Sunday, 17 June 2012

Digging deeper into the winter pattern


Taking a look at why we need a spike to warm in the PDO in order to get a cold and snowy winter in the east.

If we look at 1951-2 and 1972-3 we can see lower heights in the west due to the cold PDO that hangs on and this causes higher heights in the east and warm winters.

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This contributed to warm surface temperatures, drier and less snowy conditions in the east and central part of the country. The west, meanwhile stays cold in this type of pattern.
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Looking at the pressure pattern in those years we can see lower then normal pressures where the el nino develops while we also see higher then normal pressures where the pool of warm water is found in a cold PDO. This is surrounded by lower pressures where the horse shoe of cold water develops. Because of this we see the compensation of lower then normal pressures in the arctic and across greenland (which is what caused the low amounts of blocking in those years) and higher then normal pressures across eastern canada. This type of pattern does not support strong highs coming down from north west canada which would bring very strong cold air masses to the east. This is quite the opposite and it makes it very hard for that to occur. Also, we see a drier pattern in the eastern U.S because we are not seeing lower then normal pressures developing in the area.
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If we look at 4 years that were in a cold PDO overall and were el ninos after double or triple la ninas we can see the higher heights over the west(caused but the warm PDO which would favour more ridging in the west as opposed to the constant troughiness and cold weather we are seeing there now) and we are seeing a strong trough in the east. This produces a very cold temperature pattern in the East. We can also see the higher then normal heights across the arctic and greenland which allows blocking to develop and this brings cold air masses down into the eastern part of the country and allows strong snow storms to develop.

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If we also look at the pressure pattern in those winters we can see lower then normal pressures where the el nino develops as well as where the pocket of cold water develops due to the cold PDO. We see a ring of higher pressures around it. This creates a compensating rise in pressures across northwestern canada as well as the arctic and in tern we see lowering of pressures in the eastern part of the country. This is an exact OPPOSITE pattern of the other two years I showed you caused by the difference in the PDO! This is the perfect recipe to bringing strong highs down from northwestern canada. This, combined with a strong sub tropical jet that the moderate el nino produces allows for more storms to meet up with very cold air masses. In one of my blog posts I mentioned the fact that the spike warm in the PDO is the key to producing blocking for north america across Greenland. The reason that the blocking develops is due to the rise in pressures around Greenland (created by the compensation needed in the pressure pattern because of the Warm PDO that developed) and this also allows for the lowering of pressures in the east. These relationships I am showing you are going to be the keys to unlocking what occurs next winter and why.

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This type of el nino pattern along with a spike in the PDO to just slightly warm(it doesn't need to be very warm) translated to some of the most severe winters ever for the eastern part of the country! This story is not told by the temperature departure map at the surface but we all know how brutal these winters were both snow and cold wise!

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The CFS and the CFSv2 are showing this weakening in the PDO occurring but I believe as we go on in time it will even become more bullish on this idea. The modelling right now isn't really picking up on the specifics of what might happen in this pattern but they will come around as the SST's for next winter become more certain. I believe that I have a really good handle on the pattern that is going to develop next winter whether a warm PDO develops or not and I hope you guys enjoyed my post! Feel free to comment!

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