Climate teleconnections in the Euro-Atlantic region Dr. David B. Stephenson Department of Meteorology University of Reading www.met.rdg.ac.uk/cag Overview of Euro-Atlantic teleconnection patterns An example: European rainfall in Autumn 2000 North Atlantic ocean effects (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 Sir Gilbert Walker Correlations with pressure “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 temp (c) D.B.Stephenson@reading.ac.uk 2002 Dr. David Stephenson

Positive Phase of the NAO +CoaalUpwelling Min. Baltic Ice Max. Baltic Inflow Small Calanus fin. stoc NwAc Narrow Fast FST MAX Warm 65 Mts NAC +LSW PRODN. Storm Centre in Lab-Nordic Seas Highly idealised schematic of NAO positive phase In reality, no individual year looks like this – but variations on it. For more details about the NAO refer to: http://www.met.rdg.ac.uk/cag/NAO (c) D.B.Stephenson@reading.ac.uk 2002 Dr. David Stephenson

Winter NAO index: 1864-2001 Gibraltar © David B. Stephenson www.met.rdg.ac.uk/cag/NAO (c) D.B.Stephenson@reading.ac.uk 2002 Image source: Jim HURRELL Dr. David Stephenson

Global correlations with the NAO index SLP Temperature Precipitation Zonal wind (c) D.B.Stephenson@reading.ac.uk 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 feature II. Propagation of weather systems III. Existence of changes of great extension that affects local weather: 1. Energy reaching the Earth 2. Atmospheric circulation 3. Other (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 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) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 Northern Hemisphere wintertime teleconnection patterns NAO PEU EU-1 PNA EAO EA/WR EPJ TNH WPJ Leading rotated EOFs of 700mb geopotential Height Source: Gerry Bell, CPC (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 Euro-Atlantic wintertime teleconnection patterns (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 Euroatlantic winter (DJF mean) reconstructed CPC indices CPC indices reconstructed from historical European temperature and pressure measurements Source: J. Luterbacher (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 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) D.B.Stephenson@reading.ac.uk 2002

Autumn (Sep-Nov) totals of England Wales Precipitation 1766-2001 Centered about mean of 271mm 2001 event (c) D.B.Stephenson@reading.ac.uk 2002

A “wake-up call” for climate change “Extremes will become the norm” John Prescott, Deputy Prime Minister November 2000 (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 500hPa streamfunction anomaly Sep-Nov 2000 Note the wave-train pattern extending from the Atlantic over NW Europe with a high over Scandinavia and a low over the UK. Source: Mike Blackburn (c) D.B.Stephenson@reading.ac.uk 2002

Daily Euro-Atlantic teleconnection indices 1 Sep – 31 Dec 2000 Note the 3 month persistence in the Scandinavian index (c) D.B.Stephenson@reading.ac.uk 2002

Scatter plot of Autumn EWP totals versus reconstructed EU1 index 1766-1994 r=0.36 (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 DJF MAM Correlation maps of global SLP with EWP seasonal totals 1948-2002 SON JJA © Matt Sapiano (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 Source: Matt Sapiano’s ongoing PhD thesis work at Reading (c) D.B.Stephenson@reading.ac.uk 2002

Jacob Bjerknes’s N. Atlantic studies 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 atmosphere in the North Atlantic, G. Norvegica 1964: Atlantic air-sea interaction, Advances in Geophysics, vol. 10. Jacob Aaal Bonnevie Bjerknes 1897-1975 (c) D.B.Stephenson@reading.ac.uk 2002 Dr. David Stephenson

(c) D.B.Stephenson@reading.ac.uk 2002 Variations of N. Atlantic SSTs and NAO SLP index J. Bjerknes, Geophysica Norvegica, 24, No. 3, 1962. (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 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 47.5N 0.18 42.5N 0.33 37.5N -0.37 (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 J. Bjerknes’s short term variations in SLP and SST (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 How much direct role do North Atlantic SSTs have in determining NW Europe temperature variations? NAO SST tripole ? ? DJF central england temperature anomalies 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). (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 MUTUAL CORRELATIONS NAO Central England Temperature r=0.67 r=-0.38 r=-0.30 SST tripole (c) D.B.Stephenson@reading.ac.uk 2002

The linear modelling approach To unravel the indirect from the direct effects we need to go beyond decriptive methods (correlation analysis) and introduce a model: 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 (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 2002 Direct and indirect effects NAO Central England Temperature a=0.64 r=-0.38 b-=-0.06 SST tripole 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) D.B.Stephenson@reading.ac.uk 2002

The best explanatory fields for NW Europe temperatures SLP NAO-like SLP pattern shifted eastwards explains 63% of variance SST Direct effect not the SST tripole Strong near Newfoundland Ratcliffe and Murray (1970) explains 5% of CET variance (c) D.B.Stephenson@reading.ac.uk 2002

(c) D.B.Stephenson@reading.ac.uk 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. (c) D.B.Stephenson@reading.ac.uk 2002