Understanding the rapid summer warming and changes in temperature extremes since the mid-1990s over Western Europe
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Analysis of observations indicates that there was a rapid increase in summer (June–August) mean surface air temperature (SAT) since the mid-1990s over Western Europe. Accompanying this rapid warming are significant increases in summer mean daily maximum temperature, daily minimum temperature, annual hottest day temperature and warmest night temperature, and an increase in frequency of summer days and tropical nights, while the change in the diurnal temperature range (DTR) is small. This study focuses on understanding causes of the rapid summer warming and associated temperature extreme changes. A set of experiments using the atmospheric component of the state-of-the-art HadGEM3 global climate model have been carried out to quantify relative roles of changes in sea surface temperature (SST)/sea ice extent (SIE), anthropogenic greenhouse gases (GHGs), and anthropogenic aerosols (AAer). Results indicate that the model forced by changes in all forcings reproduces many of the observed changes since the mid-1990s over Western Europe. Changes in SST/SIE explain 62.2 ± 13.0 % of the area averaged seasonal mean warming signal over Western Europe, with the remaining 37.8 ± 13.6 % of the warming explained by the direct impact of changes in GHGs and AAer. Results further indicate that the direct impact of the reduction of AAer precursor emissions over Europe, mainly through aerosol-radiation interaction with additional contributions from aerosol-cloud interaction and coupled atmosphere-land surface feedbacks, is a key factor for increases in annual hottest day temperature and in frequency of summer days. It explains 45.5 ± 17.6 % and 40.9 ± 18.4 % of area averaged signals for these temperature extremes. The direct impact of the reduction of AAer precursor emissions over Europe acts to increase DTR locally, but the change in DTR is countered by the direct impact of GHGs forcing. In the next few decades, greenhouse gas concentrations will continue to rise and AAer precursor emissions over Europe and North America will continue to decline. Our results suggest that the changes in summer seasonal mean SAT and temperature extremes over Western Europe since the mid-1990s are most likely to be sustained or amplified in the near term, unless other factors intervene.
KeywordsSurface air temperature Temperature extremes Western Europe Atmospheric general circulation model Greenhouse gas Anthropogenic aerosol
European summer climate exhibits variability on a wide range of timescales. Understanding the nature and drivers of this variability is an essential step in developing robust climate predictions and risk assessments. In the last few decades, Europe has warmed not only faster than the global average, but also faster than expected from anthropogenic greenhouse gas increases (Ruckstuhl et al. 2008; Philipona et al. 2009; van Oldenborgh et al. 2009). With the warming, Europe experienced record-breaking heat waves and extreme temperatures that imposed disastrous impacts on individuals, and society (Stott et al. 2004; Fischer and Schär 2010; Barriopedro et al. 2011; Christidis et al. 2011, 2012; Hegerl et al. 2011; Rahmstorf and Coumou 2011; Hoerling et al. 2012; Schubert et al. 2014; Sillmann et al. 2014; Vautard et al. 2007, 2013). Such climate events are often accompanied by prominent anomalies in atmospheric circulation and precipitation, as well as in the conditions of the nearby land and ocean surfaces. The extreme 2003 European and 2010 Russian heat waves have been shown to be associated with blocking regimes. However, Barnes et al. (2014) showed that there is no increasing trend of summer blocking frequency over Western Europe based on three different reanalysis data sets, suggesting that the link between rapid surface warming and changes in blocking frequency on the decadal time scale is weak.
