Winter '11 / '12 - Arctic Oscillation - On the Cusp

NYC - East 46th St. near 2nd Ave.
16-JAN-65

The Arctic Oscillation (AO) at winter's mid-point remains pegged in a persistently positive mode.  Save for a few brief flirtations with its alter ego...the AO has been above zero for 41 days since the start of meteorological winter (01-DEC) 46 days ago.

The polar vortex (PV) has been wicked strong until recently when it weaken slightly during the on-going minor stratospheric warming.  Minor warmings are not known for their association with PV-reversals...so there's little expectation this event will be reflected at the surface in the form of a negative AO.

What a difference a year makes.  Last December/s monthly AO was the second lowest (-2.631) during the period of record which began in 1950.  JAN-11 didn't stand out in the historical record (ranked 14th) but it was strongly negative.  This winter...December was the polar opposite at +2.221.  January is on-track to finish...not unexpectedly positive...around +1.

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Below is a time-series of the AO on winter's cusp throughout the 60+ year period of record (1950 - 2011).  The blue line is a trace of the daily AO index on 15-JAN.  The dark red line is a a trace of the same data after applying a nine-point binomial filter.  The filter removes low frequency noise and highlights decadal trends.

Not sure what this says about the AO on 15-JAN other than there is a discernible cycle every nine years or so.  It does have an interesting comparison to the cycles found in the 30-DEC data.

Winter '11 / '12 - North Atlantic Oscillation - About Face

NAO dips below zero for the first time in 55 days (-0.082 on 17-NOV-11).

Question now is will it last and if it does...how strong will it be?  Current GooFuS and its ensemble forecasts hold out little hope...

Arctic Oscillation (AO) forecasts are another...more promising story...

For many NEWxSFC forecast stations...a negative AO plays a bigger role than a negative NAO for contest-worthy storms.

Related posts:
Going Negative
Going Negative - How Much for Philly
Going Negative - How Much for DC?
Going Negative - How Much for Dulles?

Daily teleconnection data here.

Winter '11 / '12 - Sudden Stratospheric Warming Watch - Update 5

The initial minor warming event @10 mb...which began 25-DEC-11...is fading as a second warming gets underway on the opposite side of the globe (red shading lower left quadrant).

Note the weakening of the mean zonal wind in the upper 10 mb centered around 01-JAN-12 (upper right corner) which reflects dissipation of the initial minor warming event.

Elsewhere in the Ignorosphere...temperatures have been steadily warming at 30 mb over the North Pole and intersecting the long-term mean.

European NWP breaks down the polar vortex and reverses its flow @D+10 (blue shading = east flow) and depicts mean easterlies in the hi-latitude troposphere (lower right corner) which builds a deep anti-cyclone...

American NWP forecasts for the northern annular oscillations (AO / NAO) and the Pacific North American (PNA) indexes are all heading in a favorable direction...

Previous update here.

Winter '11 / '12 - VCP32 - Need Nine More of These

Winter '11 / '12 - Eurasia Snow Cover - December

The latest monthly data from the Rutgers Snow Lab shows the observed areal snow cover over Eurasia was -- yawn -- average during December after being at or quite close to one standard deviation above the mean the past two years.

The month's ~26,642,000 km² coverage ranked 26th in the 46-year period of record (1966 - 2011) which is slightly less than the median December coverage of ~26,786,000 km².

Solid ORANGE line:  period of record's AVERAGE areal snow cover.
Dashed YELLOW lines:  + / - one standard deviation about the mean
RED line:  nine-point binomial filter

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Snowfall data courtesy Rutgers Snow Lab.
Earlier post about Novembers' Eurasia snow cover here.

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BONUS chart...
Monthly Eurasian snow cover for 2011 (RED line; BLUE markers) and the 'period of record' median.

Winter '11 / '12 - Sudden Stratospheric Warming Watch - Update 4

Tervetuloa Sääbriefing!

Hi-latitude 10 mb temperature well above average.  These warm temperatures extend to near 30 mb.

Source:  CPC/s Stratosphere:  Global Temperature Time Series

NWP models continue the forecast trend for a significant weakening of the polar vortex (PV) and a bold reversal @D+10.   The blue area above 10 mb between 50°N to 90°N latitude (upper right corner) depicts a mean zonal wind flow from the east (-u:  into the board).  The orange area in the troposphere centered above 30°N latitude (lower left corner) is the strong sub-tropical jet blowing from the west (+u:  out of the board).

In the study of dynamic meteorology...the 'u' wind is the zonal component (negative for east / positive for west) of the 'total' wind.  The 'v' wind is the meridional component (positive for north / negative for south) of the 'total' wind.

If the forecast reversal of the PV is observed...the effects of this change would be felt at the surface about three weeks...which portends a very cold February.

Previous update here.

Winter '11 / '12 - Sudden Stratospheric Warming Watch - Update 3

The 'minor' warming event -- indicated by areas shaded in red -- having started on 25-DEC-11...has gained significant latitude as it propagated east from ~140°E longitude over Asia to a position near 80°W latitude over NOAM.

The temperature @10 mb over the North Pole has risen from -80°C in mid-DEC to almost -50°C...as of 02-JAN-12.

The lower stratosphere/s extension of the polar vortex (PV) @100 mb has split in two.  The split is forecast to upwell into the 50 mb level next week as shown in the graphic below.

