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Pattern Jarring January

This is interesting
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1300m said:
This is a solid analysis of the literal NAO index and it identifies a major problem with how it is computed. Unfortunately, a negative NAO in and of itself is analyzed over too broad of an area. The key is in the positioning of the ridging over Greenland and the tropospheric vortex.

See the examples below: in what is arbitrarily defined as an "east-based" negative NAO, the anomalies are located further east such that temperatures across the eastern U.S. are actually above-average even though the NAO is strongly negative! In the "west-based" negative NAO example, the blocking ridge over Greenland and trapped tropospheric vortex are further west, keeping a signature arctic tap straight from Alaska into the eastern and southeastern U.S.

Positional_NAO.png


In an analysis I did while working at the State Climate Office of NC I did find that the number of snow days where at least one station reported 1" or more of snowfall (excluding the mountains) increased by 25% in all El Nino winter months, and 2/4 La Nina winter months.

NAO_Interactions.png


If this same analysis was performed for west-based NAO regimes, I am confident the signal would be even stronger. And it is no surprise than in my big RDU snowstorms H5 analysis that the west-based NAO regime is clearly present. This is an important lesson to remember about climate pattern indices - not all regimes are created equal and just because a chart says an index is in a favorable phase doesn't mean the results are going to turn out as expected without further investigation.

http://climate.ncsu.edu/climate/patterns/nc-snowfall
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1300,
Great response! In light of what you're saying about the NAO index sometimes being deceiving, I was wondering if you would please post H5 anomaly maps for these days, which are 5 days before some major SE snowstorms:

2/4/1967, 2/4/1973, 2/25/1980, 3/19/1983, 2/1/1984, 1/2/1988, 2/12/1989, 2/19/2015
 
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pcbjr said:
Amen ... ;)
Glad someone with cred is preaching what this idiot has been spouting for years ... :eek:
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I agree. I don't "expect" anything in particular when an index is whatever. I use them as tools in combination with other indices to give me an idea of the tendencies. Example: a significantly higher % of +PNA days will be cold in the SE US vs that for -PNA days. Same could be said for -AO, -NAO, -EPO, and weak MJO phases on left side of chart. Does that mean any one day you look at has to work out this way? Not in the least. There are many exceptions. The indices should be used as guidelines rather than as crystal balls.
 
GaWx said:
I agree. I don't "expect" anything in particular when an index is whatever. I use them as tools in combination with other indices to give me an idea of the tendencies. Example: a significantly higher % of +PNA days will be cold in the SE US vs that for -PNA days. Same could be said for -AO, -NAO, -EPO, and weak MJO phases on left side of chart. Does that mean any one day you look at has to work out this way? Not in the least. There are many exceptions. The indices should be used as guidelines rather than as crystal balls.
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Damn, Larry,
I thought I might get the Bard pin ... exceptional and succinct post!
Best!
Phil
 
1300m said:
This is a solid analysis of the literal NAO index and it identifies a major problem with how it is computed. Unfortunately, a negative NAO in and of itself is analyzed over too broad of an area. The key is in the positioning of the ridging over Greenland and the tropospheric vortex.

See the examples below: in what is arbitrarily defined as an "east-based" negative NAO, the anomalies are located further east such that temperatures across the eastern U.S. are actually above-average even though the NAO is strongly negative! In the "west-based" negative NAO example, the blocking ridge over Greenland and trapped tropospheric vortex are further west, keeping a signature arctic tap straight from Alaska into the eastern and southeastern U.S.

Positional_NAO.png


In an analysis I did while working at the State Climate Office of NC I did find that the number of snow days where at least one station reported 1" or more of snowfall (excluding the mountains) increased by 25% in all El Nino winter months, and 2/4 La Nina winter months.

NAO_Interactions.png


If this same analysis was performed for west-based NAO regimes, I am confident the signal would be even stronger. And it is no surprise than in my big RDU snowstorms H5 analysis that the west-based NAO regime is clearly present. This is an important lesson to remember about climate pattern indices - not all regimes are created equal and just because a chart says an index is in a favorable phase doesn't mean the results are going to turn out as expected without further investigation.

http://climate.ncsu.edu/climate/patterns/nc-snowfall
Click to expand...
Great post. Wonder if we’ll ever see a west-based -NAO in the heart of winter again in our lifetimes?
 
