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NAO's Influence On Synoptics


The Eagle

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Posted
  • Location: Co Dublin, Ireland
  • Location: Co Dublin, Ireland

    Hello Everyone

    Over the last couple of months I have been figuring out what the NAO is, so ive come up with the following and i am just wondering is there something missing? :)

    My understanding is the NAO is based on pressure values in the North Atlantic. The 'normal' state of the atmosphere in this part of the world involves a zone of high pressure - the Azores high- which as the name suggests resides near the Azores and low pressure near Iceland.

    However sometimes this setup differs and you may end up with the Azores high stronger than the Iceland low or they might be equal in Intensity.

    The index used to determine the state of the setup is known as the NAO. Positive, Average and Negative. A positive conclusion suggests a strong Azores high and deep Icelandic low.

    Positive values are associated with strong Westerly winds which blow across the North Atlantic leaving us generally mild. :(

    Negative values however point to a kind of sterile atmosphere where Northern Europe suffers colder winters due to the increaded frequency of Northeasterlies and Easterly winds. :p

    Looking through the records i reckon its important to say that during the 1960s the NAO was very negative (and im sure everyones heard of those winters) :blink:

    Then it really just rose up to around the early 90s and became strongly positive. IMO that could explain the slacker Winters. Its also odd that considering its effect on the synoptic situation IMO the Met Office seasonal till January says this actually could be a generally mild winter.hmmmmmmm Is the reading of about -1 for this year likely to influence the positioning of the Greenland high I wonder, :blink: and perhaps give us a Channel low and its Easterlies....If im missing anything please tell me :)

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    Posted
  • Location: Portland, Dorset
  • Weather Preferences: Mixed winters and springs, thundery summers and meditteranean autumns
  • Location: Portland, Dorset

    Its all very complicated!

    At the least likely, or least desireable extremes;

    Positive NAO may at times, be associated with an active Atlantic opposed by a Scandinavian or Russian high, bringing cold southeasterly flow over the UK, and Atlantic systems bringing rain can turn to spells of snow or blizzards. Early 1996 may have been a good example :huh:

    Negative NAO may at times, be associated with mild and cloudy anticyclonic northwesterly flow over the UK, whilst spells of cold polar activity affect Scandinavia and central/ eastern Europe. Late January through to mid February this year paints a good picture of that. :huh:

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    Posted
  • Location: Portland, Dorset
  • Weather Preferences: Mixed winters and springs, thundery summers and meditteranean autumns
  • Location: Portland, Dorset

    A vey interesting read, cheers.

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    Posted
  • Location: Norton, Stockton-on-Tees
  • Weather Preferences: Snow and cold in winter, warm and sunny in summer
  • Location: Norton, Stockton-on-Tees
    Yeah i've always been interested in this too.

    <{POST_SNAPBACK}>

    If you are interested in the NAO and the effects on our climate then you will like this link:

    http://www.john-daly.com/theodor/naonew.htm

    It shows a correlation between NAO and our climate, specifically our winters. It also illustrates that the NAO is a cyclical thing and that we are due to enter a long term negative phase.

    AM

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    Posted
  • Location: Portland, Dorset
  • Weather Preferences: Mixed winters and springs, thundery summers and meditteranean autumns
  • Location: Portland, Dorset
    If you are interested in the NAO and the effects on our climate then you will like this link:

    http://www.john-daly.com/theodor/naonew.htm

    It shows a correlation between NAO and our climate, specifically our winters. It also illustrates that the NAO is a cyclical thing and that we are due to enter a long term negative phase.

    AM

    <{POST_SNAPBACK}>

    Thanks for that.

    Another very interesting read. Let's hope this long term negativity brings back our winters properly, for a while! :o

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    Posted
  • Location: Lincoln, Lincolnshire
  • Weather Preferences: Sunshine, convective precipitation, snow, thunderstorms, "episodic" months.
  • Location: Lincoln, Lincolnshire

    I don't think the NAO causes synoptic changes- it is the other way round, the jet and synoptics are what add up to produce the monthly NAO values.

    It is quite right to say that NAO+ months sometimes have a dominant Scandinavian High and active Atlantic, with a battleground between the two. However, while on 5-7 February 1996 this led to a widespread snow event (locally the severest since 1947), early January 1996 showed the darker side of this pattern. It was dull and mild with southerly winds, and the cold air stayed out in the continent.

