Western Climate Variability

							2009-10 Snow Siege: Role of El Niño, NAO, &
             ‘Global Change’




                    Klaus Wolter, and NOAA-CSI Team*
 NOAA-Earth System Research Lab & University of Colorado at Boulder-CIRES
                         klaus.wolter@noaa.gov
  * CSI Team, headed by Marty Hoerling, with partners at CPC, NCDC, GFDL
                                        CPDW, Raleigh, NC
                                           04 Oct ‘10
                  What happened last winter?




Washington, DC broke a seasonal snowfall record that was established more
than a century ago (so did Baltimore, Philadelphia, etc.)
Was this just a Mid-Atlantic fluke?




  No, you were not alone – Flagstaff, AZ (left) had their 2nd biggest
  snow storm ever in January (54”); Madison, WI, ditto (right, 19”)
  in December; in Colorado Front Range, this one competed with
  1997-98 (more on El Niño below).
Snow cover hit (near-) record
levels in North America, both
in December and February!
 Top 10 snow storms for DC, Baltimore, and Philadelphia
       DC                        Baltimore                     Philadelphia
Amount Dates NAO ENSO        Amount Dates NAO ENSO          Amount Dates NAO ENSO
 28.0”   Jan 27-28, 1922    28.2”    Feb 15-18, 2003    E    30.0” Jan 7-8, 1996   B
 20.5”   Feb 11-13, 1899    26.5”    Jan 27-29, 1922         28.5” Feb 5-6, 2010   B, E
 18.7”   Feb 18-19, 1979 B  24.8”    Feb 5-6, 2010   B, E    23.2” Dec 19-20, 2009 B, E
 17.8”   Feb 5-6, 2010 B, E 22.8”    Feb 11-12, 1983 B, E    21.3” Feb 11-12, 1983 B, E
 17.1”   Jan 6-8 , 1996 B   22.5”    Jan 7-8, 1996   B       21.0” Dec 25-26, 1909

 16.7”   Feb 15-18, 2003    E 22.0” Mar 29-30, 1942 E         19.4”   Apr 3-4, 1915     E
 16.6”   Feb 11-12, 1983 B, E 21.4” Feb 11-14, 1899           18.9”   Feb 12-14, 1899
 16.4”   Dec 19-20, 2009 B, E 21.0” Dec 19-20, 2009 B, E      16.7”   Jan 22-24, 1935 B
 14.4”   Feb 15-16, 1958    E 20.0” Feb 18-19, 1979 B         15.8”   Feb 10-11, 2010 B, E
 14.4”   Feb 7, 1936     B    19.5” Feb 10-11, 2010 B, E      22.0”   Mar 29-30, 1942 B, E

 B refers to the lower quartile of monthly NAO time series from 1895 through 2010 (a.k.a.
 ‘blocked’ NAO situations, verified by visual inspection of 20th century reanalysis maps);
 E refers to the upper quartile of bimonthly MEI.ext time series (back to 1871) (a.k.a. ‘El
 Niño’ phase) <if no link, we would expect to see 7.5 occurrences each, have 19 B and 17 E>

 Winter-time ENSO and NAO are poorly correlated; ¼ * ¼ gives a 6% chance to have both
 B and E at the same time (observed 12/30, or more than six times as often!)
What is the established role of ENSO for snowfall?




               Patten et al., Weather and Forecasting, 2003
What kind of circulation pattern prevailed last winter?




                                              ∆500hPa
                                              20m C.I.




                                       CSI Report, 2010
Average circulation for 30 snowiest winter months at DC




                                               ∆500hPa
                                               5m C.I.




                                         CSI Report, 2010
Anomalously high pressure near Greenland vs. below-
normal pressure southwest of Spain = negative NAO




Schematic of climate condition (contours, showing jet stream) and weather events
(color shading snowing surface temperature/precipitation) associated with the extreme
phases of the North Atlantic Oscillation.
                                   Source: http://airmap.unh.edu/background/nao.html
Impacts in Europe?




