1. No category

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
Outline
Summary of observed anomalies
• UK Ö global
• Circulation patterns
NAO & blocking
• The phenomena
• Remote influences
Predictability – forecasts
Early winter 2010/11 – comparison
Attribution studies
Reading, 6 January 2010
Winter 20009/10 in the UK
Reading: Tmax, Tmin DJF 0910
Temperature
• Mean 1.6°C
• 2.0°C below
1971-2000
average
Mike Stroud
Precipitation (%)
• Total 249mm
• 75% of
average
DJF averages
European anomalies - DJF
Temperature
NOAA Climate Prediction Center
Precipitation (% normal)
Surface air temperature
December 2009
January 2010
February 2010
DJF average
• Persistence
• Warm Arctic + sub-tropics
NOAA ESRL
Cold mid-latitudes, amid global warmth
Annual temperature anomalies
relative to 1951-1980
“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)
Westerly winds – DJF
climatology
U 250hPa
NOAA ESRL
U 850hPa
Westerly winds – DJF
2009/10
U 250hPa
NOAA ESRL
U 850hPa
Westerly winds – DJF
2009/10 anomalies
U 250hPa
NOAA ESRL
U 850hPa
Pressure – DJF
2009/10 anomalies
Geopotential height 500hPa
NOAA ESRL
MSL Pressure
NCEP reanalysis DJF 2009/10
North Atlantic Oscillation (NAO)
Z500
anomaly
Z500
Arctic Oscillation (AO)
PMSL
anomaly
PMSL
DJF average
NAO and AO time-series
North Atlantic Oscillation (NAO)
Z500
Arctic Oscillation (AO)
NOAA CPC
PMSL
DJF average
NAO and AO time-series
Iceland / Azores sea level pressure difference (NOAA ESRL)
North Atlantic Oscillation (NAO)
Z500
Arctic Oscillation (AO)
Strongest AO- on record (>1900)
NOAA CPC; Colorado State University
PMSL
DJF average
NAO impacts in winter
Positive phase
Martin Visbeck, Columbia University
Negative phase
NAO - example surface charts
NAO Positive phase
NAO Negative phase
2 February 2011
5 January 2010
UK Met Office
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)
NAO and blocking during winter 2009/10
NAO Index
NOAA Climate Prediction Center
(Tibaldi & Molteni blocking index)
Blocking frequency anomalies
Winter 2009/10
Winter 2008/9
Blocking frequency - percentage of days
(Pelly & Hoskins blocking index)
Giacomo Masato, University of Reading
Early December onset of cold weather
ECMWF analyses
Early December onset of cold weather
ECMWF analyses
Early December onset of cold weather
ECMWF analyses
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)
Ö complex interactions; multiple factors
Look at historical relationships with individual factors
What did these factors predict for DJF 2009/10?
El Niño and La Niña
NOAA, Climate Prediction Center
El Niño
SST anomaly DJF 09/10
OLR anomaly DJF 09/10
NOAA Climate Diagnostics Bulletin
NOAA ESRL
El Niño Teleconnections
Tropospheric teleconnections from a moderate El Niño
¾ Project onto negative NAO
PMSL response to
moderate El Nino,
Jan-Feb 1950-2000
PMSL DJF 09/10
Toniazzo & Scaife (2006)
¾ Expect response to be captured in seasonal forecasts
NCEP Reanalysis
Dynamical DJF Forecast
versus NCEP reanalysis
Z500
Forecast
for JFM
Z500 Anomalies: DJF 2009/10
27
Solar activity and Atlantic blocking
Woollings et al (2010)
Solar activity and Atlantic blocking
Blocking frequency – composite anomalies (1958-2001)
Woollings et al (2010)
Stratosphere – structure & variability
Stratopause ~50km
(warm)
Tropopause ~10km
(cold)
80°S
60°S
40°S
20°S
0°
20°N
40°N
60°N
80°N
Latitude
Stratospheric
variability:
Summer
easterlies
Sudden Warmings
(SSW)
Polar
vortex
waves
QBO
mean
wind
Planetary waves
Quasi-Biennial
Oscillation (QBO)
ERA-40 reanalysis
80°S
60°S
40°S
20°S
0°
Latitude
20°N
40°N
60°N
80°N
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)
Solar activity – mechanism 2
Equatorial heating
5K
0K
Idealised model response to
lower stratospheric heating
Feedback between winds
and weather systems in the
storm-track
Zonal wind climatology
Temperature anomaly DJF 2009/10
Zonal wind response
Haigh et al (2005), Simpson et al (2009)
Cold equatorial stratosphere in
winter 2009/10 (solar-min + QBO?)