The impacts of temperature extremes have highlighted the urgency of improved understanding of their physical causes and to what extent they are a manifestation of a warming world (e.g., Dole et al. 2011; Trenberth and Fasullo 2012; Otto et al. 2012; Christidis et al. 2014; Perkins 2015). A number of studies have investigated the factors contributing to such extreme events (Schär et al. 2004; Otto et al. 2012; Hanlon et al. 2013; Christidis et al. 2014) and how extremes will change in response to anthropogenic forcings (Kim et al. 2013; Lindvall and Svensson 2014; Cattiaux et al. 2015; Vavrus et al. 2015). These factors include the role of soil–atmosphere feedbacks during the hot summers. In summer, dry soil induces fewer clouds, presumably through reduced upward latent heat fluxes, which in turn increases the amount of incident solar energy at the surface and further enhances heat fluxes and the ratio of sensible over latent heat fluxes. This causes a positive feedback to soil drying (Vautard et al. 2007; Fischer and Schär 2010; Seneviratne et al. 2010; Fischer et al. 2012; Kim et al. 2013; Boé and Terray 2014; Lindvall and Svensson 2014; Miralles et al. 2014; Cattiaux et al. 2015; Whan et al. 2015), highlighting the importance of local land–atmosphere interactions in driving the regional temperature extremes. Other factors include SST anomalies affecting heat waves in Western Europe through boundary forcing induced large scale circulation (e.g., Cassou et al. 2005; Sutton and Hodson 2005; Black and Sutton 2007; Della-Marta et al. 2007; Carril et al. 2008; Feudale and Shukla 2011a, b; Kamae et al. 2014).
Due to air quality legislation, AAer precursor emissions in Europe and North America have continuously decreased since the 1980s (Smith et al. 2011; Kühn et al. 2014). This decrease in AAer precursor emissions and the accompanied increase in downward surface solar radiation (solar “brightening”) is considered as the likely cause of the rapid warming over Western Europe (Wild 2009, 2012; Folini and Wild 2011; Nabat et al. 2014). However, it is still not clear to what extent the rapid warming over Western Europe is a result of a fast land–atmosphere response to the direct impact of changes in GHGs and AAer and to what extent the warming is mediated by the SST changes induced by these forcings. In particular, the relative roles of these forcing factors on the temperature extremes have not been investigated.
The main aims of this work are to determine the relative roles of changes in: (i) SST/SIE, (ii) GHGs, and (iii) AAer forcings in shaping the changes in the summer mean SAT and temperature extremes since the mid-1990s over Western Europe. This will be achieved by performing a set of experiments using the atmospheric component of the state-of-the-art HadGEM3 global climate model. In this paper, we do not address the role of land use change (e.g., Christidis et al. 2013). The structure of the paper is as follows. Section 2 describes observed changes since the mid-1990s. In Sect. 3, the model and experiments are described briefly. Section 4 presents simulated changes in response to different forcings. Section 5 elucidates the physical processes for the responses. Conclusions are in Sect. 6.
2 Observed changes since the mid-1990s over Western Europe
2.1 Observational data sets
The monthly mean SAT data sets used are the University of Delaware (UD) land SAT (1901–2010) (Legates and Willmott 1990a, b), the CRU TS3.21 data sets (1901–2013) on a 0.5° × 0.5° grid (Harris et al. 2014), the NASA GISS Surface Temperature Analysis (GISTEMP) (1880–2013) on a 2° × 2° grid (Hansen et al. 2010), and the HadCRUT4 data set (Morice et al. 2012). Temperature extreme indices used are daily maximum temperature (Tmax), daily minimum temperature (Tmin), and annual hottest day temperature (TXx), warmest night temperature (TNx), the diurnal temperature range (DTR), the frequency of summer days (SU, annual number of days when Tmax > 25 °C), and tropical nights (TR, annual number of days when Tmin > 20 °C). Tmax, Tmin, and DTR are based on CRU TS3.21 data sets. TXx, TNx, SU, and TR are based on HadEX2 data set (Donat et al. 2013) and the E-OBS gridded data set (Haylock et al. 2008). Monthly mean SSTs used are HadISST on a 1° × 1° grid (Rayner et al. 2003). Monthly mean variables of NCEP/NCAR reanalysis (1979–2013) (Kalnay et al. 1996) and monthly sea level pressures of HadSLP2 date (Allan and Ansell 2006) are also used.
2.2 Observed changes in summer mean temperature and temperature extremes
The rapid summer mean warming over Western Europe was also accompanied by changes in temperature extremes. The time evolutions of area averaged temperature extremes (Fig. 1b–d) show a rapid increase in summer mean Tmax, Tmin, TXx and TNx over Western Europe. The change in DTR, however, is small. Analysis also shows a rapid increase in the frequency of SU, and TR over Western Europe since the mid-1990s. SU and TR have increased by 8 and 3 days respectively over the last 16 years relative to 30 year mean (1964–1993).