                     Avignon,
                     France,
                     8jan10 (top);
                     Great
                     Britain,
                     7jan10 (left);
                     Oslo tied all-
                     time
                     minimum
                     temperature
                     record (right)
Eurasia
4th highest Dec-Feb Eurasian
snowcover on record teams up
with highest on record for North
America to give Northern
Hemisphere 2nd highest winter
snowcover in over 40 years of
satellite records
2010 had lowest seasonal value of NAO on record (1823+)




                                                   2010
Combining El Niño with negative NAO explains much of
                           what happened this winter

                           December-February 2010 observed near-
                           surface temperatures anomalies (top left),
                           and the components linearly related to El
                           Niño (bottom left), negative NAO phase
                           (bottom right), and the linear combination
                           of El Niño and NAO (top right), based on
                           monthly NCEP/NCAR Reanalysis 1000mb
                           temperatures. Contour interval in lower
                           panels is ½ that in the upper panels.

                           El Niño impact is calculated by regressing
                           Niño3.4 ∆SST on wintertime 1000mb
                           temperatures during 1951-2010, and then
                           scaling by observed Niño 3.4 seasonal
                           index value for 2009-2010). NAO impact is
                           calculated in similar fashion.


                                             CSI Report, 2010
            What about ‘Global Warming’?




Underwhelming warming signal in Mid-Atlantic states over
last century (but matches NAO fairly well, except for 1940s)


                                             CSI Report, 2010
            What about ‘Global Warming’?




Seasonal snowfall has actually been declining over last
century in Mid-Atlantic states (right), part of a larger
pattern (left; Changnon et al., J. Appl. Meteor. Clim., 2006)


                                               CSI Report, 2010
                   What about ‘Global Warming’?




Have snowstorms gotten heavier (higher ratio of moisture to snow), and/or
warmer in Mid-Atlantic region? Apparently not, but this is complicated by
freezing rain/sleet (‘83, ’96), or windy conditions (undercatch, such as Feb’10).

Most top-10 storms have high temperatures near freezing, not much room for
warmer temperatures (only two exceeded this level, in ‘42 and ‘79)
                   Concluding remarks
1. Record-breaking snow season for Mid-Atlantic states was
   anchored by unprecedented combination of ‘Top 10 snowstorms’,
   and was part of hemispheric pattern;
                    Concluding remarks
1. Record-breaking snow season for Mid-Atlantic states was
   anchored by unprecedented combination of ‘Top 10 snowstorms’,
   and was part of hemispheric pattern;
2. El Niño provided fuel, as in the past, to numerous heavy snow
   storms in the U.S. from last fall right through the winter, and
   into spring 2010;
                    Concluding remarks
1. Record-breaking snow season for Mid-Atlantic states was
   anchored by unprecedented combination of ‘Top 10 snowstorms’,
   and was part of hemispheric pattern;
2. El Niño provided fuel, as in the past, to numerous heavy snow
   storms in the U.S. from last fall right through the winter, and
   into spring 2010;
3. While currently not predictable(?), the NAO provided decisive
   focus to a southward shifted storm track, to the tune of the
   lowest NAO values in almost 200 years of record-keeping;
                     Concluding remarks
1. Record-breaking snow season for Mid-Atlantic states was
   anchored by unprecedented combination of ‘Top 10 snowstorms’,
   and was part of hemispheric pattern;
2. El Niño provided fuel, as in the past, to numerous heavy snow
   storms in the U.S. from last fall right through the winter, and
   into spring 2010;
3. While currently not predictable(?), the NAO provided decisive
   focus to a southward shifted storm track, to the tune of the
   lowest NAO values in almost 200 years of record-keeping;
4. There is no clearly discernible temperature or snowfall trend in the
   Mid-Atlantic region, nor do we find any systematic shifts in the
   temperatures or moisture content of the most extreme snowfall
   events. Thus, we place ourselves firmly in between those who want
   to deny the existence of climate change and those who want to
   blame every extreme event on exactly that forcing factor.