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 (weaker Brewer-Dobson circulation) (Charlton et al
…all fit weaker
than average
polar vortex
2008; Bell et al 2010)
QBO‐ east
90N 30hPa Temperature
QBO – equatorial winds
DJF 09/10
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
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/
Pressure
Dec‐01
Dec‐15
Jan‐01
Jan‐15
Feb‐01
Feb‐15
Mar‐01
NAM index
Pressure
ECMWF operational analysis
Zonal mean zonal wind 60‐90N
Dec‐01
Dec‐15
lower stratospheric split
08‐12‐2009
Jan‐01
Jan‐15
Strong mid‐winter
Feb‐01
Feb‐15
Mar‐01
displacement SSW event 01‐02‐2010
Tropospheric precursors?
EP-flux vector
(10hPa, 60N)
16 NOV
16 DEC
16 JAN
16 FEB
16 MAR
16 APR
EQ
Vertical EP-flux
(100hPa)
NOV 500hPa Z’
90N
16 NOV
16 DEC
16 JAN
16 FEB
• 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 MAR
16 APR
• 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
Eurasian snow cover in October
Cohen et al (2007, 2009)
Solar minimum
Stratospheric
vortex
QBO east
ve
Wa
s
NAM
height
NAO -
AO -
Blocking
Planetary
waves
Tropical
Atlantic
PNA
Eurasian snow
cover (October)
El Niño
latitude
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)
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
Attribution studies (3)
Surface Air Temperature anomalies
• US east coast snow
• Past snowstorms related to
El Niño and NAO-
• Combine El Niño + NAOregressions to account for
temperature anomalies
NOAA Attribution Team (2010), ESRL
Compare early Winter 2010/11
Surface Air Temperature anomaly
December 2009
NOAA ESRL
December 2010
Compare early Winter 2010/11
250hPa wind speed
• Circulation change in midNovember across N.Hemisphere
• Strong NAO- again
• Pacific differs, short jet (La Niña)
NOAA CPC and ESRL
• Differing factors: La Niña, QBO west
• Solar activity remains low
• Strong stratospheric vortex
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
Thank You
Z500 – persistence….
December 2009
Percentage of days with significant
anomaly of same sign (>15m)
January 2010
February 2010
Monthly
Mean
(dm)
Persistence
NOAA Climate Diagnostics Bulletin
(%)
…enhanced medium-range forecast skill
™ First month for which 60% anomaly correlation reached 10 days
for Europe and northern hemisphere
48
NAO impacts (January)
Opposite sign for negative NAO anomaly
NOAA CPC
NAO index
Tropopause potential temperature
Weekly averages
Greenland / Atlantic blocking is a strong NAO‐ flow regime
Woollings et al (2008, JAS; 2010, JCL)
Negative NAO, AO – potential factors
1) Blocking as the mechanism of negative NAO
NAO- associated with Greenland blocking, Woollings et al (2008)
2) Eurasian Snow Cover in October
3) Moderate El Niño
(a) Rossby wave propagation
(b) Via stratosphere, weak polar vortex
4) Stratosphere
(a) Solar minimum (equatorial cooling)
(b) Easterly QBO (Holton-Tan mechanism)
(c) Weak polar vortex, sudden warmings
(2) Response to stratospheric heating
E5 Equatorial heating (5K)
U5 Uniform heating (5K)
5K
5K
0K
E5
Zonal wind response
U5
P10 Polar heating (10K)
0K
10K
P10
Control zonal wind
Equatorward jet shift for:
•Tropical cooling
•Polar heating
Idealised GCM
Haigh et al (2005)
See also
Lorenz & de Weaver (2007)