hadley centre © crown copyright 2004 the summer north atlantic oscillation since the early...

© 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


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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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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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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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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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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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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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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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Additional slides

Additional Slides


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

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