Associations Between ENSO and AO in Winter

A moderately warm ENSO event seems to be a lock this winter. The multi-variate ENSO Index (MEI) is bullish as are a majority of numerical and statistical ENSO forecast models...even though the Southern Oscillation (data) hasn/t made up its mind.

Conventional wisdom suggests this scenario favors a split-flow regime with an active...moisture-laden sub-tropical jet (STJ) and opportune phasing with the polar jet (PJ) and producing strong cyclogenesis along the eastern seaboard. Add arctic HIGH pressure over eastern Canada and it/s snow-time.

The graphic below was extracted from NCEP/Climate Prediction Center/s ATLAS No. 8 - Relationships Between El Niño-Southern Oscillation and the Arctic Oscillation: A Climate-Weather Link

It shows composite fields for 500 mb geo-potential height...surface temperarture...and precipitation during +ENSO and three phases (positive...neutral...and negative) of the Arctic Oscillation (AO).

Best-case for snow crows during warm ENSO is a negative AO (bottom row of charts).

Otherwise...the mid-level long-wave trof axis is suppressed to the south...(neutral AO) or there/s no trof at all (+AO) and temperatures are above normal with the best moisture contained across the Deep South and the Mississippi and Tennessee River valleys.

Winter '09 / '10 - WSI

Todd Crawford - Seasonal Forecaster:

"A moderate El Nino event will likely persist through the winter, resulting in cool weather across the Southeast”[...]. “However, ocean temperatures in the North Pacific are not currently supportive of a typical El Nino pattern this upcoming winter. If this persists, this winter will likely be colder in the western US than is typical during El Nino events.

"In the Northeast, we expect a relatively mild fall, with increased chances of sharply colder weather after the New Year. The combination of various climate indices suggests that the winter will likely arrive late but loudly in much of the East, and that a mild start will not necessarily persist through the winter.

"In December, WSI forecasts:
Northeast – Warmer than normal"

More...

North Atlantic Oscillation - September '09

+1.51

2007...2000...1956...1974...and 1999 are still the best NAO-analog years after the addition of September/s data point. Only change was '74 and '99 switching places from August/s rankings.

What/s it all mean...Mr. Peabody?

2007: -ENSO...+AO...-PDO...-QBO...sun spots below normal
2000: -ENSO...-AO...+PDO...-QBO...sun spots near normal
1956: -ENSO...+AO...-PDO...-QBO...sun spots above normal
1974: -ENSO...+AO...-PDO...-QBO...sun spots below normal
1999: -ENSO...+AO...-PDO...+QBO...sun spots near normal

El Niño more than likely this winter...so how much utility is there using NAO analog years given their unanamous association with -ENSO?

Winter '09 / '10 - Quasi-biennial Oscillation

The Quasi-biennial Oscillation (QBO) is characterized by alternating easterly and westerly descending wind regimes at the equator. It dominates the variability of the tropical stratosphere. Although the QBO is a tropical phenomenon...it affects the stratospheric flow from pole to pole by modulating the effects of extra-tropical waves. [M. P. Baldwin et.al., 2001 (.pdf)]

The period of oscillation varies between 20 and 36 months...with an average period of about 28 months...as measured over the past half-century. The QBO index at each stratospheric level is the zonally-averaged zonal (east-west) wind. It is the most predictable inter-annual climate fluctuation on the planet.



August QBO @30mb is -14.46 (data)...three months into its nascent easterly phase. It/s expected to remain easterly through spring 2010...reaching its peak amplitude (< -25) during the heart of meteorological winter.

QBO is important b/c it determines the character of the early winter. The east phase leads to a more disturbed and warmer Arctic in December and January. During winter...QBO appears to excite the 'northern annular mode' (aka Arctic Oscillation [Thompson and Wallace, 1998 (.pdf), 2000]. Its easterly phase is represented in composites geo-potential...wind...and temperature fields by a weaker polar vortex (PV)...warmer polar temperatures...and higher geo-potential heights at hi-latitudes. In years with low solar activity...the winter PV tends to be disturbed and weak when the QBO is easterly.

Monthly sunspot activity YTD (data) has been anomalously quiet (lowest 10th percentile) 24 of the past 32 months and every month since April 2008. Top-5 sunspot analog years: 1913...1823...1811...1810...1912.

Reconstructed NAO index (Li) for '12/'13 is 1.44 and '13/'14 is 1.63... neither of which lend any support to a correlation between weak PVs with its implied -AO/-NAO and extremely low sunspot activity.

Top-5 sunspot analogs for years beginning 1950 and their D-J-F AO/NAO: 2008 (0.259/-0.08)...1954 (-0.717/-0.76)...1996 (-0.096/-0.07)...1964 (-1.125/-0.61)...1976 (-2.617/-1.04) supports the correlation.

Polar temperature varies out of phase with sunspot activity (low sunspot activity ==> warmer temperature ==> higher geo-potential heights) for winters when the QBO is easterly. Weak...'warm' PVs are less able to contain arctic air masses at hi-latitudes than cold...strong...less disturbed PVs. This suggests potentially favorable conditions this winter for arctic outbreaks leading to strong baroclinic zones developing between land and water which represent prime conditions for cyclogenesis and block-buster Nor'easters.

Easterly QBO also favors the occurrence of major winter stratospheric warmings [Holton and Tan (1980)(.pdf)]. Stratosphere warmings propagate from 10mb to the surface in ~3 weeks. Should such a warming occur mid-winter during QBO minimum...it could very well signal a cold end to meteorological winter.
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Should QBO play a dominant role this winter...

