The North Atlantic Oscillation

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

2. Acknowledgements • Pierre-Philippe Mathieu • Martina Junge • Thomas Raddatz • Abdel Hannachi • Malcolm Brooks • Kevin Hodges Thanks for help from the following colleagues: Note: all mistakes are my own ! (c) 2000 D.B.Stephenson@reading.ac.uk

3. Some important NAO questions … What exactly is it ? What makes it tick ? How well can it be forecast ? How does it affect things ? (c) 2000 D.B.Stephenson@reading.ac.uk

4. What should we call it ? • Druck Schwankungen • Northern Oscillation • North Atlantic Oscillation • Arctic Oscillation • Annular mode • Not the Arctic Oscillation • Al Moubarak • El Nao • Le YoYo Meteo • Northern Wazaaaaaa ! (c) 2000 D.B.Stephenson@reading.ac.uk

5. World weather correlations Sir Gilbert T. Walker “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) (c) 2000 D.B.Stephenson@reading.ac.uk

6. Early history of NAO research … • Saabye 1770-8 • Teisserence de Bort 1883 • Hann 1890 • Hildebrandsson 1897 • Meinardus 1898 • Pettersson 1905 • Exner 19?? • Walker 1908, 1924, 1932 • Defant 1924 • Loewe 1937 • Van Loon and Rogers 1978 • + many others … (c) 2000 D.B.Stephenson@reading.ac.uk

7. NAO related publications1980-2000 All articles with “North Atlantic Oscillation” either in the title of abstact Source: web of science bibliographic database (19,213,946 documents) (c) 2000 D.B.Stephenson@reading.ac.uk

8. 1988/89 1989/90 1990/91 1993/94 1991/92 1992/93 1995/96 1996/97 1994/95 Wintertime SLP VariabilitySLP 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. (c) 2000 D.B.Stephenson@reading.ac.uk

9. Small Calanus fin. stock NwAc Narrow Fast Storm Centre in Lab-Nordic Seas Min. Baltic Ice Max. Baltic Inflow +LSW PRODN. FST MAX 65 Mts NAC Warm +Coastal Upwelling Positive Phase of the NAO For more details about the NAO refer to: http://www.met.rdg.ac.uk/cag/NAO (c) 2000 D.B.Stephenson@reading.ac.uk

10. Negative phase of the NAO For more details about the NAO refer to: http://www.met.rdg.ac.uk/cag/NAO (c) 2000 D.B.Stephenson@reading.ac.uk

11. What is NAO ? NAO + NAO - Source: Martin Visbeck and Heidi Cullen (c) 2000 D.B.Stephenson@reading.ac.uk

12. NAO Indices Gibraltar Source: Jim HURRELL (c) 2000 D.B.Stephenson@reading.ac.uk

13. NAO varies on all time scales … (c) 2000 D.B.Stephenson@reading.ac.uk

14. Power spectrum of daily NAO 1948-2000 (c) 2000 D.B.Stephenson@reading.ac.uk

15. What makes it tick … • Tropospheric processes ? • Stratospheric processes ? • Oceanic processes ? • Coupled processes ? • Anthropogenic forcing ? (c) 2000 D.B.Stephenson@reading.ac.uk

16. North Atlantic trends Jacob Aaal Bonnevie Bjerknes 1897-1975 NAO related tripole in sea surface temperatures Source: Rowan Sutton (c) 2000 D.B.Stephenson@reading.ac.uk

17. Is it a coupled process ? Source: M.C. Escher (c) 2000 D.B.Stephenson@reading.ac.uk

18. Is it a coupled process ? Source: M.C. Escher (c) 2000 D.B.Stephenson@reading.ac.uk

19. 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. • Amount of skill depends on the lead time ! • Even low skill may be very useful ! • Impacts can lag NAO  e.g. wheat quality (c) 2000 D.B.Stephenson@reading.ac.uk

20. 1989 Mean=1.78 1990 Mean=1.02 1991 Mean=0.67 Jan30 1989 Jan29 1990 Dec3 1990 Jan28 1991 Dec5 1988 Dec4 1989 1992 Mean=1.24 1993 Mean=1.43 1994 Mean=0.26 Dec2 1991 Jan27 1992 Dec7 1992 Feb1 1993 Dec6 1993 Jan31 1994 1996 Mean=-0.7 1997 Mean=0.32 1995 Mean=1.03 Jan30 1995 Dec4 1995 Jan29 1996 Dec2 1996 Jan27 1997 Dec5 1994 Daily Values of the NAO Index Daily values of the NAO index obtained by projecting daily sea-level pressure onto the NAO pattern. - In collaboration with T. Strunk. (c) 2000 D.B.Stephenson@reading.ac.uk

21. a)Daily NAO index (1 Jan. 1989 to 30 Dec. 1997) 3 1 -1 -3 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 b)Accumulated Daily NAO index (1 Jan. 1989 to 30 Dec. 1997) 400 200 0 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 Persistence in the NAO Time Series Note the persistence in a) and the slow predictable movement in b). (c) 2000 D.B.Stephenson@reading.ac.uk

22. b) Decorrelation Time vs Maximum Lag a) Autocorrelation Function 50 40 30 20 10 0 1.0 0.8 0.6 0.4 0.2 0.0 decorrelation time Autocorrelation Function 0 200 600 1000 0 20 40 60 80 Lag Lag in days Persistence in daily NAO … Autocorrelations persist up until very long lags => early warning system may be possible (few week's lead). (c) 2000 D.B.Stephenson@reading.ac.uk

23. Some wise words … “Those who have knowledge, do not predict, Those who predict, do not have knowledge.” • Lao Tzu Chinese philospher 604-531 B.C (c) 2000 D.B.Stephenson@reading.ac.uk

24. How does it affect things ? The NAO is a major factor controlling • Surface conditions • Physical environment • Terrestrial ecosystems • Marine ecosystems • Human activities Note: the impacts are often very complex and not well understood (c) 2000 D.B.Stephenson@reading.ac.uk

25. Surface impacts of NAO … (c) 2000 D.B.Stephenson@reading.ac.uk

26. NAO and the sex life of Scandinavian ungulates (c) 2000 D.B.Stephenson@reading.ac.uk

27. Future events … OA26.03 Climate Variability “The Northern Oscillations: NAO and AO” European Geophysical Society XXVI General Assembly 25-30 March 2001 Nice, France Deadline for abstracts 1 Dec 2000 !!! More information: www.copernicus.org (c) 2000 D.B.Stephenson@reading.ac.uk

28. Consensus of specialists … (c) 2000 D.B.Stephenson@reading.ac.uk