Supposed to hit 70 here Friday. I expect to see some wintry weather coming a week later.
 
1300m said:
I'm going to let the cat out of the bag a little here - I am pretty close to a climate change skeptic. I do not deny that some warming has occurred over the past 30 years, but I do not believe in the magnitude of change that is generally accepted, and I definitely don't believe that anthropogenic forcing is the primary driver. I have done research on the degree of warming in NC and it is less than half at non-urban locations as it is at rural sites like RDU. I struggle to see any significant correlations between a "warming Earth" and our current weather.

Why am I putting this unpopular, and very criticized, opinion out here in a discussion about the NAO?! Because to me, the lack of winter-time negative NAO regimes is probably one of the largest climate-related changes I see and I don't have an answer for it. Is it something about the change in ice cover over the Greenland region? Even in strongly negative winter-time AO regimes over the past 5 years the NAO really has not responded. There have been numerous occasions where NWP suggest a major negative west-based NAO episode only to greatly diminish it as verification nears. I do believe that something in the climate has shifted to cause the winter-time west-based neg. NAO to disappear, but I don't have the answer to what that is exactly.
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I’m largely in your camp here. Regarding the wintertime -NAO, I haven’t heard any compelling rationale as to why we don’t see it much anymore other than having to do with climate change. I guess we have to blame it on that since there’s no other obvious reason. I don’t really like settling for that answer, but it’s also hard to accept that we’re just in an unprecedented period of bad luck. Something has to be causing it.
 
Rain Cold said:
I’m largely in your camp here. Regarding the wintertime -NAO, I haven’t heard any compelling rationale as to why we don’t see it much anymore other than having to do with climate change. I guess we have to blame it on that since there’s no other obvious reason. I don’t really like settling for that answer, but it’s also hard to accept that we’re just in an unprecedented period of bad luck. Something has to be causing it.
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I’m thinking the enormous amount of sea ice loss has something to do with it by disturbing the polar vortex.


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I'm on GW side of the coin as 1300m. In my opinion the reason why the west based Neg Nao( or any neg Nao for that matter) has been so absent is because of a lesser amount of sea ice in that region. However on the the Pacific side you have the same thing happening and the net effect is seeing the EPO behave the way it has. The EPO is in turn deflating and influencing the NAO even when you get stretches of neg AO. The NAO is downstream of the results being produced in the Pacific and even though a neg AO argues we should see a neg nao, the EPO signal trumps orverwhelms any neg AO trickle down effect. Hope that makes sense and just an opinion as to possibly why we don't see a neg nao due to the cycle we are currently in.
 
Looks like we need another mechanism to get deep snow in central NC.


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1300m said:
I'm going to let the cat out of the bag a little here - I am pretty close to a climate change skeptic. I do not deny that some warming has occurred over the past 30 years, but I do not believe in the magnitude of change that is generally accepted, and I definitely don't believe that anthropogenic forcing is the primary driver. I have done research on the degree of warming in NC and it is less than half at non-urban locations as it is at sites like RDU. I struggle to see any significant correlations between a "warming Earth" and our current weather.

Why am I putting this unpopular, and very criticized, opinion out here in a discussion about the NAO?! Because to me, the lack of winter-time negative NAO regimes is probably one of the largest climate-related changes I see and I don't have an answer for it. Is it something about the change in ice cover over the Greenland region? Even in strongly negative winter-time AO regimes over the past 5 years the NAO really has not responded. There have been numerous occasions where NWP suggest a major negative west-based NAO episode only to greatly diminish it as verification nears. I do believe that something in the climate has shifted to cause the winter-time west-based neg. NAO to disappear, but I don't have the answer to what that is exactly.
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Enjoy your posts, they are very informative.

As your original post indicated the NAO is the single most significant feature that Raleigh depends on for big snows.

The last 5 6”+ events for Raleigh all had -NAO. Let’s hope climate change isn’t the cause of the recent period of +NAO winters, rather a long term cycle.

