February AO/PNA Scenarios

[donsutherland1]

With February approaching and the La Niña likely to persist, it is useful to examine some scenarios concering AO/PNA combinations during La Niña events. Since 1950, there have been 18 occasions during which the February ENSO Region 3.4 anomaly came to -0.50 or below: 1950, 1956, 1963, 1968, 1971, 1974, 1976, 1981, 1985, 1986, 1989, 1996, 1999, 2000, 2001, 2006, 2008, and 2009.

During such cases, the PNA, in general, typically exerted greater influence over the nationwide temperature anomalies than the AO. However, severely negative AO regimes could overcome a weaker PNA regime. The following two tables show the various AO/PNA regimes (monthly averages):

For purposes of examining the temperature anomalies associated with each combination from the aforementioned 18 cases, I took the composites where each figure was 0.50 or more standard deviations from the mean. The temperature anomalies were as follows:

PNA -0.50 or below/AO -0.50 or below:

Note: PNA-/AO- regimes with figures less than 0.50 standard deviations from the mean also resulted in a cool west/warm east scenario.

PNA -0.50 or below/AO +0.50 or above:

Note: PNA-/AO+ regimes with figures less than 0.50 standard deviations from the mean resulted only in a smaller area of cool anomalies in the west.

PNA +0.50 or below/AO -0.50 or below:

Note: PNA+/AO- regimes with figures less than 0.50 standard deviations from the mean resulted in almost nationwide warm anomalies.

PNA +0.50 or above/AO +0.50 or above:

At this point, while there remains a lot of uncertainty, I suspect that the combination of the development of a long-duration PNA+ regime in the face of the La Niña/PDO- regime and experience with severe to extreme December blocking episodes suggests that the third combination (PNA+/AO-) might be more likely than not. That does not mean that either index cannot reach the opposite state. In fact, I suspect that the AO will likely be predominantly positive into and perhaps through the first week in February before heading lower (possibly sharply lower). For now, though, these combinations merely provide some insight into various combinations with the AO/PNA for February during cases when the ENSO Region 3.4 anomaly was -0.50 or below.

[weatherdude]

Nice work Don!

[Reale WX]

Thanks Don... very useful stuff. Looks like nice correlations too

[wxmx]

Winter of extremes, looks like

[Organizing Low]

i was surprised that the +PNA / +AO combo would yield such warm anomalies up here.

i figured it would be a wash.

[donsutherland1]

i was surprised that the +PNA / +AO combo would yield such warm anomalies up here.

i figured it would be a wash.

2000 skews the composite. 2008 actually had cool anomalies around the Great Lakes region/Ohio Valley.

[usedtobe]

Don, as always, an interesting post. I was surprised by how warm the positive pna and nao looked like.

[Organizing Low]

2000 skews the composite. 2008 actually had cool anomalies around the Great Lakes region/Ohio Valley.

interesting to that 2000 was warm under that scenario.....mustve been somethign else going on?

[Snow Wizard]

It certainly doesn't appear that a -AO hurts the chances for a negative PNA. There is still a very decent chance the PNA could flip in February.

[Chinook]

I recently read that in 2010,

A negative Arctic Oscillation (AO) in January and February helped usher in very cold Arctic air to much of the Northern Hemisphere. Record cold and major snowstorms with heavy accumulations occurred across much of eastern North America, Europe and Asia. The February AO index reached -4.266, the largest negative anomaly since records began in 1950.

This is copied directly from this web site

http://www.noaanews....lobalstats.html

[pcbjr]

Can someone explain why the NAO per GFS is forecast to go down on the 7 and 10 day forecast, but then be up at the same exact time on the 14 day (towards the end of the month)?

[Mitchel Volk]

Using persistence, I believe the NAO will return to be strongly negative this February I also think it will be very stormy and cold since there is a lot of cold air in Canada to tap into the pattern. The +PNA pattern may swatch back towards a -PNA, but it should not make much of a difference. I truly beleave this winter is a good analog to the 95/96 winter.

