The Summer North Atlantic Oscillation since the early eighteenth century

1. The Summer North Atlantic Oscillation since the early eighteenth century Chris Folland, Hans Linderholm, Sarah Ineson, Jeff Knight, Jim Hurrell, Peter Baines, David Fereday, Steve Warrenand Adam Scaife Hadley Centre, Met Office, Univ. of Goteborg, Sweden, NCAR, USA, Univ. of Melbourne, Australia and Univ. of Washington, USA • What is the Summer North Atlantic Oscillation (SNAO)? • Temporal characteristics • Links to temperature, rainfall, cloudiness and SST • Links to Hadley circulation changes • Reconstructions of SNAO back to 1706 • SNAO simulations under enhanced CO2. • Conclusions

2. Annual cycle of the NAO using EOF analyses Summer NAO (3 month) From Hurrell et al, 2003, pp1-34, AGU Geophysical Monograph 134

3. Main definition of summer NAO used here First area weighted covariance EOF of daily pressure at mean sea level, 1881-2003 Based on new daily EMSLP data, Ansell et al (2006)

4. Daily K-means cluster analysis of summer NAO, July and August Clusters based on daily data, 1850-2003, new EMSLP SLP data set. Nearly equiprobable, 30% of all days explained by positive and negative clusters together. Other cluster analyses give similar results

5. Regression of EMSLP EOF1 time series and HadSLP2, 1948-2003 A dipole pattern is clear

6. Time series of 1 day to 2 month EMSLP SNAO calculated over 1881-2003. EOF1 of JA in all cases Patterns and time series are almost identical

7. Vertical Structure of daily SNAO 300hPa 500hPa The daily SNAO is nearly equivalent barotropic

8. Time series (non normalised) of high summer daily SNAO index, 1850-2006

9. Correlation of SNAO with surface temperature (land air and ocean surface from HadCRUT3v), 1900-1998 Strong positive correlation over N. W Europe, negative near Greenland and E. Mediterranean/Middle East.

10. Correlation of SNAO with precipitation (land) (from Hulme data set), 1900-1998 Strong negative correlations over N.W. Europe, moderate positive correlations over Mediterranean and positive over Sahel

11. Correlation of SNAO and high summer cloudiness An area of positive correlation from UK to north west Russia shows clearly

12. Summer versus winter North Atlantic Oscillations The winter and summer NAOs seem to be independent

13. Regression of observed SST (HadISST) and SNAO. Decadal time scales. 1950-2002 1900-2002 A link with the AMO pattern is indicated

14. Regression of model SNAO versus SST 1950-2002, ensemble mean of 6 integrations of HadAM3 with all forcings A quasi-hemispheric AMO like pattern emerges again

15. Modelled and Observed Summer North Atlantic Oscillation, 1871-2002 (HadAM3, 6 members all forcings) Model emphasises positive SNAO in early cool AMO phase, observations emphasise the latter phase

16. Sahel rainfall and the southern node of the summer NAO Second Covariance EOF of global sea surface temperature 1911-1995. Based on Folland et al, 1999 SST EOF - blue SAHEL - green Summer NAO - dark red AMO pattern may be largely related to natural variations in the thermohaline circulation (Knight et al, 2005). Note decrease in1960s. Link of SNAO and Sahel rainfall strong on decadal time scales but intriguingly but joint link to AMO is starting to fail.

17. Evidence of worldwide climate changes in 1960s possibly related to AMO 10E 20-40S Baines and Folland (in press, J. Climate)

18. Changes in July-Sept Meridional and Zonal 200hPa winds, 1976-95 minus 1958-67 (NCEP Reanalysis) Meridional wind climatology, 1958-67 Change, 1976-95 minus 1958-67 Change, 1976-95 minus 1958-67 Zonal wind climatology, 1958-67 From Baines and Folland (in press, J. Climate)

19. Tree Ring Paleo-reconstruction of SNAO • Based on an analysis of conifer tree ring data in Scotland and Western Norway (Hans Linderholm) • Reconstruction of SNAO back to 1706 • Calibrated over both 1880-1912 and 1913-1976 and tested on independent period • Reconstruction 1880-1912 based on 1913-1976 regression equation. 38% of variance explained. • Compared to Central England temperature and England and Wales Precipitation

20. Reconstruction of the SNAO back to 1706 Observed SNAO (grey), Reconstruction (black)

21. Standardised High summer Central England Temperature and SNAO 1706-1976

22. Predicted SNAO changes under 4 times pre-industrial CO2 in HadGEM1 coupled model CO2 increased over 140 years at 1%/year, then stabilised Time series of two point SNAO index: UK minus Greenland EOF1 and time series of 30 year filtered data SNAO type pattern increases positively (UK hot and dry mode) strongly as CO2 increases. Would locally reinforce direct effects of global warming

23. Conclusions • The high Summer North Atlantic Oscillation is a useful concept and can be identified in more than one way. • Affects N.W. Europe rainfall and temperature in high summer. • Likely to be influenced by SST on decadal time scales. • Decadal variations may relate to worldwide changes in atmospheric circulation involving the N. African monsoon. • A paleo-reconstruction of SNAO seems to have significant skill in temperature, and possibly rainfall, back to the early eighteenth century. • HadGEM1 predicts a strong increase in N.W Europe drought mode of SNAO under strong greenhouse warming. Is this starting to happen?

24. Additional slides Additional Slides

25. Changes in European climate across the 1960s July and August Rainfall 1967-1998 as a Percentage of 1921-1960 Surface Pressure Difference July and August 1967-1998 relative to 1921-1960 Weather over the UK became more settled and drier in high summer in recent decades

26. High summer England and Wales precipitation and SNAO, 1766-1976