12/26/2010
02/27/2004
01/01/2002
01/26/2000
01/07/1996
 
1300m said:
I'm honestly not convinced it is entirely due to climate change. If it were, how did we go from one of the strongest west-based neg. NAO episodes ever in December 2010 to January 2011 to basically a non-existent NAO since then? It's hard for me to believe something that dramatic shifted from January 2011 onward related to the climate change issue that caused such a dramatic change in the NAO. I'm not saying it may not be contributing, and maybe there was a critical amount of Greenland ice cover that was surpassed after 2011 that has resulted in this - but for it to be so sudden and dramatic raises questions.
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Yep, agree. It’s possible that a tipping point was reached, as you say, but I haven’t seen any research on it that has resulted in a definitive conclusion. Tipping points are hard to forecast and also hard to see in real-time. But they should be easier to identify in hindsight. Maybe someone will do a study on it someday.
 
Didn't we snap on Webb for just mentioning a warming background state, while these guys have a nice discussion on Climate change in the wrong thread? Fair is fair folks.. Take this discussion to the Climate Change thread.
 
whamby said:
Didn't we snap on Webb for just mentioning a warming background state, while these guys have a nice discussion on Climate change in the wrong thread? Fair is fair folks.. Take this discussion to the Climate Change thread.
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I don't remember that happening exactly. It may have been a more lengthy conversation the other time, and once it gets too long off it can be an issue. If it happened then I'm not sure when.
 
1300m said:
I'm honestly not convinced it is entirely due to climate change. If it were, how did we go from one of the strongest west-based neg. NAO episodes ever in December 2010 to January 2011 to basically a non-existent NAO since then? It's hard for me to believe something that dramatic shifted from January 2011 onward related to the climate change issue that caused such a dramatic change in the NAO. I'm not saying it may not be contributing, and maybe there was a critical amount of Greenland ice cover that was surpassed after 2011 that has resulted in this - but for it to be so sudden and dramatic raises questions.
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There's a considerable amount of interannual variability that masks long-term changes, and clearly in 2010-11 the major icelandic volcano eruption likely had a lot to do w/ why the -NAO was so incredibly impressive. The relationships wrt NAO/AO variability and extratropical volcanic eruptions are more elusive however it could be argued that the placement of the eruption in the context of the NAO changed the radiative budget of the stratosphere s.t. the circulation anomalies generated by persistent absorption of incoming solar radiation by sulfate aerosols (& thus warming of the lower-mid stratosphere) may have persisted and propagated down into the troposphere by the subsequent winter of 2010-11 over the far North Atlantic where the volcanic ash was most persistent and intense. Observing a dramatic change like this from year-to-year or even a large -NAO episode or multi-year/decade period w/ increasing negative regimes doesn't necessarily negate the long-term signal in the NAO attributable to AGW which is towards an increasingly positive regime as was the case during the Medieval Warming Period. The stout +NAO signal in the MWP completely reversed in the subsequent cooling period during the Little Ice Age. I think a substantial pathway wrt how the NAO responds to the climate has to do w/ background changes in ENSO behavior attributable to large-scale global temperature change on timescales of a few-several centuries or so. During warmer regimes in the Quartnerary period, the upper ocean warms more quickly than the deeper ocean, which increases the thermal stratification that suppresses stronger warm ENSO events as suggested by Kohyama (2017) (who presented on this at the AMS conference in Austin earlier today) and favors a La Nina mean background state. This La Nina mean state modulates the trajectory and placement of an extratropical Rossby Wave train that emanates from the mean convective center in the tropics s.t. a weaker Aleutian Low & stronger Icelandic Vortex/+NAO is favored during a NINA and vis versa. The warmer climate also pushes the extratropical storm track further north due to Hadley Cell expansion and on the whole is more conducive to poleward EP flux which strengthens the mid-latitude jet stream and the exit regions of the jet (which again is conducive to a stronger Icelandic Vortex).
 
whamby said:
Didn't we snap on Webb for just mentioning a warming background state, while these guys have a nice discussion on Climate change in the wrong thread? Fair is fair folks.. Take this discussion to the Climate Change thread.
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Climate variability and weather aren't mutually exclusive especially on timescales beyond a few days or so, thus the +NAO climate change discussion is applicable to this thread.
 
Depending on the source, the first 8 days of this month have been -13 to -15 on the average here. Of course, today skewed it warmer some.

Unless it gets really warm, January will probably end up finishing below average.
 
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