(c) [email protected] 2002 Climate teleconnections in the Euro-Atlantic region 1.Overview...

(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


“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


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


Winter NAO index: 1864-2001

Gibraltar

Image source: Jim HURRELL

© David B. Stephenson www.met.rdg.ac.uk/cag/NAO


Global correlations with the NAO index

SLP Temperature

Precipitation Zonal wind


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


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


Northern Hemisphere wintertime teleconnection patternsPEU EU-1NAO

PNA EAO EA/WR

TNH WPJEPJ

Leading rotatedEOFs of 700mbgeopotential Height Source: Gerry Bell, CPC


Euro-Atlantic wintertime teleconnection patterns


Euroatlantic winter (DJF mean) reconstructed CPC indices

Source: J. Luterbacher

CPC indicesreconstructed from historical Europeantemperature and pressure measurements


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


Autumn (Sep-Nov) totals of England Wales Precipitation

1766-20012001event

Centered about mean of 271mm


A “wake-up call” for climate change

“Extremes will become the norm”

John Prescott, Deputy Prime Minister November 2000


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.


Daily Euro-Atlantic teleconnection indices 1 Sep – 31 Dec 2000

Note the 3 monthpersistence in the Scandinavian index


Scatter plot of Autumn EWP totals versus reconstructed EU1 index1766-1994

r=0.36


Correlation mapsof global SLP with

EWP seasonal totals1948-2002

DJF

SON JJA

MAM

© Matt Sapiano


Source: Matt Sapiano’s ongoing PhD thesis work at Reading


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.


Variations of N. Atlantic SSTs and NAO SLP index

J. Bjerknes, Geophysica Norvegica, 24, No. 3, 1962.


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


J. Bjerknes’s short term variations in SLP and SST


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?


NAO

SST tripole

Central EnglandTemperature

r=-0.38

r=-0.30

r=0.67

MUTUAL CORRELATIONS


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


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.


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


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.

(c) [email protected] 2002 Climate teleconnections in the Euro-Atlantic region 1.Overview...

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