1. 3Institute of Geography, Masaryk University, Brno, Czech Republic
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European Geosciences Union General Assembly 2019, Vienna, Austria, 7–12 April 2019
Role of teleconnections in establishing European drought variability patterns
Jiří Mikšovský1,4, Miroslav Trnka2,4, Rudolf Brázdil3,4 , Michal Belda 1
1Department of Atmospheric Physics, Charles University, Prague, Czech Republic
2Department of Agrosystems and Bioclimatology, Mendel University, Brno, Czech Republic
3Institute of Geography, Masaryk University, Brno, Czech Republic
4Global Change Research Institute, Czech Academy of Sciences, Brno, Czech Republic

2. Standardized Precipitation Evapotranspiration Index (SPEI):
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Standardized Precipitation Evapotranspiration Index (SPEI):
Data source, calculation and general properties
Separation of components correlated with individual climate variability modes:
Point-wise application of multiple linear regression
SPEI responses to
NAO, SO, AMO and PDO
Role of temperature
and precipitation in establishing the SPEI response patterns
Deviations from linearity:
How justified is purely linear approach?
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

3. Standardized Precipitation Evapotranspiration Index (SPEI)
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Charles University Prague
Standardized Precipitation Evapotranspiration Index (SPEI)
Index of meteorological drought defined from standardized difference between precipitation and potential evapotranspiration
Positive values for wet conditions, negative for dry
SPEI data here: calculated from monthly CRU TS gridded precipitation and potential evapo- transpiration series, in 0.5°x0.5° horizontal resolution, over the period
SPEI series for three sample locations (central Spain, south England, east Austria)
40.25°N, 5.25°W
51.75°N, 0.25°W
48.25°N, 16.25°E
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

4. /18
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Analysis setup
Multiple linear regression between local values of SPEI, temperature or precipitation and a set of five explanatory variables; standardized regression coefficients shown
Monthly data, division into cold (October-March) and warm (April-September) half-year
Statistical significance evaluated by moving-block bootstrap and shown for the 95% level
Explanatory variables:
Linear trend
North Atlantic Oscillation (NAO) index by Jones et al. (normalized pressure difference between Reykjavik and Gibraltar)
Atlantic Multidecadal Oscillation (AMO) index from NOAA ESRL (detrended mean sea surface temperature (Kaplan SST v2) in northern Atlantic)
Southern Oscillation (SO) index by Ropelewski & Jones (normalized pressure difference between Darwin (Australia) and Tahiti)
Pacific Decadal Oscillation (PDO) index from JISAO (detrended first principal component of sea surface temperature in northern Pacific)
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

5. Imprints of variability modes in SPEI: North Atlantic Oscillation
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Imprints of variability modes in SPEI: North Atlantic Oscillation
Distinct multipolar structure, shifting between cold and warm season
SPEI responses less prominent in transitory zones, making NAO less influential in some regions, e.g. near southern Baltic Sea
Regression coefficients between SPEI and NAO
October-March
Regression coefficients between SPEI and NAO
April-September
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

6. Role of temperature and precipitation:
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Role of temperature and precipitation:
NAO-related responses, October to March
Temperature vs. NAO
Effect of precipitation boosted by temperature in most of central and southern Europe, but weakened in the north and north-west
Local combination of responses
Precipitation vs. NAO
standardized regression coefficient
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

7. Role of temperature and precipitation:
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Charles University Prague
Role of temperature and precipitation:
NAO-related responses, April to September
Temperature vs. NAO
Effect of precipitation boosted by temperature in most of Europe, except in much of Scandinavia, Scotland and Ireland
Local combination of responses
Precipitation vs. NAO
standardized regression coefficient
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

8. Imprints of variability modes in SPEI: Atlantic Multidecadal Osc.
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Imprints of variability modes in SPEI: Atlantic Multidecadal Osc.
During cold part of the year, scattered imprint of AMO in the Balkans and eastern Europe (more clear when just spring is considered)
During warm part of the year, significant responses especially in Iberia and France
Regression coefficients between SPEI and AMO
October-March
Regression coefficients between SPEI and AMO
April-September
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

9. Role of temperature and precipitation:
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Role of temperature and precipitation:
AMO-related responses, October to March
Temperature vs. AMO
Effect of precipitation amplified by temperature in several small regions with significant SPEI response, but weakened in the Balkans and southern Russia
Local combination of responses
Precipitation vs. AMO
standardized regression coefficient
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

10. Role of temperature and precipitation:
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Charles University Prague
Role of temperature and precipitation:
AMO-related responses, April to September
Temperature vs. AMO
Precipitation responses largely non-significant in Iberia and France, but their combination with response in temperature results in a more significant SPEI anomaly
Local combination of responses
Precipitation vs. AMO
standardized regression coefficient
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

11. Imprints of variability modes in SPEI: Southern Oscillation
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Imprints of variability modes in SPEI: Southern Oscillation
Only weak and scattered responses, statistically significant mostly in eastern Europe and the adjacent parts of Asia
Somewhat stronger patterns associated with SST-based NINO indices instead of SO, but collinearity with PDO increases in such configurations
Regression coefficients between SPEI and SO
October-March
Regression coefficients between SPEI and SO
April-September
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

12. Imprints of variability modes in SPEI: Pacific Decadal Oscillation
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Imprints of variability modes in SPEI: Pacific Decadal Oscillation
Cold season: Strong geographical contrast of significant responses in Scandinavia, intermittent signal in the rest of Europe
Warm season: Significant signal mostly in central Balkans and southern Russia
Regression coefficients between SPEI and PDO
October-March
Regression coefficients between SPEI and PDO
April-September
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

13. Role of temperature and precipitation:
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Charles University Prague
Role of temperature and precipitation:
PDO-related responses, October to March
Temperature vs. PDO
SPEI responses largely following the precipitation pattern, bolstered by temperature especially in Sweden and England
Local combination of responses
Precipitation vs. PDO
standardized regression coefficient
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

14. Role of temperature and precipitation:
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Charles University Prague
Role of temperature and precipitation:
PDO-related responses, April to September
Temperature vs. PDO
Temperature-related strengthening of SPEI responses over southern Russia and central Balkans, but suppression over central France
Local combination of responses
Precipitation vs. PDO
standardized regression coefficient
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

15. Deviations from linearity?
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Deviations from linearity?
Responses in linear regression assumed to be directly proportional to the explanatory variables - but are they really?
Even simple nonlinearity-sensitive statistics reveal differences from strict linearity, although highly variable between different regions
Difference of SPEI-to-NAO regression coefficients
pertaining to NAO+ and NAO- phases, October-March
Difference of SPEI-to-AMO regression coefficients
pertaining to AMO+ and AMO- phases, October-March
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

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Conclusions
The role NAO is generally strong, but its imprint in SPEI is diminished in some locations by mutually opposite contributions from precipitation and temperature
Other variability modes are less prominent overall, but project significant influences regionally, including:
AMO especially in eastern Europe during the cold half-year, in south-western Europe during the warm season
PDO especially in Scandinavia during the cold season, in the Balkans and southern Russia during the warm season
Notable deviations from linearity can only be found in some areas and seasons, but for these regions, application of nonlinear mappings has potential to provide more realistic picture of the local effects of teleconnections and of their possible interactions
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns

17. Thank you for your attention
Charles University Prague
Thank you for your attention
Mikšovský et al.: Role of teleconnections in establishing European drought variability patterns