The upwelling is forecast to accompany a broad...deep column of frigid air imported from Siberia into Canada's Hudson Bay region around D+10.  Of particular interest is the short-wave ridge downstream of the cold pool suggesting the feature may b/come a closed circulation.  The ECMWF 500 mb heights within this feature approach 492 dm....GooFuS ~120 meters deeper.

Finally...the PV continues to lose strength...decreasing to less than 40 kts (20 m/s) by the end of the period.  Also note 1) the mean zonal wind from the east throughout the depth of the polar troposphere...quite suggestive for a developing anti-cyclone and a long awaited return of the Arctic Oscillation to its negative state and 2) the robust 80kt (40 m/s) sub-tropical jet (STJ) INVOF 30°N latitude.

All the ingredients for severe winter weather in the eastern CONUS are forecast to come together during the start of winter's second half.

Previous update here.

Winter '11 / '12 - Arctic Oscillation and Chi Square

Geo. Washington...Valley Forge...PA (1928)

Wes Junker...of the WaPo/s Capital Weather Gang blog...crafted an interesting post last month about January/s temperature and precipitation prospects.  He included a rough analysis on the likelihood of December/s persistently positive Arctic Oscillation (AO) carrying over through January.  The article concluded the chances were high for such an outcome.

One measure of whether two events -- such as the AO for two months -- are independent of each other or whether a strong association exists between them is the chi-square test.  The distribution of observed events (positive one month, negative the next or vice-versa) is compared to the distribution of the 'expected' events.  Small differences between the 'observed' and 'expected' are interpreted as evidence of an association.  Large differences mean they are independent.

No statistical testing was presented in Junker/s post; however...strong evidence for an association does exist...as shown in the table below at the intersection of DEC and JAN.  The probability is 0...to three decimal places...the association between  December/s and January/s AO state occurs by chance.

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What about the other 'cool season' months?  Can more predictive information be mined from these data?  Is there a statistically significant association between the AO state lurking elsewhere?

Monthly AO data between 1950 and 2010 from CPC were analyzed using the 'chi-square' statistic to determine if there were statistically significant associations between months.

There is a fairly strong association between SEP/s AO state and that of FEB (a 5.5% probability the association occurs by chance) and a good association between JAN and FEB (8% probability)

What does AO state for SEP-11 'predict' for FEB-12?  The SEP-11 AO was 0.665...so FEB-12 'should' follow suit and be positive.

What does AO state for JAN-12 'predict' for FEB-12?  The JAN-12 AO is unknown but since JAN follows DEC....JAN-12 AO 'should' also follow suit...which implies FEB-12 will be positive.

Winter's where the AO ran hot between DEC and FEB have occurred nine times since 1950 (1972/3...1974/5...1975/6...1988/9...1991/2...1992/3...1998/9...1999/00...2007/08).

Not lookin'good for the home-team...altho all may not be lost this winter.

The so called 'Storm of the Century' roared out of the GOM and tore up the east coast in MAR-93 after +AO in D-J-F (1.627; 3.495; 0.184) and MAR was 0.764!  There was a 'notable' NESIS storm in MAR-99 and the unforgettably infamous JAN-00 storm.

Winter '11 / '12 - Meteorological Winter - Month One

One down.
Two to go...altho the Contest does run until the storms stop -- assuming they do get started.

As bad as it/s been for areal snow cover over the forecast area so far this winter...DEC-03 was worse with DEC-06 a close second.

December/s end-of-month snow cover between 2002 (upper left panel) and 2011 (lower right panel).  Images courtesy NOHRSC.

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Disappointed snow crows blame the villainous duo of la Nina...which favors a storm track west of the Appalachian mountains...and the persistently positive state of the Arctic Oscillation from a paucity of hi-latitude blocking.

The above table -- sorted by descending 'percent of climo' -- shows December snowfall totals for the 'season-total' forecast contest stations.  Trace amounts are reported as 0.05.  Green (red) cells are the top (bottom) 25th percentile of values in each column.

Not a single station is even close to normal snow...altho the distribution is typical for a la Nina winter.  December's total 'normal' snowfall is 214".  The month's observed snowfall is 44", a deficit of 169" and a mere 21% of normal.

Winter '11 / '12 - Arctic Oscillation - Day 30

500 mb height anomalies
associated with
three states of the
Arctic Oscillation

Thirty days into the season...the Arctic Oscillation (AO) remains well above zero despite a weakening polar vortex (PV) and hi-latitude easterly winds.

The 500 mb height anomalies (below right) have looked more like the top chart which has kept arctic cold away from the mid-latitudes.  The ECMWF continues to depict a weak anti-cyclone building over the Pole; however the on-set is always at least seven days away. 

Today/s 500 mb height anomalies

The minor stratospheric warming event @10 mb has contributed to a weakened PV but is occurring well away from the pole INVOF northeast Asia and has not propagated below 30 mb.

Minor warmings do not cause the PV to break down or cause a wind reversal from west to east so it's debatable how much influence this event will have on establishing hi-latitude blocking.  Best guess:  very little.

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Below is a time-series of the AO on 30-DEC throughout the 60+ year period of record (1950 - 2011).  The blue line is a trace of the daily AO index on 30-DEC.  The dark red line is a a trace of the same data after applying a nine-point binomial filter.  The filter removes low frequency noise and highlights decadal trends.

Not sure what this says about the AO at the end of December other than there is a decernable cycle every seven years or so.