• Easterly phase during low solar activity ==> favors weak PV
• Weak PV ==> high heights @ hi-latitudes
• High heights @ hi-latitudes ==> negative northern annular modes (-AO / -NAO)
• -AO / -NAO ==> arctic outbreaks + increased east coast snowfalls
• Easterly phase favors major stratospheric warmings ==> lagged arctic outbreaks

QBO analogs YTD.

1991 is currently the leading analog. Winter '91 / '92 featured +ENSO (albeit one that became strong by spring)...+AO...+NAO...and -SOI...all of which go against type for -QBO. Did anonymously high (96th percentile) monthly sunspots play an active and deciding role?

EDIT: corrected sign of Winter '91 SOI.

Winter '09 / '10 - No Negative NAO

Eastern snow crows are often heard aching for a negative North Atlantic Oscillation (NAO) come winter b/c of its negative correlation to season-total snowfall. Winters where the NAO is...on average...negative (positive)...higher (lower) snowfalls and colder (milder) temperature are more likely.

Buzz-killing evidence is beginning to mount that is sure to disappoint those pining for predictions of a coming negative NAO.

The Developing Case Against Negative NAO
1) The British MET Office's 'positive NAO' forecast is based on SST anomalies in the Northern Atlantic during the month of May. The technique's correlation skill is about 0.45 with the correct sign of the NAO predicted for 66% of all winters. Details here.

2) Trend analysis supports a positive NAO this winter. NAO/s yearly 'winter' average (D-J-F-M) has been positive 21 out of the last 26 years. NAO/s five-year moving-average for the same four months has been positive every year since 1983. Data.

3) Analog years point to positive NAO this winter. NAO has been negative five months this year...three of which occured during meteorological summer. This was also the case the past two summers. July observed its third lowest value (-2.15) since 1950.

Analog years selected using a 'least-squares' regression technique (shown below) are 2007...2000... 1956...1999...and 1974. Monthly NAO values for the analog years are displayed through year/s end.



When monthly NAO index values for the analog years are extended through the winter months of D-J-F...three of the five analog years had a positive monthly NAO index. One analog year/s NAO monthly index was positive during J-F. Another analog year/s NAO monthly index was positive during D-J. NAO analog analysis does not support a forecast for negative NAO this winter. Similar story from the Arctic Oscillation (AO).



4) Contravening evidence is found in the Quasi-biennial oscillation (QBO)...which is beginning its negative phase and will persist throughout the coming winter. QBO/s east phase and solar minimums are correlated with positive high-latitude height anomalies. More about this winter/s QBO here.

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Negative NAO is not a be all and end all for decent season-total snows...nor is it required for severe winter storms along the eastern seaboard. NAO was positive when The 'Storm of the Century' raked the east coast in mid-March of 1993.

Positive NAO image courtesy Lamont-Doherty Earth Observatory...Columbia U.

Winter '09 / '10 - AccuWeather - Update

Joe Bastardi...Senior Meteorologist:

"...similar historic weather trends...could herald a bitterly cold winter, particularly for the Northeast and South.

"The closest comparison we’re using for this upcoming winter is the 2002-03 winter season..." Then you have the extreme case of the 1977-78 winter, which is also a possibility. The latter winter held the record for subfreezing temperatures for an unrelenting 51 days."

The Market Oracle

Winter '09 / '10 - Commodity Weather Group

Matt Rogers...Meteorologist:

"Some forecasters see the potential for the coldest US winter in a decade. "We do think there are opportunities for a potentially colder than normal winter coming up, one of the coldest we've seen in the past decade,"...

The Market Oracle

Winter '09 / '10 - CPC Outlook (SEPT)

CPC/s 2.5 month long-lead forecast for D-J-F...issued yesterday...continues to reflect their expectation for a strengthening an el Niño event. Last month/s outlook here.



A moderate strength +ENSO event (3-month SSTa between 1°C and 1.5°C) now supported by 11 dynamic and statistical forecast models...up from 9 models in August. This may be a result of the 12-week moving average in Niño 3.4 (data) having reached +0.84°C following three weeks of +0.9°C.



+ENSO favor above-normal snowfall over the mid-Atlantic...especially along coastal regions and northern New England. Throw in an easterly QBO...which favors positive height anomalies at high latitudes during solar minimums...and the ingredients for good snows are beginning to come together.



Climate Prediction Center/s 'ENSO Impact' page here.

Winter '09 / '10 - AccuWeather

Joe Bastardi, Chief Hurricane and Long-range Forecaster:

*Chance of colder winter in Northeast, Europe and Far East
*Midwest outlook warmer than last year

"Not only does it have a chance to be colder in the U.S. Northeast but also in the Far East and probably Europe.

"But the EL Nino is not really 'causing' anything in my view. It's a by-product of larger scale forces going on, some of them cyclical, like very low Sun spot activity, which can cause a decrease in radiation from the Sun. This weaker El Nino could fade, as it did in '06-'07, when we went from one of the warmest winters to very cold.

"I expect New York and Washington D.C. to have more snow and to be colder. I'm expecting it to be colder than last year in southern New England more than in the northern part.

"The Midwest should be warmer, like in Ohio and the parts that got blasted with so much snow last year."

Reuters

Winter '09 / '10 - Earthsat

Travis Hartman, Energy Weather Manager:

*December and January warmer than normal, but cooler than the average over the past 10 years
*February temps to fall below normal, including along the eastern seaboard.

"We see the main driver for the winter being a moderate strength El Nino in the central tropical Pacific. With that we see a propensity for warmer-than-normal temperatures -- normal being the 30 year normal from 1971 to 2000 -- across the northern U.S. tier, specifically the Northern Plains, the Midwest and the Great Lakes region."

Reuters