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Brendon Hills Bandit
23 February 2015 22:18:50

What with the Azores high and the resultant positive NAO making it difficult to get a cold spell for the UK this winter, I thought it might be interesting to discuss what the future holds.


As I am sure many of you will know, the NAO has been largely positive for the last 30 years or so. I've read a few scientific articles, and it seems that this may be because of global warming, some scientists suspect, something to do with rainfall in the Indian Ocean IIRC. 


However of course in the winter of 2009-10 the NAO was the lowest it has ever been in the 190-year record. This might just a blip in the overall trend I suppose, but it still takes some explaining I think, and I haven't been able to find much scientific opinion on the NAO since that winter, the articles I found on the web about global warming causing the NAO to go positive where from the early 2000s. 


So I was wondering if forum members had any theories about what the NAO will be like in future winters, or have read any recent research.


 


 


 

220m asl, edge of Brendon Hills
KevBrads1
23 February 2015 22:33:53

Isn't the natural state of NAO is to be positive?

The North Atlantic average pressure pattern in winter is a +ve NAO.

Icelandic low and Azores high are semi permanent features, I have a map from 1913 which shows them on.

Shouldn't the debate be on whether the pressure gradient between Iceland and Azores has increased overall in the last 25 years?

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Brendon Hills Bandit
23 February 2015 23:05:02
http://earthobservatory.nasa.gov/Features/NAO/NAO_4.php 

It's worth reading all of this article I'd say, it explains why the MO use SSTs from May to forecast the coming winter's NAO.
220m asl, edge of Brendon Hills
Gavin P
24 February 2015 09:50:29
http://earthobservatory.nasa.gov/Features/NAO/NAO_4.php 

It's worth reading all of this article I'd say, it explains why the MO use SSTs from May to forecast the coming winter's NAO.


Thanks. That will be acquired reading with my morning coffee.

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Rob K
24 February 2015 12:00:44
Does "negative NAO" actually mean the pressure in the Azores (or wherever they measure it) is lower than in Greenland (or wherever)? I would have thought that very rarely happens. Or is the baseline "zero NAO" taken as, for example, a 10mb or 20mb pressure difference? For such a commonly used marker, it is remarkably difficult to find out exactly what the definition is!
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Maunder Minimum
24 February 2015 12:16:14

"The winter of 2009-10 the NAO was the lowest it has ever been in the 190-year record."


My view on that is that it was related to the long period of very low solar activity during the extended solar minimum between solar cycles 23 and 24.

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Brendon Hills Bandit
24 February 2015 13:31:19

 


Thanks for the replies.


 



"The winter of 2009-10 the NAO was the lowest it has ever been in the 190-year record."


My view on that is that it was related to the long period of very low solar activity during the extended solar minimum between solar cycles 23 and 24.


Originally Posted by: Maunder Minimum 


Yeah, I think there might some credence to that view, low solar activity tends to make the jet stream weaker, and I think this might affect the pressure gradient between Iceland and the Azores, though it is probably a complex two-way thing. I also read this Daily mail article that was quite well informed, in which NASA scientists where quoted as saying that excessive polar ice melt had forced that NAO into a hyper-negative state. 


As for the future of the NAO, I personally think that global warming will have all sorts of conflicting results for global weather, so it is probably very difficult to work out what will happen in the future, of course there will also be the influence of natural variations, solar activity, AMO, etc. 


Also it possible for temps to be below average in the UK with a +NAO, and vice versa! Though it does seem to be one of the strongest, if not the strongest, influences. 


 

220m asl, edge of Brendon Hills

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