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Why was Winter 2009/10 so cold?

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)

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Why was Winter 2009/10 so cold?

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  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. Reading: Tmax, Tmin DJF 0910 Winter 20009/10 in the UK DJF averages • Temperature • Mean 1.6°C • 2.0°C below 1971-2000 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. Annual temperature anomalies relative to 1951-1980 Monthly comparison, 2010 vs. previous years Cold mid-latitudes, amid global warmth “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) • 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. U 250hPa U 850hPa Westerly winds – DJF 2009/10 NOAA ESRL

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

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

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

  12. North Atlantic Oscillation (NAO) Z500 Arctic Oscillation (AO) PMSL DJF average NAO and AO time-series 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. 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: http://www.met.reading.ac.uk/Data/CurrentWeather/archive/20091211/anims/Z250.large.gif Animation for 11 – 18 Dec 2009: http://www.met.reading.ac.uk/Data/CurrentWeather/archive/20091218/anims/Z250.large.gif 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. PMSL response to moderate El Nino, Jan-Feb 1950-2000 Toniazzo & Scaife (2006) El Niño Teleconnections • Tropospheric teleconnections from a moderate El Niño • Project onto negative NAO PMSL DJF 09/10 • Expect response to be captured in seasonal forecasts

  23. NCEP Reanalysis 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 (1958-2001) Woollings et al (2010)

  27. QBO waves mean wind 60°S 60°S 40°S 40°S 20°S 20°S 0° 0° 20°N 20°N 40°N 40°N 60°N 60°N 80°N 80°N 80°S 80°S Stratosphere – structure & variability Stratopause ~50km (warm) Tropopause ~10km (cold) Latitude Stratospheric variability: Sudden Stratospheric Warmings (SSW) Planetary waves Quasi-Biennial Oscillation (QBO) Summer easterlies Polar vortex ERA-40 reanalysis 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. Equatorial heating 5K 0K Zonal wind climatology Zonal wind response Solar activity – mechanism 2 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. 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 QBO – equatorial winds 30hPa height, North Pole DJF 09/10 Did we expect a disturbed stratospheric vortex in winter 2009/10? 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 1958-2001 were preceded by blocking • SSW type is related to blocking location Blocking frequency preceding stratospheric sudden warming (SSW) events, 1958-2001

  32. 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 http://www.met.reading.ac.uk/research/stratclim/current/

  33. Zonal mean zonal wind 60-90N Pressure NAM index Pressure Dec-01 Dec-15 Jan-01 Jan-15 Feb-01 Feb-15 Mar-01 lower stratospheric split Strong mid-winter displacement SSW event 08-12-2009 01-02-2010 ECMWF operational analysis Dec-01 Dec-15 Jan-01 Jan-15 Feb-01 Feb-15 Mar-01

  34. Tropospheric precursors? EP-flux vector (10hPa, 60N) 16 NOV 16 DEC 16 JAN 16 FEB 16 MAR 16 APR NOV 500hPa Z’ EQ Vertical EP-flux (100hPa) 90N • 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)? 16 NOV 16 DEC 16 JAN 16 FEB 16 MAR 16 APR Japan Meteorological Agency; Freie Universtät Berlin

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

  36. height NAO- Blocking latitude Stratospheric vortex Solar minimum QBO east Waves NAM AO- Planetary waves Tropical Atlantic 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. 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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