[JBG]

At this point, while there remains a lot of uncertainty, I suspect that the combination of the development of a long-duration PNA+ regime in the face of the La Niña/PDO- regime and experience with severe to extreme December blocking episodes suggests that the third combination (PNA+/AO-) might be more likely than not. That does not mean that either index cannot reach the opposite state. In fact, I suspect that the AO will likely be predominantly positive into and perhaps through the first week in February before heading lower (possibly sharply lower). For now, though, these combinations merely provide some insight into various combinations with the AO/PNA for February during cases when the ENSO Region 3.4 anomaly was -0.50 or below.

Is there any way of cranking such winters as 1916-17 and 1909-10, which seem from the point of view of sensible weather, into the equation?

[donsutherland1]

Is there any way of cranking such winters as 1916-17 and 1909-10, which seem from the point of view of sensible weather, into the equation?

1916-17 is a better fit. 1909-10 had a PDO+ (something that isn't the case today).

[nzucker]

Using persistence, I believe the NAO will return to be strongly negative this February I also think it will be very stormy and cold since there is a lot of cold air in Canada to tap into the pattern. The +PNA pattern may swatch back towards a -PNA, but it should not make much of a difference. I truly beleave this winter is a good analog to the 95/96 winter.

It's amazing how much cold air there is in Canada right now...I don't think I've ever seen so much territory inside the -30C contour:

Very impressive and bodes well for the pattern in the next two weeks.

[donsutherland1]

To date, the 2010-11 La Niña event has been starkly different from recent moderate/strong La Niña events in terms of cold anomalies. However, against the larger historical backdrop, it is not unique.

Prior to 1920, 4 of 11 (36%) moderate/strong La Niña events featured widespread cold in the East. For purposes of definitions, I classify a "moderate" event as having a December-February average ENSO Region 3.4 anomaly of -0.75 or below. The brutally cold Winter of 1917-18 just missed that threshold, averaging -0.707. A "strong" La Niña is one during which the December-February average ENSO Region 3.4 anomaly came to -1.00 or below. The four La Niña winters featuring the widespread cold in the East were 1872-73, 1903-04, 1909-10, and 1916-17. Three of those winters were clustered over a thirteen-year span.

The composite temperature anomalt for Winters 1903-04, 1909-10, and 1916-17 was as follows:

Winter 1873-73 featured widespread cold over the eastern half of the U.S. The database I used to construct the composite only goes back to 1895, but allows for 0.5° intervals. Given that Winter 1872-73 had a similar profile, I chose to use that more detailed database.

For some time, I have suggested that February will likely wind up with cold anomalies in a large portion of the East, particularly the Mid-Atlantic, Northeast, and Ohio Valley. I expect something reasonably close to the AO-/PNA+ composite in the first message in this thread, except that the Southeast might wind up milder than normal.

FWIW, here's the composite February temperatue anomaly for the above-noted three winters:

Notice how the February composite anomaly mirrors the overall winter anomaly.

FWIW, Winter 1872-73 was cold in both the Mid-Atlantic and Northeast.

[A-L-E-X]

Don, some wonderful research right there. I wonder what caused those winters to be so different back then, especially with such strong la ninas? Was it primarily solar? It's especially shocking we had long duration la ninas in back to back winters like 1916-17 and 1917-18 and they were the only two years to feature back to back 50 inch plus snowfalls at NYC.

Also, when you have time, can you give your thoughts for next week's storm as modeled by the 12z Euro? It seems to be showing the reincarnation of PD2

[donsutherland1]

Don, some wonderful research right there. I wonder what caused those winters to be so different back then, especially with such strong la ninas? Was it primarily solar? It's especially shocking we had long duration la ninas in back to back winters like 1916-17 and 1917-18 and they were the only two years to feature back to back 50 inch plus snowfalls at NYC.

Also, when you have time, can you give your thoughts for next week's storm as modeled by the 12z Euro? It seems to be showing the reincarnation of PD2

Right now, I don't know the reason those Niñas differed so much from most of the recent moderate/strong ones.

