• Observational analysis supports link between leading fall Eurasian snow cover and Northern Hemisphere winter climate.
• The dominant Northern Hemisphere winter circulation pattern can be shown to often originate with a two-way stratosphere-troposphere
interaction forced by high latitude Eurasian land surface and lower
tropospheric atmospheric conditions (i.e. high pressure) during autumn.
• This cycle occurred has contributed to recent decadal winter temperature trends for the extratropical Northern Hemisphere.
• Lower tropospheric conditions differ prior to polar vortex displacements than prior to polar vortex splits.
NCEP/NCAR Reanalysis 1948-2010 Eurasian snow cover 1948-2009-
a. Remotely sensed snow cover from 1967-2009 (Robinson’s NOAA dataset)
b. Reconstructed snow cover from 1948-1966 (Brown’s reconstructured dataset)
CRUTEM3 for land surface temperature data 1950- 2009
WCRP Coupled Model Intercomparison Project
phase 3 (CMIP3) multi-model dataset.
r (SON Eurasian Snow, DJF Z500)
Cohen and Entekhabi [1999]
H L
L H
H
Increased snow cover in the fall over Siberia leads to rapid development of Siberian High.
Cold dense air needs to spread out, it is blocked to the south and east by high topographical barriers in Asia, so it spreads over the poles into North America and west into Europe.
Once cold air is pooled over Canada it enters the lower 48 resulting in extended periods of increased frequency of Arctic outbreaks.
Less snow means more low pressure over Siberia, which contracts and no cold air build-up over North America and extended periods where Arctic outbreaks do not occur.
H L L
L
H
1988/89 - low October Siberian snow cover and strongly positive winter AO
2009/10 – high October Siberian snow cover
and strongly negative winter AO
Large changes in fall snow cover can have
significant impact on earth’s energy balance:
Shading represents sea level pressure anomalies averaged for 45 day periods Winter AO events are preceded by same signed regional precursors related to the development of the Siberian High.
Shading represents sea level pressure anomalies averaged for 20 day periods
Shading represents temperature anomalies averaged for 20 day periods
Shading represents accumulative anomalies through the shown date
Polar vortex displacements – where the polar vortex is displaced off the Pole
Polar vortex splits – where the polar vortex is
split into two
Splits
Displacements
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Models completely miss link between snow cover variability and vertical EP flux.
Models also poorly simulate link between leading vertical EP flux and lagging
surface response. Hardiman et al. JGR 2008
22/02/16
Power and Cross Spectrum:
Power and Cross Spectrum:
Eurasian snow cover and the Eurasian snow cover and the AO AO
70-120 mo
CRU
Most of the observed annual warming is driven by warming in winter, with the greatest warming in the mid to high latitudes of the interior North American and Asian continents and has been linked to positive trend in the AO/NAM.
Regional Perturbation
over Siberia
Sept Oct Nov Dec Jan Feb
Increased Eurasian snow cover
Background Westerlies
Stratospheric Polar Vortex Weakens
Negative Arctic Oscillation Upward Energy
Flux
Downward propagation of High pressure and southward
displacement of jet.
High Pressure over the Arctic and frequent
cold air outbreaks
warming
1
2
3
4
5
6
1 22
Oct Snow Cover Trend (1988-2007)
44 3
66
• Land and ocean are key predictors when trying to produce a skillful seasonal forecast.
• For winter forecasts, Siberian snow cover and lower tropospheric precursors are skillful predictors.
• Early fall anomalies result in large-scale wave propagation during the late fall/early winter, which is instrumental in forcing the dominant mode of wintertime, extratropical NH climate variability.
• Differentiating between polar vortex displacements and splits better identifies tropospheric precursors to stratospheric warmings and negative tropospheric AO events.
•The strong relationship between the AO and antecedent snow cover suggest that snow cover variability on decadal time scales can
influence winter climate also on decadal time scales.
•An unexpected trend reversal between spring, summer, fall and winter surface temperatures has been observed over the past two decades
•At least part of the trend reversal is associated with increased
stratosphere-troposphere coupling and is closely related to the
surface AO.
•Increasing our ability to understand and explain natural variability will improve our predictions and message of climate change.
•A more careful understanding of the link between stratosphere-
troposphere coupling, snow cover and the AO/NAM could help in
making interannual to decadal predictions.
Observed and forecast (issued in November) temperatures in °C for December-February 2010/11 for the Northern Hemisphere.
