1 (c) 2000 Dr. David B. Stephenson Department of Meteorology University of Reading AGU Chapman conference 28 Nov – 1 Dec 2000, Orense, Spain The North Atlantic Oscillation

2 (c) 2000 Acknowledgements Thanks for help from the following colleagues: Pierre-Philippe Mathieu Martina Junge Martina Junge Thomas Raddatz Thomas Raddatz Abdel Hannachi Abdel Hannachi Malcolm Brooks Malcolm Brooks Kevin Hodges Kevin Hodges Note: all mistakes are my own !

3 (c) 2000 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 ?

4 (c) 2000 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 !

5 (c) 2000 “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 correlations World weather correlations Sir Gilbert T. Walker

6 (c) 2000 Early history of NAO research … Saabye Saabye 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 many others …+ many others …

7 (c) 2000 NAO related publications All articles with “North Atlantic Oscillation” either in the title of abstact Source: web of science bibliographic database (19,213,946 documents)

8 (c) 2000 Wintertime SLP Variability Wintertime SLP Variability SLP Anomalies Dec/Jan/Feb ( ) Anomalies in wintertime mean sea-level pressure (contours every 2mb). Note the recurrent NAO dipole pattern in the Atlantic sector. 1991/921992/ /891989/901990/ / / /961996/97

9 (c) 2000 Positive Phase of the NAO For more details about the NAO refer to: +Coastal Upwelling Min. Baltic Ice Max. Baltic Inflow Small Calanus fin. stock NwAc Narrow Fast FST MAX Warm 65 Mts NAC +LSW PRODN. Storm Centre in Lab-Nordic Seas

10 (c) 2000 Negative phase of the NAO Negative phase of the NAO For more details about the NAO refer to:

11 (c) 2000 What is NAO ? What is NAO ? NAO + NAO - Source: Martin Visbeck and Heidi Cullen

12 (c) 2000 NAO Indices NAO Indices Gibraltar Source: Jim HURRELL

13 (c) 2000 NAO varies on all time scales …

14 (c) 2000 Power spectrum of daily NAO

15 (c) 2000 What makes it tick … Tropospheric processes ?Tropospheric processes ? Stratospheric processes ?Stratospheric processes ? Oceanic processes ?Oceanic processes ? Coupled processes ?Coupled processes ? Anthropogenic forcing ?Anthropogenic forcing ?

16 (c) 2000 December 1999 Storms upper level jet speed sea-level pressure low-level relative vorticity Synoptic conditions on the 23rd December 1999 showing the upper level jet speed (blue contours every 20m/s), sea-level pressure (black contours every 5mb), and low-level relative vorticity (mauve contours.) - Meteo-France

17 (c) 2000 Storm Tracks and NAO Storm Tracks and NAO Winter 1989/1990 NAO positive Winter 1995/1996 NAO negative Source: Kevin HODGES (ESSC)

18 (c) 2000 North Atlantic trends North Atlantic trends NAO related tripole in sea surface temperatures Source: Rowan Sutton Jacob Aaal Bonnevie Bjerknes

19 (c) 2000 Is it a coupled process ? Source: M.C. Escher

20 (c) 2000 Is it a coupled process ? Source: M.C. Escher

21 (c) 2000 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  e.g. wheat qualityImpacts can lag NAO  e.g. wheat quality

22 (c) 2000 Daily values of the NAO index obtained by projecting daily sea-level pressure onto the NAO pattern. - In collaboration with T. Strunk Mean= Mean= Mean= Mean= Mean= Mean= Mean= Mean= Mean=0.32 Jan Dec Dec Dec Dec Dec Dec Dec Dec Dec Jan Jan Jan Jan Jan Feb Jan Jan Daily Values of the NAO Index

23 (c) 2000 Persistence in the NAO Time Series Note the persistence in a) and the slow predictable movement in b). a) Daily NAO index (1 Jan to 30 Dec. 1997) b) Accumulated Daily NAO index (1 Jan to 30 Dec. 1997)

24 (c) a) Autocorrelation Function Autocorrelation Function Lag 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 decorrelation time Lag in days

25 (c) 2000 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 Chinese philospher B.C

26 (c) 2000 How does it affect things ? Surface conditionsSurface conditions Physical environmentPhysical environment Terrestrial ecosystemsTerrestrial ecosystems Marine ecosystemsMarine ecosystems Human activitiesHuman activities The NAO is a major factor controlling Note: the impacts are often very complex and not well understood

27 (c) 2000 Surface impacts of NAO …

28 (c) 2000 NAO and the sex life of Scandinavian ungulates

29 (c) 2000 Future events … OA26.03 Climate Variability “The Northern Oscillations: NAO and AO” European Geophysical Society XXVI General Assembly March 2001 Nice, France Deadline for abstracts 1 Dec 2000 !!! More information:

30 (c) 2000 Consensus of specialists … Consensus of specialists …