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(c) 2000 D.B.Stephenson@reading.ac.uk

Dr. David B. Stephenson

Department of Meteorology

University of Reading

AGU Chapman conference 28 Nov – 1 Dec 2000, Orense, Spain

The North Atlantic OscillationThe North Atlantic Oscillation

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(c) 2000 D.B.Stephenson@reading.ac.uk

AcknowledgementsAcknowledgements

Thanks for help from the following colleagues:

• Pierre-Philippe MathieuPierre-Philippe Mathieu• Martina JungeMartina Junge• Thomas RaddatzThomas Raddatz• Abdel HannachiAbdel Hannachi• Malcolm BrooksMalcolm Brooks• Kevin HodgesKevin Hodges

Note: all mistakes are my own !

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Some important NAO questions …Some important NAO questions …

• What exactly is it ?What exactly is it ?

• What makes it tick ?What makes it tick ?

• How well can it be forecast ?How well can it be forecast ?

• How does it affect things ?How does it affect things ?

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What should we call it ?What should we call it ?

• Druck SchwankungenDruck Schwankungen• Northern OscillationNorthern Oscillation• North Atlantic OscillationNorth Atlantic Oscillation• Arctic OscillationArctic Oscillation• Annular modeAnnular mode• Not the Arctic OscillationNot the Arctic Oscillation• Al MoubarakAl Moubarak• El NaoEl Nao• Le YoYo MeteoLe YoYo Meteo• Northern Wazaaaaaa ! Northern Wazaaaaaa !

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“The relationships between weather over the Earth are so complex that it seems useless to try to derive them from

theoretical considerations; and the only hope at present is that of ascertaining the facts and of arranging them in

such a way that interpretation shall be possible.” (1908)

World weather correlationsWorld weather correlationsSir Gilbert T. WalkerSir Gilbert T. Walker

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Early history of NAO research …Early history of NAO research …

• Saabye 1770-8Saabye 1770-8• Teisserence de Bort 1883Teisserence de Bort 1883• Hann 1890Hann 1890• Hildebrandsson 1897Hildebrandsson 1897• Meinardus 1898Meinardus 1898• Pettersson 1905Pettersson 1905• Exner 19??Exner 19??• Walker 1908, 1924, 1932Walker 1908, 1924, 1932• Defant 1924Defant 1924• Loewe 1937Loewe 1937• Van Loon and Rogers 1978Van Loon and Rogers 1978• + many others …+ many others …

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NAO related publicationsNAO related publications1980-20001980-2000

0

20

40

60

80

100

120

1981 1984 1987 1990 1993 1996 1999

Articles

All articles with “North Atlantic Oscillation” either in the title of abstactSource: web of science bibliographic database (19,213,946 documents)

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Wintertime SLP VariabilityWintertime SLP Variability SLP Anomalies Dec/Jan/Feb (1988-1997)

Anomalies in wintertime mean sea-level pressure (contours every 2mb).

Note the recurrent NAO dipole pattern in the Atlantic sector.

1991/92 1992/93

1988/89 1989/90 1990/91

1993/94

1994/95 1995/96 1996/97

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Positive Phase of the NAOPositive Phase of the NAO

For more details about the NAO refer to: http://www.met.rdg.ac.uk/cag/NAO

+CoastalUpwelling

Min. Baltic IceMax. BalticInflow

SmallCalanus fin.

stock

NwAc Narrow Fast

FSTMAX

Warm

65 Mts NAC

+LSWPRODN.

Storm Centre inLab-Nordic Seas

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Negative phase of the NAONegative phase of the NAO

For more details about the NAO refer to: http://www.met.rdg.ac.uk/cag/NAO

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What is NAO ?What is NAO ? NAO + NAO -

Source: Martin Visbeck and Heidi Cullen

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NAO IndicesNAO Indices

Gibraltar

Source: Jim HURRELL

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NAO varies on all time scalesNAO varies on all time scales … …

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Power spectrum of daily NAO 1948-2000Power spectrum of daily NAO 1948-2000

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What makes it tick …What makes it tick …

• Tropospheric processes ?Tropospheric processes ?

