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

Summary of observed anomalies

• UK global

• Circulation patterns

NAO & blocking

• The phenomena

• Remote influences

Predictability – forecasts

Early winter 2010/11 – comparison

Attribution studies

Outline

Reading, 6 January 2010

Winter 20009/10 in the UK DJF averages

Reading: Tmax, Tmin DJF 0910

Temperature

• Mean 1.6°C

• 2.0°C below 1971-2000 average

Precipitation (%)

• Total 249mm

• 75% of average

Mike Stroud

European anomalies - DJF

NOAA Climate Prediction Center

Temperature Precipitation (% normal)

Surface air temperatureDecember 2009 January 2010 February 2010

DJF average

• Persistence

• Warm Arctic + sub-tropics

NOAA ESRL

Cold mid-latitudes, amid global warmth

NASA Goddard Institute for Space Studies – Hansen et al (2010)

Annual temperature anomalies relative to 1951-1980

Monthly comparison, 2010 vs. previous years

• DJF 2010 global average Ts second warmest on record to 2007

• Contribution from El Niño

“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)

Westerly winds – DJF climatology

U 250hPa U 850hPa

NOAA ESRL

U 250hPa U 850hPa

NOAA ESRL

Westerly winds – DJF 2009/10

NOAA ESRL

Pressure – DJF 2009/10 anomalies

MSL PressureGeopotential height 500hPa

PMSL anomaly

NCEP reanalysis DJF 2009/10

Z500 anomaly

DJF average

Arctic Oscillation (AO)

North Atlantic Oscillation (NAO)

Z500

PMSLNOAA CPC

DJF average

Arctic Oscillation (AO)

North Atlantic Oscillation (NAO)

Z500

PMSL

NAO and AO time-series

NOAA CPC

DJF average

Arctic Oscillation (AO)

North Atlantic Oscillation (NAO)

Z500

PMSL

NAO and AO time-series

NOAA CPC; Colorado State University

Strongest AO- on record (>1900)

Iceland / Azores sea level pressure difference (NOAA ESRL)

NAO impacts in winter

Martin Visbeck, Columbia University

Positive phase Negative phase

NAO - example surface charts

UK Met Office

NAO Positive phase

2 February 2011

NAO Negative phase

5 January 2010

Upper troposphere height at 250hPaWeekly averages

NAO index

Greenland / Atlantic blocking is a strong NAO- flow regimeWoollings et al (2008, JAS; 2010, JCL)

ECMWF analyses

NOAA Climate Prediction Center(Tibaldi & Molteni blocking index)

NAO and blocking during winter 2009/10

NAO Index

Winter 2008/9Winter 2009/10

Giacomo Masato, University of Reading

Blocking frequency - percentage of days

(Pelly & Hoskins blocking index)

Blocking frequency anomalies

Early December onset of cold weather

ECMWF analyses

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

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?

Influences on NAO & Atlantic blocking

El Niño and La Niña

NOAA, Climate Prediction Center

Tropical convection generates Rossby waves that propagate into the extra-tropics, creating “teleconnections” with weather around the globe.

El Niño

NOAA ESRL

SST anomaly DJF 09/10

OLR anomaly DJF 09/10

NOAA Climate Diagnostics Bulletin

El Niño Teleconnections

PMSL DJF 09/10

PMSL response to moderate El Nino, Jan-Feb 1950-2000

Toniazzo & Scaife (2006)

Tropospheric teleconnections from a moderate El Niño

Project onto negative NAO

Expect response to be captured in seasonal forecasts

Dynamical DJF Forecast versus NCEP reanalysis

Z500

NCEP Reanalysis

Forecast for JFM

24

Z500 Anomalies: DJF 2009/10

Solar activity and Atlantic blocking

Woollings et al (2010)

Solar activity and Atlantic blocking

Woollings et al (2010)

Blocking frequency – composite anomalies (1958-2001)

Stratosphere – structure & variability

20°S80°S 60°S 40°S 20°N 40°N 60°N 80°N0°

20°S80°S 60°S 40°S 20°N 40°N 60°N 80°N0°

ERA-40 reanalysis

Tropopause ~10km (cold)

Stratopause ~50km (warm)

Polar vortex

Summer easterlies

Stratospheric variability:

Sudden Stratospheric Warmings (SSW)

Planetary waves

Quasi-Biennial Oscillation (QBO)

QBO waves

Latitude

Latitude

mean wind

Solar activity – mechanism 1

Kodera & Kuroda (2002)

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)

Solar activity – mechanism 2

Haigh et al (2005), Simpson et al (2009)

Idealised model response to lower stratospheric heating

Feedback between winds and weather systems in the storm-track

5K0K

Equatorial heating

Zonal wind climatology

Zonal wind response

Cold equatorial stratosphere in winter 2009/10 (solar-min + QBO?)

Temperature anomaly DJF 2009/10

Did we expect a disturbed stratospheric vortex in winter 2009/10?

DJF 09/10

QBO – equatorial winds

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

Met Office analyses, University of Reading Labitzke and Kunze (2010, JGR)

QBO- east

30hP

a he

ight

, Nor

th P

ole

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

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

North Pole temperature at 10hPa Zonal wind at 60N 10hPa

Courtesy Andrew Charlton-Perez http://www.met.reading.ac.uk/research/stratclim/current/

Minor Major

Stratospheric vortex, DJF 2009/10

Pres

sure

Dec-01 Jan-01 Feb-01 Mar-01Dec-15 Jan-15 Feb-15

NAM index

Zonal mean zonal wind 60-90N

Pres

sure

Dec-01 Jan-01 Feb-01 Mar-01Dec-15 Jan-15 Feb-15

ECM

WF

oper

ation

al a

naly

sis

lower stratospheric split Strong mid-winter displacement SSW event

08-12-2009 01-02-2010

• 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)?

• 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

16 NOV 16 DEC 16 JAN 16 FEB 16 MAR 16 APR

EP-flux vector (10hPa, 60N)

16 NOV 16 DEC 16 JAN 16 FEB 16 MAR 16 APR

Vertical EP-flux (100hPa)

EQ

90N

Tropospheric precursors?

NOV 500hPa Z’

Japan Meteorological Agency; Freie Universtät Berlin

Eurasian snow cover in October

Cohen et al (2007, 2009)

AO -NAO -

Blocking

El NiñoEurasian snow cover (October)

Solar minimum QBO eastStratospheric

vortex

Tropical Atlantic

Planetary waves

PNA

Waves

NAM

latitude

height

Atlantic SST?

Attribution studies (1)

Cattiaux et al (2010)

• Use past relationship to predict magnitude of Winter 2010 anomaly

• European temperature

• Flow analogues

• Warm residual

• (1963 similar dynamics but colder)

Attribution studies (2)

Osborn (2011) in Weather

• Surface temperature

• Linear regression to estimate contribution of NAO-

• Residual shows European warmth

• Unexplained cold anomalies, USA, Siberia

Attribution studies (3)

NOAA Attribution Team (2010), ESRL

• US east coast snow

• Past snowstorms related to El Niño and NAO-

• Combine El Niño + NAO- regressions to account for temperature anomalies

Surface Air Temperature anomalies

Compare early Winter 2010/11

NOAA ESRL

Surface Air Temperature anomaly

December 2009 December 2010

• Remarkably similar temperature pattern

• Europe colder than 2009

• North Pacific & western USA differ

Compare early Winter 2010/11

NOAA CPC and ESRL

• 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

250hPa wind speed

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

Conclusions