1. What is the Summer North Atlantic Oscillation (SNAO)?
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 Warren and 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
© Crown copyright 2004

2. Annual cycle of the NAO using EOF analyses
Summer
NAO (3 month)
From Hurrell et al, 2003, pp1-34, AGU Geophysical Monograph 134
© Crown copyright 2004

3. Main definition of summer NAO used here
First area weighted covariance EOF of daily
pressure at mean sea level,
Based on new daily EMSLP data, Ansell et al (2006)
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4. Daily K-means cluster analysis of summer NAO, July and August
Clusters based on daily data, , new EMSLP SLP data set.
Nearly equiprobable, 30% of all days explained by positive and negative clusters together. Other cluster analyses give similar results
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5. Regression of EMSLP EOF1 time series and HadSLP2, 1948-2003
A dipole pattern is clear
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6. Patterns and time series are almost identical
Time series of 1 day to 2 month EMSLP SNAO calculated over EOF1 of JA in all cases
Patterns and time series are almost identical
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7. Vertical Structure of daily SNAO
300hPa
500hPa
The daily SNAO is nearly equivalent barotropic
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8. Time series (non normalised) of high summer daily SNAO index, 1850-2006
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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.
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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
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11. Correlation of SNAO and high summer cloudiness
An area of positive correlation from UK to north west Russia shows clearly
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12. Summer versus winter North Atlantic Oscillations
The winter and summer NAOs seem to be independent
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13. Regression of observed SST (HadISST) and SNAO. Decadal time scales.
A link with the AMO pattern is indicated
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14. A quasi-hemispheric AMO like pattern emerges again
Regression of model SNAO versus SST , ensemble mean of 6 integrations of HadAM3 with all forcings
A quasi-hemispheric AMO like pattern emerges again
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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
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16. Sahel rainfall and the southern node of the summer NAO
Second Covariance
EOF of global
sea surface temperature
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.
© Crown copyright 2004

17. Evidence of worldwide climate changes in 1960s possibly related to AMO
Baines and Folland (in press, J. Climate)
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18. From Baines and Folland (in press, J. Climate)
Changes in July-Sept Meridional and Zonal 200hPa winds, minus (NCEP Reanalysis)
Meridional wind climatology,
Change, minus
Change, minus
Zonal wind climatology,
From Baines and Folland (in press, J. Climate)
© Crown copyright 2004

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 and and tested on independent period
Reconstruction based on regression equation. 38% of variance explained.
Compared to Central England temperature and England and Wales Precipitation
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20. Reconstruction of the SNAO back to 1706
Observed SNAO (grey), Reconstruction (black)
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21. Standardised High summer Central England Temperature and SNAO 1706-1976
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22. CO2 increased over 140 years at 1%/year, then stabilised
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
© Crown copyright 2004

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?
© Crown copyright 2004

24. Additional slides
Additional Slides
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25. Changes in European climate across the 1960s
July and August Rainfall
as a Percentage of
Surface Pressure Difference
July and August
relative to
Weather over the UK
became more settled and
drier in high summer in
recent decades
© Crown copyright 2004

26. High summer England and Wales precipitation and SNAO, 1766-1976
© Crown copyright 2004