1. Why was Winter 2009/10 so cold? Mike Blackburn
National Centre for Atmospheric Science, University of Reading
With input from
Chris Bell, Thomas Toniazzo, Tim Woollings (NCAS; University of Reading);
Brian Hoskins (Grantham Institute, Imperial College)
Thomas Jung, Laura Ferranti, Frederic Vitart (ECMWF)
Royal Meteorological Society, 5 February 2011

2. Outline Summary of observed anomalies UK  global Circulation patterns
NAO & blocking
The phenomena
Remote influences
Predictability – forecasts
Early winter 2010/11 – comparison
Attribution studies
Reading, 6 January 2010

3. Winter 20009/10 in the UK Temperature Mean 1.6°C
DJF averages
Reading: Tmax, Tmin DJF 0910
Temperature
Mean 1.6°C
2.0°C below average
Mike Stroud
Precipitation (%)
Total 249mm
75% of average

4. European anomalies - DJF
Temperature
Precipitation (% normal)
NOAA Climate Prediction Center

5. Surface air temperature
December 2009
January 2010
February 2010
DJF average
Persistence
Warm Arctic + sub-tropics
NOAA ESRL

6. Cold mid-latitudes, amid global warmth
Annual temperature anomalies relative to
“Global temperature is rising as fast in the past decade as in the prior two decades, despite year-to-year fluctuations associated with the El Niño-La Niña cycle of tropical ocean temperature”, Hansen et al (2010)
Monthly comparison, 2010 vs. previous years
DJF 2010 global average Ts second warmest on record to 2007
Contribution from El Niño
NASA Goddard Institute for Space Studies – Hansen et al (2010)

7. Westerly winds – DJF climatology
U 250hPa
U 850hPa
NOAA ESRL

8. Westerly winds – DJF 2009/10
U 250hPa
U 850hPa
NOAA ESRL

9. Pressure – DJF 2009/10 anomalies
Geopotential height 500hPa
MSL Pressure
NOAA ESRL

10. NCEP reanalysis DJF 2009/10 North Atlantic Oscillation (NAO)
DJF average
Arctic Oscillation (AO)
North Atlantic Oscillation (NAO)
Z500
PMSL
Z500 anomaly
PMSL anomaly
NOAA CPC

11. NAO and AO time-series North Atlantic Oscillation (NAO) Z500
DJF average
Arctic Oscillation (AO)
North Atlantic Oscillation (NAO)
Z500
PMSL
NOAA CPC

12. NAO and AO time-series Strongest AO- on record (>1900)
DJF average
Arctic Oscillation (AO)
North Atlantic Oscillation (NAO)
Z500
PMSL
Iceland / Azores sea level pressure difference (NOAA ESRL)
Strongest AO- on record (>1900)
NOAA CPC; Colorado State University

13. NAO impacts in winter Positive phase Negative phase
Martin Visbeck, Columbia University

14. NAO - example surface charts
NAO Positive phase
2 February 2011
NAO Negative phase
5 January 2010
UK Met Office

15. Upper troposphere height at 250hPa Weekly averages
NAO index
Upper troposphere height at 250hPa
Weekly averages
Greenland / Atlantic blocking is a strong NAO- flow regime
Woollings et al (2008, JAS; 2010, JCL)
ECMWF analyses

16. NAO and blocking during winter 2009/10
NAO Index
(Tibaldi & Molteni blocking index)
NOAA Climate Prediction Center

17. Blocking frequency anomalies
Winter 2009/10
Winter 2008/9
Blocking frequency - percentage of days
(Pelly & Hoskins blocking index)
Giacomo Masato, University of Reading

18. Early December onset of cold weather
Animation for 04 – 11 Dec 2009:
Animation for 11 – 18 Dec 2009:
ECMWF analyses

19. Influences on NAO & Atlantic blocking
NAO and blocking are natural variations of Atlantic weather
What factors influence them – alter the chance of high/low values?
Low extended range predictability (weeks, months)
 complex interactions; multiple factors
Look at historical relationships with individual factors
What did these factors predict for DJF 2009/10?

20. El Niño and La Niña
Tropical convection generates Rossby waves that propagate into the extra-tropics, creating “teleconnections” with weather around the globe.
NOAA, Climate Prediction Center

21. El Niño SST anomaly DJF 09/10 OLR anomaly DJF 09/10
NOAA Climate Diagnostics Bulletin
NOAA ESRL

22. El Niño Teleconnections
Tropospheric teleconnections from a moderate El Niño
Project onto negative NAO
PMSL response to moderate El Nino, Jan-Feb
Toniazzo & Scaife (2006)
PMSL DJF 09/10
Expect response to be captured in seasonal forecasts

23. Dynamical DJF Forecast versus NCEP reanalysis Z500
Forecast for JFM

24. Z500 Anomalies: DJF 2009/10 24

25. Solar activity and Atlantic blocking
Woollings et al (2010)

26. Solar activity and Atlantic blocking
Blocking frequency – composite anomalies ( )
Woollings et al (2010)

27. Stratosphere – structure & variability
Stratopause ~50km (warm)
20°S
80°S
60°S
40°S
20°N
40°N
60°N
80°N 0°
Tropopause ~10km (cold)
Latitude
Stratospheric variability:
Sudden Stratospheric Warmings (SSW)
Planetary waves
Quasi-Biennial Oscillation (QBO)
Summer easterlies
Polar vortex
waves
mean wind
QBO
ERA-40 reanalysis
20°S
80°S
60°S
40°S
20°N
40°N
60°N
80°N 0°
Latitude

