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© Crown copyright 2004
Hadley Centre
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
Hadley Centre
Annual cycle of the NAO using EOF analyses
From Hurrell et al, 2003, pp1-34, AGU Geophysical Monograph 134
SummerNAO (3 month)
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Main definition of summer NAO used here
First area weighted covariance EOF of dailypressure at mean sea level, 1881-2003
Based on new daily EMSLP data, Ansell et al (2006)
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Hadley Centre
Daily K-means cluster analysis of summer NAO, July and August
Nearly equiprobable, 30% of all days explained by positive and negative clusters together. Other cluster
analyses give similar results
Clusters based on daily data, 1850-2003, new EMSLP SLP data set.
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Hadley Centre
Regression of EMSLP EOF1 time series and HadSLP2, 1948-2003
A dipole pattern is clear
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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
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Vertical Structure of daily SNAO
500hPa 300hPa
The daily SNAO is nearly equivalent barotropic
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Time series (non normalised) of high summer daily SNAO
index, 1850-2006
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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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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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Correlation of SNAO and high summer cloudiness
An area of positive correlation from UK to north west Russia shows clearly
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Summer versus winter North Atlantic Oscillations
The winter and summer NAOs seem to be independent
Winter NAO versus summer NAO. Correlation (all time scales) over 1867-2006 exactly zero .
(1961-2006 still only 0.05)
1850 1870 1890 1910 1930 1950 1970 1990 2010-3
-2
-1
0
1
2
3
Sta
ndard
ised o
ver
1867-2
006
Standardised winter NAO (Standardised over 1867-2006) (Iceland minus Azores) Standardised high summer NAO (Standardised over 1867-2006)
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Regression of observed SST (HadISST) and SNAO. Decadal time scales.
A link with the AMO pattern is indicated
1900-2002
1950-2002
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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
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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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Second CovarianceEOF of global sea surface temperature1911-1995.
Sahel rainfall and the southern node of the summer NAO
SST EOF - blue
SAHEL - green
Summer NAO - dark red
Based on Follandet al, 1999
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.
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Hadley Centre
Evidence of worldwide climate changes in 1960s possibly related to AMO
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
(-) Sahel rainfall
Brazil rainfall
Subtropical jet stream transport, 10S,10E
Summer NAO
(-)African Hadley circ transport 20S
Baines and Folland (in press, J. Climate)
10E 20-40S
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Hadley Centre
Changes in July-Sept Meridional and Zonal 200hPa winds, 1976-95 minus 1958-67 (NCEP Reanalysis)
From Baines and Folland (in press, J. Climate)
Meridional wind climatology, 1958-67
Change, 1976-95 minus 1958-67
Change, 1976-95 minus 1958-67
Zonal wind climatology, 1958-67
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Hadley Centre
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
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Reconstruction of the SNAO back to 1706
Observed SNAO (grey), Reconstruction (black)
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Standardised High summer Central England Temperature and SNAO 1706-1976
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Predicted SNAO changes under 4 times pre-industrial CO2 in HadGEM1 coupled model
CO2 increased over 140 years at 1%/year, then stabilised
EOF1 and time series of30 year filtered data
Time series of two point SNAO index: UK minus
Greenland
SNAO type pattern increases positively (UK hot and dry mode) strongly as CO2 increases. Would locally reinforce direct effects of global warming
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Hadley Centre
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?
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Hadley Centre
Additional slides
Additional Slides
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Hadley Centre
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 UKbecame more settled
anddrier in high summer
inrecent decades
Changes in European climate across the 1960s
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Hadley Centre
High summer England and Wales precipitation and SNAO, 1766-1976
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