    Another possible snowy NAO+ setup is when there is high pressure over Greenland deflecting much cold air into the mid Atlantic, giving us many returning polar maritime snowfalls (e.g. January 1978, January 1984, March 1995), but the problem here is that if pressure is often low to the west of Iceland, the air has to take a very long track over the ocean such that it isn't very cold any more (e.g. early February 1988, January 1994, January & December 1999)

    As for snowless NAO- setups, you need to look no further than winter 2002/03. Interestingly, winter 2004/05 was strongly positive until the second half of February, according to recent NAO updates.

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    • 11 months later...
    Posted
  • Location: North Kenton (Tyne-and-Wear)6miles east from newcastle airport
  • Location: North Kenton (Tyne-and-Wear)6miles east from newcastle airport

    Morning Guys and Gals

    I apologise if this is in the wrong thread but i need a bit of help ,[ and this is a tough subjet to learn]

    question is

    im trying to learn and understand Teleconnections , {and is it possible and if so How] can we predict Temperatures for the Uk just by looking at the PNA and the NAO , Looking at the Latest forecast for the NAO its showing that it will be on the positive side prob from around mid september , now leading up to that it is on the Negative side ,How can we gain temperature readings from this at

    hope ive made some sense

    Nigel

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    Posted
  • Location: Steeton, W Yorks, 270m ASL
  • Location: Steeton, W Yorks, 270m ASL
    Morning Guys and Gals

    I apologise if this is in the wrong thread but i need a bit of help ,[ and this is a tough subjet to learn]

    question is

    im trying to learn and understand Teleconnections , {and is it possible and if so How] can we predict Temperatures for the Uk just by looking at the PNA and the NAO , Looking at the Latest forecast for the NAO its showing that it will be on the positive side prob from around mid september , now leading up to that it is on the Negative side ,How can we gain temperature readings from this at

    hope ive made some sense

    Nigel

    The short answer is that you can't. The best a forecast of NAO can do is indicate a general pattern of mildness of cold, but that pattern will be no more than a forecast. As TWS says further up it's important to understand that the NAO as measured is not a signal, it's a LAGGING measure of mean pressure differential. The logic is straightforward: A +ve NAO means pressure is higher over the Azores than it is around Iceland. The mean flow is therefore W-E, i.e. off the Atlantic. This doesn't have to mean mild - though it will tend to, this general pressure gradient would still allow for a polar flow. A -ve NAO puts pressure higher over Iceland than the Azores, this means a mean flow from E-W, suggesting continental air. Again, not forced to be cold (in winter), but that will tend to be the case, though this set up could allow, say, a straight southerly (at a push).

    To reitterate though, NAO is NOT a teleconnection or in any way a specific synoptic feature. All it is is a measure of average pressure gradient between two points west of the UK.

    Looking through the records i reckon its important to say that during the 1960s the NAO was very negative (and im sure everyones heard of those winters) :whistling:

    Then it really just rose up to around the early 90s and became strongly positive. IMO that could explain the slacker Winters. Its also odd that considering its effect on the synoptic situation IMO the Met Office seasonal till January says this actually could be a generally mild winter.hmmmmmmm Is the reading of about -1 for this year likely to influence the positioning of the Greenland high I wonder, :) and perhaps give us a Channel low and its Easterlies....If im missing anything please tell me :)

    ...therefore, the NAO doesn't really EXPLAIN change in winter weather, it's just another (very simplified) way of describing the average surface synoptics. It certainly doesn't point to drivers, and if we want to get better at long term predictions then it's the drivers that we need to be (far) better at understanding.

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    Posted
  • Location: North Kenton (Tyne-and-Wear)6miles east from newcastle airport
  • Location: North Kenton (Tyne-and-Wear)6miles east from newcastle airport

    Morning Stratos Ferric

    Thankyou for that information ,i think i understand now ,so if their is ie high pressure over iceland if im not mistaken the winds would be virtually approaching from the north East or from the north- East part of the compass ,and if there is a a high over the azores the winds approaching from the south - West part of the compass

    Think i have that the right way round ,

    thankyou again you have been very helpful

    nigel

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