What has caused these rapid changes in both summer mean SAT and temperature extremes over Western Europe since the mid-1990s? By performing a set of numerical experiments using the atmospheric component of the state-of-the-art HadGEM3 global climate model with different forcings we will investigate the drivers for those observed changes since the mid-1990s.
3 Model and model experiments
3.1 Model and model experiments
Summary of numerical experiments
Monthly climatological sea surface temperature (SST) and sea ice extent (SIE) averaged over the period 1964 to 1993, using HadISST (Rayner et al. 2003) with greenhouse gas (GHG) concentrations set at mean values over the same period, and anthropogenic aerosol (AAer) precursor emissions (Lamarque et al. 2010) at mean values over the period 1970–1993
Monthly climatological SST/SIE averaged over the period of 1996 to 2011, with GHG concentrations set at mean values over the period 1996–2009, and AAer precursor emissions at mean values over the period 1996–2010
Monthly climatological SST/SIE averaged over the period of 1996 to 2011, with GHG concentrations set at mean values over the period 1996–2009, but with AAer precursor emissions at mean values over the period 1970–1993
Monthly climatological SST/SIE averaged over the period of 1996 to 2011, with GHG concentrations and AAer precursor emissions the same in the CONTROL experiment
3.2 Model climatology
Observations show a strong meridional SAT gradients around 45°N over East North Atlantic and Western Europe (Fig. 4a–c). The circulation is characterized by the subtropical anticyclone over subtropical North Atlantic and with a ridge extending eastward over the Mediterranean region (Fig. 4b). Associated with circulation are strong westerlies across the UK into Western Europe and northwesterlies over the east part of the Mediterranean region. Observed precipitation indicates a band of large precipitation of >2.0 mm day−1 extending from North Atlantic into Western Europe with precipitation over the Iberian Peninsula and over the Mediterranean Sea being <1.0 mm day−1. The model reproduces the spatial patterns of the summer climate fairly well over the North Atlantic and the European sector (Fig. 4d–f), suggesting that it is an appropriate tool for the investigation of the response of regional climate to different forcings.
4 Model simulated responses to different forcings
5 Physical processes for the model simulated responses to different forcings
In this section, the physical processes responsible for the model responses to different forcings are elucidated in detail by focusing on changes in the surface energy components and related variables over land.
5.1 Response to changes in SST/SIE
5.2 Response to the direct impact of changes in GHGs forcing
5.3 Response to the direct impact of changes in AAer precursor emissions
The decrease in AAer precursor emissions over Europe induces a significant local warming through aerosol-radiation and aerosol-cloud interactions (e.g., Twomey 1977; Rosenfeld et al. 2008; Nabat et al. 2014; Dong et al. 2015). Changes in surface energy budget are characterized by large increase in the net SW over Europe (Fig. 12b). Cloud cover changes show a large decrease over Central and Southern Europe (Fig. 12f) with an increase to the north, consistent with the changes in soil moisture over Europe (Fig. 12i). The decrease in cloud cover gives rise to positive SW CRE of 2–6 W m−2 and negative LW CRE of 2–4 W m−2 (Fig. 12h, i). These lead to larger changes in the net SW of about 8–16 W m−2 and LW of about 4–8 W m−2 (Fig. 12b, c) than change in the clear sky net SW of about 6-8 and LW at the surface (not shown). The increase in clear sky SW at the surface is about 2–3 times as large as the increase in SW CRE (Fig. 12b, g), indicating the dominant role of aerosol-radiation interaction in the surface solar radiation change over Western Europe. This is in line with Ruckstuhl et al. (2010) who found a small contribution of cloud impact to the recent solar brightening over Europe. The increase in surface net SW and the decrease in upward latent heat flux (Fig. 12d) due to soil drying (Fig. 12i) over Southern Europe give rise to large warming in surface air temperature over Western Europe and large changes in temperature extremes.
