1. European summer heat of 2015 in a long-term perspective and atmospheric/oceanic circulation
Andreas Hoy1,2, Stephanie Hänsel 2
Hessian Agency of Nature Conservation, Environment and Geology, Rheingaustr. 186, Wiesbaden, Germany
TU Bergakademie Freiberg, Interdisciplinary Environmental Research Centre, Brennhausgasse 14, Freiberg, Germany
1) Introduction
Hot summers inhere diverse social, economic and ecologic side effects. During 2015 large areas of Europe experienced a number of extreme heat episodes (fig. 1a), accompanied by very dry conditions mainly in Eastern Europe. We analyse the extremity of the 2015 summer and link recent hot summers to large-scale atmospheric and oceanic drivers.
2) Data and methods
Study area: central European latitudes (44 to 52 N; fig. 1b)
Stations: 42 temperature time series (with daily mean/extreme values)
Data source: ECA&D climate dataset (eca.knmi.nl/)
Observation start: mostly 19th century, latest from 1946 (fig. 1c, d)
Homogeneity: spatial coverage minimizes problems of single stations
Climate indices: threshold- and percentile-based indices (table 1)
Teleconnections: cpc.ncep.noaa.gov/data/teledoc/telecontents.shtml
Atlantic Multidecadal Oscill. esrl.noaa.gov/psd/data/timeseries/AMO/
Table 1: List of investigated temperature indices
Abbreviation
Definition
Unit
MAX
Tmax
Highest annual daily temperature maximum °C
MAX99/90
Tmax above 99th/90th percentile (Jun–Aug; )
days
MAX99/90sum
Temperature sum of temperatures above MAX99/90 K
VHD
Days with maximum temperatures ≥35°C HD
Days with maximum temperatures ≥30°C SU
Days with maximum temperatures ≥25°C
MINmax
Tmin
Highest annual daily temperature minimum
 °C
MIN99/90
Tmin above 99th/90th percentile (Jun–Aug; )
MIN99/90sum
Temperature sum of temperatures above MIN99/90 TN
Days with mainimum temperatures ≥20°C c) a) d) b)
Figure 1: a) temperature anomaly of summer 2015 (gridded dataset: GHCN v3), b) study area with regional division (West, Central, East), c) start year of daily mean temperature and d) maximum/minimum temperature time series
3) Climatology
Strong general increase of unusually hot summer conditions in recent 25 years (compared to the past years), especially after 2003 (table 2)
Especially pronounced for temperature minima in 20th century (table 2)
2015 summer was extreme even considering this warming background
Previous records in heat extremes were exceeded in large areas (fig. 3)
All indices indicating extremely hot summer conditions (apart the more moderate “Mean” and “SU”) show a new maximum in 2015 (table 2).
Example: VHD (Tmax≥35°C ) occurred with an regionally averaged frequency of 9 days almost twice as often as in previous record years (1946 & 2012: 5 d).
Central-eastern Europe affected most severely: almost all heat records broken in Prague (since 1775), Budweis (1884) and Vienna (1855; fig. 3) a) c)
Figure 4: all lines represent11-year means; a) air temperature anomalies from 1961–1990 within the study area and 3 CPC; telecon-nection indices (summer1951–2015; see fig. 5; WP: West Pacific Pattern); b) average air pressure anomalies: red=higher; blue=lower c) de-trendend time series of air temperature anomalies like in fig. a and and the Atlantic Multidecadal Oscillation (summer 1901–2015)
East Atlantic Pattern (EA)
4) Circulation
2015 major heat episodes linked to reoccurring atmospheric circulation patterns with frequent advection of very hot subtropical air masses (not shown)
Strong relation between temperature increase from the 1990s and condition of certain teleconnection indices impacting the study area (fig. 4a, b)
Warmest summers in the study region (2003, 2010, 2012 & 2015 with anomalies of >2 K) correlate with the highest values of the East Atlantic Pattern (EA)
Atlantic Multidecadal Oscillation (AMO) is a long-term cycle in North Atlantic SST with positive (warmer) and negative (cooler) phases of ca years duration
Strong correlation of r>0.8 of de-trended AMO-values and summer temperatures within the study area (fig. 4c)
Assuming a similarly long positive AMO-period like in the past, another 10 to 15 years of enhanced hot summer potential related to the AMO is possible
NB! Despite strong link between AMO and European summer temperatures such forecasts are quite speculative, as underlying mechanisms are not fully understood b)
Table 2: Most extreme summers (top 10) within 1901–2015 (11 indices); summers after 1990 are marked in bold, 2015 in red
Mean
MAX
VHD
MAX99 HD
MAX90 SU
MIN´max TN
MIN99
MIN90
2003
2015
2012
1947
2010
1952
1946
2011
1992
2007
1994
2006
1921
1911
2009
2013
2000
1983
2002
2005
1928
1998
2001
East Atlantic/ Western Russia Pattern (EAWR)
Fig. EA/EAWR after Rust et al. 2015
doi : /metz/2015/0642
5) Conclusions
The summer of 2015 has shown the potential of the projected further increase in heatwave intensity. Tremendous impacts are to be expected, if such conditions of extremely intensive heat would combine with a longer duration over the same region. This has already be experienced in 2003 and 2010 in parts of western Europe and European Russia, respectively. Preparing society and economy for such heatwaves is therefore essential.
Figure 3: Number (summation of lines) and kind (color of lines) of record values in 2015 at meteorological stations (1b) since start of observations (fig. 1c, d) for 11 temperature indices (table 1); black circles: no new records
Publication
This poster contains results of the following study: Hoy, A., Hänsel, S., Skalak, P., Ustrnul, Z. and Bochníček, O. (2016), The extreme European summer of 2015 in a long-term perspective. Int. J. Climatol.. doi: /joc.4751