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Why was Winter 2009/10 so cold? Mike Blackburn

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Presentation on theme: "Why was Winter 2009/10 so cold? Mike Blackburn"— Presentation transcript:

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 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 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 2
5K 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

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