28. Solar activity – mechanism 1
At solar maximum:
Stratopause heating (UV+O3)
Alters planetary wave forcing
Weakens Brewer-Dobson circulation
Cools polar stratosphere, strengthens polar vortex
Downward influence on troposphere (Northern Annular Mode, NAM)
Potential interaction with tropical winds (QBO)
(Gray et al (2001)
Kodera & Kuroda (2002)

29. Solar activity – mechanism 25K 0K
Equatorial heating
Zonal wind climatology
Zonal wind response
Idealised model response to lower stratospheric heating
Feedback between winds and weather systems in the storm-track
Temperature anomaly DJF 2009/10
Cold equatorial stratosphere in winter 2009/10 (solar-min + QBO?)
Haigh et al (2005), Simpson et al (2009)

30. Did we expect a disturbed stratospheric vortex in winter 2009/10?
Modified mean state:
Solar minimum (Labitzke & Van Loon 1988; Kodera and Kuroda 2002)
QBO East (Holton & Tan 1980)
Trend (stronger Brewer-Dobson circulation) (Charlton et al 2008; Bell et al 2010)
…all fit weaker than average polar vortex
QBO- east
30hPa height, North Pole
DJF 09/10
QBO – equatorial winds
Met Office analyses, University of Reading
Labitzke and Kunze (2010, JGR)

31. Did we expect a disturbed stratospheric vortex in winter 2009/10?
Increased planetary wave activity
El Niño (Ineson and Scaife, 2009; Bell et al 2009)
Blocking (Martius et al 2010)
25 of 27 SSWs in the period were preceded by blocking
SSW type is related to blocking location
Blocking frequency preceding stratospheric sudden warming (SSW) events,

32. North Pole temperature at 10hPa
Stratospheric vortex, DJF 2009/10
North Pole temperature at 10hPa
Zonal wind at 60N 10hPa
Minor
Major
Key features of stratospheric flow:
Minor warming – early December
Strong vortex until mid-January – major stratospheric sudden warming (SSW)
Weak, disturbed vortex late winter – persistent easterlies over polar cap
Courtesy Andrew Charlton-Perez

33. Zonal mean zonal wind 60-90N
Pressure
Dec-01
Jan-01
Feb-01
Mar-01
Dec-15
Jan-15
Feb-15
ECMWF operational analysis
Pressure
Dec-01
Jan-01
Feb-01
Mar-01
Dec-15
Jan-15
Feb-15
NAM index
lower stratospheric split
Strong mid-winter
displacement SSW event

34. Tropospheric precursors?
16 NOV
16 DEC
16 JAN
16 FEB
16 MAR
16 APR
EP-flux vector (10hPa, 60N)
Vertical EP-flux (100hPa) EQ
90N
NOV 500hPa Z’
January blocking precedes SSW
Also strong PNA-like pattern, no NAO
Low EP flux (10hPa) in late winter due to persistence of easterly anomalies in lower stratosphere
Early December wave-2 simultaneous with blocking, confined to lower stratosphere
Growth in vertical EP-flux is pre-blocking
Was it driven by PNA-type pattern (El Niño)?
Japan Meteorological Agency; Freie Universtät Berlin

35. Eurasian snow cover in October
Cohen et al (2007, 2009)

36. Eurasian snow cover (October)
Stratospheric vortex
Solar minimum
QBO east
Waves
NAM
latitude
height
NAO -
Blocking
AO -
Planetary waves
Tropical Atlantic
Importance of stratosphere increasingly recognised in last decade -> Include additional forcing processes in extended-range forecast models.
PNA
Eurasian snow cover (October)
El Niño
Atlantic SST?

37. Attribution studies (1)
Use past relationship to predict magnitude of Winter 2010 anomaly
European temperature
Flow analogues
Warm residual
(1963 similar dynamics but colder)
Cattiaux et al (2010)

38. Attribution studies (2)
Surface temperature
Linear regression to estimate contribution of NAO-
Residual shows European warmth
Unexplained cold anomalies, USA, Siberia
Osborn (2011) in Weather

39. Attribution studies (3)
Surface Air Temperature anomalies
US east coast snow
Past snowstorms related to El Niño and NAO-
Combine El Niño + NAO- regressions to account for temperature anomalies
NOAA Attribution Team (2010), ESRL

40. Surface Air Temperature anomaly
Compare early Winter 2010/11
Surface Air Temperature anomaly
December 2009
December 2010
Remarkably similar temperature pattern
Europe colder than 2009
North Pacific & western USA differ
NOAA ESRL

41. Compare early Winter 2010/11
250hPa wind speed
Circulation change in mid-November across N.Hemisphere
Strong NAO- again
Pacific differs, short jet (La Niña)
Differing factors: La Niña, QBO west
Solar activity remains low
Strong stratospheric vortex
NOAA CPC and ESRL

42. Conclusions Persistent equatorward shift of jet stream
Blocking, negative NAO
Increased forecast skill – El Niño signal + persistence
Possible origins – factors working together?
….or chance, unpredictable?
Attempts to attribute cold & snow anomalies
Further work
Forecast & hindcast diagnosis; anomalous forcing