As for next week's storm, I believe something more significant than the GFS shows is likely. Moreover, a colder solution than the 0z Euro is more likely. I'm not sure whether we'll have a blockbuster event as depicted on the 12z Euro, but a significant event is a distinct possibility.

[A-L-E-X]

Right now, I don't know the reason those Niñas differed so much from most of the recent moderate/strong ones.

As for next week's storm, I believe something more significant than the GFS shows is likely. Moreover, a colder solution than the 0z Euro is more likely. I'm not sure whether we'll have a blockbuster event as depicted on the 12z Euro, but a significant event is a distinct possibility.

As always, thanks your thoughts, Don. What really catches my eye is those "aberrant" la ninas all occurred in a short time frame (13 years, as you pointed out.) Which makes me think they all had some other factor in common which the more recent ones hadnt-- until now.

[donsutherland1]

Upon further inspection, it appears that the cluster (1903-04, 1909-10, and 1916-17), occurred during and just after a span of three abnormally weak solar cycles.

http://en.wikipedia.org/wiki/List_of_solar_cycles

Currently, we're in the midst of just the first abnormally weak solar cycle in quite some time.

IPCC, noted, "Stratospheric aerosols from large volcanic eruptions have led to important, but brief-lived, negative forcings (particularly the periods 1880-1920 and 1960-1994)..." However, as the moderate/strong La Niñas during the 1960-94 period did not exhibit characteristics similar to the current one, I don't believe stratospheric aerosols from volcanic eruptions provided the single explanation for the cluster in question.

Right now, I can only hypothesize that the combination of a sequence of weak solar cycles and stratospheric aerosols together faciliated the outcome. A similar situation existed during the early 19th century. But data is more sparse. Nonetheless, the following winters may have had moderate/strong La Niñas based on annual ENSO reconstructions: 1800-01, 1801-02, 1804-05, 1805-06, 1818-19, and 1819-20. My record of major 19th century snowstorms indicates that 1800-01 was quite snowy in New England, 1804-05 had a KU-type storm from New Jersey into New England, 1818-19 was mild but there was a substantial interior snowstorm, and 1819-20 had a KU-type snowstorm that extended from the Mid-Atlantic region into New England. I'll have to check the David Ludlum books for some select temperature data e.g., the New Haven record might extend into the timeframe in question.

[A-L-E-X]

Don, thanks for the info! The following info is from Unc, and note how there is another cluster from the 1870s......

according to those numbers here is a list of the strongest la nina winters...

1916-17...-22

1892-93...-15

1942-43...-15

1973-74...-15

1909-10...-14

1970-71...-14

1975-76...-14

1949-50...-13

1955-56...-13

1988-89...-13

1999-00...-13

2010-11...-13? estimated

1873-74...-12

2007-08...-12

1875-76...-11

1945-46...-11

1872-73...-10

1908-09...-10

1967-68...-10

I highlighted some popular analogs.

[A-L-E-X]

source:

ftp://www.coaps.fsu.edu/pub/JMA_SST_Index/jmasst1868-today.filter-5

[donsutherland1]

Don, thanks for the info! The following info is from Unc, and note how there is another cluster from the 1870s......

1872-73 was a cold, moderate La Niña winter. I didn't include it in the composite map, even as I mentioned it, because the dataset I used goes back to 1895. 1886-87 (moderate) saw a cold winter in the Northeast/Mid-Atlantic. 1892-93 (strong) featured cold winter anomalies in the Northeast/northern Mid-Atlantic.

[A-L-E-X]

1872-73 was a cold, moderate La Niña winter. I didn't include it in the composite map, even as I mentioned it, because the dataset I used goes back to 1895. 1886-87 (moderate) saw a cold winter in the Northeast/Mid-Atlantic. 1892-93 (strong) featured cold winter anomalies in the Northeast/northern Mid-Atlantic.

Don, I find it interesting that some of these cold winters are amongst the strongest la ninas on record (specifically 1892-93, 1916-17 and 1942-43). March 1893 was our former snowiest month on record.

I remember you mentioned 1892-93 and 1942-43 awhile back when making comparisons to how this winter has been unfolding.

