(c) [email protected] 2002
Climate teleconnections in the Euro-Atlantic region
1. Overview of Euro-Atlantic teleconnection patterns
2. An example: European rainfall in Autumn 2000
3. North Atlantic ocean effects
Dr. David B. Stephenson
Department of MeteorologyUniversity of Reading www.met.rdg.ac.uk/cag
(c) [email protected] 2002
“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.”
Correlations with pressure
Sir Gilbert Walker
Correlations with temp
(c) [email protected] 2002
Positive Phase of the NAO
For more details about the NAO refer to: http://www.met.rdg.ac.uk/cag/NAO
+CoaalUpwelling
Min. Baltic IceMax. BalticInflow
SmallCalanus fin.
stoc
NwAc Narrow Fast
FSTMAX
Warm
65 Mts NAC
+LSWPRODN.
Storm Centre inLab-Nordic Seas
(c) [email protected] 2002
Winter NAO index: 1864-2001
Gibraltar
Image source: Jim HURRELL
© David B. Stephenson www.met.rdg.ac.uk/cag/NAO
(c) [email protected] 2002
Global correlations with the NAO index
SLP Temperature
Precipitation Zonal wind
(c) [email protected] 2002
Anders Knut Ångström’s word “Teleconnection”
A. Ångström (1935) Teleconnections of climatic changes in present time, Geogr. Annal., 17, 243-258
“the weather at a given place is not an isolated phenomenon but is intimately connected with the weather at adjacent places”
Chief causes of teleconnections:I. Local extension of a given featureII. Propagation of weather systemsIII. Existence of changes of great extension that affects local weather:
1. Energy reaching the Earth
2. Atmospheric circulation3. Other
(c) [email protected] 2002
PCth k' (t)X~
EOFth k')(e
(t)X~
(s)e),(X
k
k
k
rank
1kk
s
st
Empirical Orthogonal Function analysis
• Grid point variables Principal Components• PCs ranked in order of how much variance they explain• Allows dimension reduction
Known as Principal Component Analysis (PCA) in statistics
(c) [email protected] 2002
Northern Hemisphere wintertime teleconnection patternsPEU EU-1NAO
PNA EAO EA/WR
TNH WPJEPJ
Leading rotatedEOFs of 700mbgeopotential Height Source: Gerry Bell, CPC
(c) [email protected] 2002
Euro-Atlantic wintertime teleconnection patterns
(c) [email protected] 2002
Euroatlantic winter (DJF mean) reconstructed CPC indices
Source: J. Luterbacher
CPC indicesreconstructed from historical Europeantemperature and pressure measurements
(c) [email protected] 2002
Autumn 2000 rainfall (Sep-Nov) percent of long-term mean
England Wales Precipitation (EWP) is the average of rainfall over the 5 UK regions south of Scotland. It is obtained by taking the weighted average of about 60 rain gauges.
• Wigley et al. 1984• Wigley and Jones 1986• Gregory et al. 1991• Jones and Conway 1996• Osborn et al. 2000• Alexander and Jones 2000
(c) [email protected] 2002
Autumn (Sep-Nov) totals of England Wales Precipitation
1766-20012001event
Centered about mean of 271mm
(c) [email protected] 2002
A “wake-up call” for climate change
“Extremes will become the norm”
John Prescott, Deputy Prime Minister November 2000
(c) [email protected] 2002
500hPa streamfunction anomaly Sep-Nov 2000
Source: Mike Blackburn
Note the wave-trainpattern extending from the Atlanticover NW Europe witha high over Scandinaviaand a low over the UK.
(c) [email protected] 2002
Daily Euro-Atlantic teleconnection indices 1 Sep – 31 Dec 2000
Note the 3 monthpersistence in the Scandinavian index
(c) [email protected] 2002
Scatter plot of Autumn EWP totals versus reconstructed EU1 index1766-1994
r=0.36
(c) [email protected] 2002
Correlation mapsof global SLP with
EWP seasonal totals1948-2002
DJF
SON JJA
MAM
© Matt Sapiano
(c) [email protected] 2002
Source: Matt Sapiano’s ongoing PhD thesis work at Reading
(c) [email protected] 2002
Jacob Bjerknes’s N. Atlantic studies
Jacob Aaal Bonnevie Bjerknes 1897-1975
• 1959: The recent warming of the North Atlantic, Rossby Mem. Vol.
• 1960: Ocean temperatures and atmospheric circulation, WMO Bull.
• 1961: Climatic change as an ocean-atmosphere problem, Proc. of Rome Symposium.
• 1962: Synoptic survey of the interaction of sea and atmospherein the North Atlantic, G. Norvegica
• 1964: Atlantic air-sea interaction,Advances in Geophysics, vol. 10.
(c) [email protected] 2002
Variations of N. Atlantic SSTs and NAO SLP index
J. Bjerknes, Geophysica Norvegica, 24, No. 3, 1962.
(c) [email protected] 2002
J. Bjerknes’s long-term changes in SLP and SST
Lat r(NAO,T)
61.5N -0.34
57.5N -0.82
52.5N -0.82
47.5N 0.18
42.5N 0.33
37.5N -0.37
(c) [email protected] 2002
J. Bjerknes’s short term variations in SLP and SST
(c) [email protected] 2002
M. Junge and D.B. Stephenson (2002) "Mediated and direct effects of the North Atlantic ocean on winter temperatures in northwest Europe“, Int. Journal of Climatology (submitted).
DJF central england temperature anomalies
NAO SST tripole
? ?
How much direct role do North Atlantic SSTs have in determining NW Europe temperature variations?
(c) [email protected] 2002
NAO
SST tripole
Central EnglandTemperature
r=-0.38
r=-0.30
r=0.67
MUTUAL CORRELATIONS
(c) [email protected] 2002
Using data from 1900-1994, we obtain estimates of:
The fit explains 45% of the total CET variance and is statistically significant at p<0.001
The linear modelling approach
SSTbNAOaCET
To unravel the indirect from the direct effects we need to go beyond decriptive methods
(correlation analysis) and introduce a model:
08.006.0ˆ
08.064.0ˆ
b
a
(c) [email protected] 2002
NAO
SST tripole
Central EnglandTemperature
r=-0.38
b-=-0.06
a=0.64
Direct and indirect effects
r(CET,NAO)=0.67=0.64+(-0.38)x(-0.06)
r(CET,SST)=-0.30=-0.06+(-0.38)x(0.64)
So most of correlation between SST and
CET is coming indirectly via the NAO’s
influence on both variables.
(c) [email protected] 2002
The best explanatory fields for NW Europe temperatures
• NAO-like SLP pattern• shifted eastwards• explains 63% of variance
• Direct effect not the SST tripole• Strong near Newfoundland • Ratcliffe and Murray (1970)• explains 5% of CET variance
SLP
SST
(c) [email protected] 2002
Main Conclusions
5 main teleconnections for Europe in winter: PEU,NAO,EAO, SCA,EAWR,PNA
Teleconnections are of use for understanding climate events e.g. Autumn 2000
The correlation between North Atlantic SSTs and European climate is mainly an indirect effect mediated by the ocean’s response to the atmospheric flow.