• Stratospheric processes ?Stratospheric processes ?

• Oceanic processes ?Oceanic processes ?

• Coupled processes ?Coupled processes ?

• Anthropogenic forcing ?Anthropogenic forcing ?

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North Atlantic trendsNorth Atlantic trends

NAO related tripole in sea surface temperatures

Source: Rowan Sutton

Jacob Aaal Bonnevie BjerknesJacob Aaal Bonnevie Bjerknes 1897-19751897-1975

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Is it a coupled process ?Is it a coupled process ?

Source: M.C. Escher

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Is it a coupled process ?Is it a coupled process ?

Source: M.C. Escher

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How well can we forecast it ?How well can we forecast it ?

NAO = some trends + a lot of noise

X(t) f(t) e(t)

• Persistent trend part f(t) gives some skill. Persistent trend part f(t) gives some skill. • Amount of skill depends on the lead time !Amount of skill depends on the lead time !• Even low skill may be very useful !Even low skill may be very useful !• Impacts can lag NAO Impacts can lag NAO e.g. wheat quality e.g. wheat quality

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Daily values of the NAO index obtained by projecting daily sea-level pressure onto the NAO pattern. - In collaboration with T. Strunk.

1989 Mean=1.78 1990 Mean=1.02 1991 Mean=0.67

1992 Mean=1.24 1993 Mean=1.43 1994 Mean=0.26

1995 Mean=1.03 1996 Mean=-0.7 1997 Mean=0.32

Jan301989

Dec51988

Dec41989

Dec31990

Dec61993

Dec21996

Dec21991

Dec71992

Dec51994

Dec41995

Jan291990

Jan281991

Jan311994

Jan271997

Jan271992

Feb11993

Jan301995

Jan291996

Daily Values of the NAO IndexDaily Values of the NAO Index

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Persistence in the NAO Time SeriesPersistence in the NAO Time Series

Note the persistence in a) and the slow predictable movement in b).

a) Daily NAO index (1 Jan. 1989 to 30 Dec. 1997)

b) Accumulated Daily NAO index (1 Jan. 1989 to 30 Dec. 1997)

3

1

-1

-3

400

200

0

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

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0 20 40 60 80

a) Autocorrelation Function

Aut

ocor

rela

tion

Fun

ctio

n

Lag

1.0

0.8

0.6

0.4

0.2

0.0

Persistence in daily NAO …Persistence in daily NAO …

Autocorrelations persist up until very long lags => early warning system may be possible (few week's lead).

b) Decorrelation Time vs Maximum Lag

deco

rrel

atio

n tim

e

Lag in days

50

40

30

20

10

00 200 600 1000

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Some wise words …Some wise words …“Those who have knowledge, do not predict,Those who have knowledge, do not predict,

Those who predict, do not have knowledge.” Those who predict, do not have knowledge.” - Lao Tzu Lao Tzu

Chinese philospher 604-531 B.CChinese philospher 604-531 B.C

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How does it affect things ?How does it affect things ?

• Surface conditionsSurface conditions

• Physical environmentPhysical environment

• Terrestrial ecosystems Terrestrial ecosystems

• Marine ecosystemsMarine ecosystems

• Human activitiesHuman activities

The NAO is a major factor controllingThe NAO is a major factor controlling

Note: the impacts are often very complex and not well understood

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Surface impacts of NAO …Surface impacts of NAO …

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NAO and the sex life of Scandinavian ungulatesNAO and the sex life of Scandinavian ungulates

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Future events …Future events …

OA26.03 Climate VariabilityOA26.03 Climate Variability““The Northern Oscillations: NAO and AO”The Northern Oscillations: NAO and AO”

European Geophysical SocietyEuropean Geophysical SocietyXXVI General AssemblyXXVI General Assembly25-30 March 2001 Nice, France25-30 March 2001 Nice, France

Deadline for abstracts 1 Dec 2000 !!!Deadline for abstracts 1 Dec 2000 !!!More information: www.copernicus.orgMore information: www.copernicus.org

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Consensus of specialists … Consensus of specialists …