[Jebman]

Spot on analysis as usual Don and others.

This has already been one freezing cold winter in northern VA. I've suffered thru TWO severe viral episodes in two months.

I can already hear the cries of "I told you so, I told you so" after I prayed for a modern Maunder Minimum.

It has been some time since I can remember a winter with SO MUCH BELOW NORMAL weather here in Woodbridge. You can count the number of normal to above normal days in this winter ON ONE HAND lol.

If the +PNA don't cool us down, that raging -NAO/-AO Couplet will definitely make one hell of a comeback and we'll chill out in Feb. Or - we'll endure BOTH - +PNA AND the negative AO/NAO Couplet simultaneously.

Brrr.

[pcbjr]

Upon further inspection, it appears that the cluster (1903-04, 1909-10, and 1916-17), occurred during and just after a span of three abnormally weak solar cycles.

http://en.wikipedia....of_solar_cycles

Currently, we're in the midst of just the first abnormally weak solar cycle in quite some time.

IPCC, noted, "Stratospheric aerosols from large volcanic eruptions have led to important, but brief-lived, negative forcings (particularly the periods 1880-1920 and 1960-1994)..." However, as the moderate/strong La Niñas during the 1960-94 period did not exhibit characteristics similar to the current one, I don't believe stratospheric aerosols from volcanic eruptions provided the single explanation for the cluster in question.

Right now, I can only hypothesize that the combination of a sequence of weak solar cycles and stratospheric aerosols together faciliated the outcome. A similar situation existed during the early 19th century. But data is more sparse. Nonetheless, the following winters may have had moderate/strong La Niñas based on annual ENSO reconstructions: 1800-01, 1801-02, 1804-05, 1805-06, 1818-19, and 1819-20. My record of major 19th century snowstorms indicates that 1800-01 was quite snowy in New England, 1804-05 had a KU-type storm from New Jersey into New England, 1818-19 was mild but there was a substantial interior snowstorm, and 1819-20 had a KU-type snowstorm that extended from the Mid-Atlantic region into New England. I'll have to check the David Ludlum books for some select temperature data e.g., the New Haven record might extend into the timeframe in question.

Don,

Sort of way south of you, but in early Feb 1899 (the 4th, I think) Tallahassee FL went to -2ºF (coldest in state history) and there was a pretty significant snow storm into Fla (upwards of 6" I've read and heard from my Great G-Pa) - any idea of the AO/NAO/PNA/EPO set up then? I'd have no way of knowing how or where to research that - but apparently it was a cold SE period (one of our local courthouses built in the very early 1900's has a mural depiction of that Feb it was so unusual).

Thanks!

[uncle W]

Don,

Sort of way south of you, but in early Feb 1899 (the 4th, I think) Tallahassee FL went to -2ºF (coldest in state history) and there was a pretty significant snow storm into Fla (upwards of 6" I've read and heard from my Great G-Pa) - any idea of the AO/NAO/PNA/EPO set up then? I'd have no way of knowing how or where to research that - but apparently it was a cold SE period (one of our local courthouses built in the very early 1900's has a mural depiction of that Feb it was so unusual).

Thanks!

here's a link to some old data...

http://www.cgd.ucar.edu/cas/jhurrell/indices.data.html#naostatdjfm

[donsutherland1]

here's a link to some old data...

http://www.cgd.ucar....tml#naostatdjfm

Thanks, Uncle W. That's a great resource.

[donsutherland1]

With respect to the 1800-01, 1801-02, 1804-05, 1805-06, 1818-19, and 1819-20 La Niña winters, here are the New Haven temperature anomalies for those winters:

1800-01: +2.6°

1801-02: +4.5°

1804-05: -1.1° (February was warmer than normal)

1805-06: +5.3°

1818-19: +0.7° (December was 4.8° below normal)

1819-20: -0.7° (December was normal and January was 4.2° below normal)

Source: David M. Ludlum's Early American Winters 1604-1820

[pcbjr]

Thanks, Uncle W. That's a great resource.

Being the "asker", I'd like to ditto Don's reply - Thanks!