, the NAO goes positive, not as much cold air.  

What does this have to do with hurricane season?  Look at the pressure patterns over the Atlantic in the spring following a strongly NEGATIVE NAO winter. The Azores High (shown in light green) is smaller and weaker.

Dec, Jan and Feb 2009-2010
Dec, Jan and Feb 2010-2011
The steering currents around the high stay weak.  These weak trade winds from the east don't cause upwelling of deeper colder water. Here are the steering winds over the last 2 winters into early spring.

Now this early spring

The turnover of water is much smaller which allows the sea surface temperature to increase.  Here are the Atlantic sea surface temperatures in early spring in 2010 and 2011 after a STRONGLY NEGATIVE NAO winter.  Very warm water is located in the tropical storm development areas.

Now look at this winter's NAO signature.  Remember our mild pattern?  Look at the Icelandic Low and the Azores High.  The High is very strong which kept cold air locked up in Canada.   

The Icelandic Low stays further north which kept the flow across the US and the Great Lakes out of the west NOT THE NORTHWEST.  The Azores High allowed the steering currents to stay strong which is now causing the upwelling of water off of Africa and into the central Atlantic development region.

Currently, the Atlantic Sea Surfaces are much cooler than in springs past covering a larger area.  
So the positive NAO this past winter was a result of a stronger High in the central Atlantic. This strengthened the trade winds which caused upwelling of cooler water which has led to a cooler Atlantic development area.

Will this result in fewer tropical storms developing earlier in the season?  Will this result in less tropical storm or hurricanes affecting the east coast or Gulf coast?  We shall see...

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