These results further suggest that decreased AAer precursor emissions over Europe might also play an important role for the decrease in summer cloud cover during the recent decades over Europe (e.g., Tang et al. 2012) in which the deficit of soil moisture limits evapotranspiration, which results in an increased upward sensible heat fluxes (Fig. 12e) due to the energy conservation constraint. The shift towards higher sensible heat fluxes in turn produces drier and warmer air and increases evaporative demand, which dries the soil further (Mueller and Seneviratne 2012; Boé and Terray 2014; Miralles et al. 2014; Cattiaux et al. 2015; Whan et al. 2015). This positive feedback loop generates less cloud and increased surface SW radiation, which again causes even more drying, illustrating the importance of soil processes in driving the regional responses of temperature and temperature extremes (Dai et al. 1999; Jaeger and Seneviratne 2011; Mueller and Seneviratne 2012; Stegehuis et al. 2012; Whan et al. 2015) and resulting in increases in Tmax, TXx, and the frequency in summer days as well as an enhancement of DTR (Fig. 9b). Various studies have demonstrated the role of precipitation deficit in spring for hot temperature extremes in summer (Mueller and Seneviratne 2012; Whan et al. 2015). We have analysed precipitation and soil moisture changes in spring season (MAM) in response to AAer changes and results indicate that there are no significant pre-summer soil moisture anomalies that persist into summer (not shown). These suggest that summer drying (Fig. 12i) over Central Europe is mainly the outcome of reduced precipitation in summer, rather than any pre-summer signals, and that the impact of pre-summer land surface conditions on summer surface temperature and temperature extremes in our model simulation is weak. Therefore, three factors in response to AAer changes in the recent decade cause elevated summer SAT and change in temperature extremes: (1) increased solar radiation reaching the surface because of aerosols-radiation interaction due to the reduction of AAer precursor emissions, (2) further enhanced surface solar radiation by the reduction in cloud cover through aerosol-cloud interaction (e.g., Twomey 1977; Rosenfeld et al. 2008), and land–atmosphere interaction (e.g., Zampieri et al. 2009; Jaeger and Seneviratne 2011; Mueller and Seneviratne 2012; Stegehuis et al. 2012; Cattiaux et al. 2015; Whan et al. 2015), and (3) less energy going toward changing the phase of soil water from liquid to vapor due to soil drying and more energy going toward increasing the air temperature through increased upward sensible heat flux (Boé and Terray 2014; Miralles et al. 2014; Cattiaux et al. 2015; Whan et al. 2015). However, the magnitude of this positive SW CRE (Fig. 12g) on surface SW radiation change in our model experiments is about 1/3 of the clear sky SW radiation change (Fig. 12b minus 12g), highlighting the dominant role of aerosol-radiation interactions for the summer mean warming and changes in temperature extremes over Western Europe. Similar conclusions were reached based on observations for the role of aerosols on recent solar brightening over Europe (e.g., Ruckstuhl et al. 2008, 2010).
The analysis of observations indicates that there was a rapid increase in summer mean surface air temperature since the mid-1990s over Western Europe. The area averaged summer mean SAT change over the region during the period 1996–2011 increased by 0.93 to 1.10 °C relative to the period 1964–1993 for the four surface temperature data sets.
Accompanied by this rapid summer mean surface warming are rapid changes in temperature extremes. The changes in temperature extremes are characterized by a rapid increase in summer mean Tmax, Tmin, TXx (1.2 °C) and TNx (1.0 °C). The change in DTR is small. Observations also show a rapid increase in frequency of summer days and tropical nights by 8 and 3 days respectively.
The HadGEM3A model response to changes in SST/SIE, anthropogenic GHG, and AAer forcings reproduce many of observed changes in summer mean SAT and temperature extremes over Western Europe. The area averaged changes in SAT (1.16 °C) and temperature extremes are quantitatively similar to observed changes except that the model overestimates the increase in DTR (Fig. 9a).
Responses to different forcing factors indicate a significant role for the direct impact (in addition to an impact via SST/SIE changes) of changes in anthropogenic forcings, especially aerosols, in the rapid warming of European summer temperatures. The drying of the land surface and reduction of cloud cover exert important positive feedbacks. Changes in SST/SIE explain 62.2 ± 13.0 % of the area averaged seasonal mean warming signal over Western Europe, with the remaining 37.8 ± 13.6 % signal explained by the direct impact of changes in anthropogenic GHGs and AAer.
The direct impact of changes in AAer is a key factor for the increases in TXx and frequency of summer days. It explains 45.5 ± 17.6 % and 40.9 ± 18.4 % of area averaged signals for these temperature extremes.
The direct impact of AAer changes act to increase DTR, but change in DTR is countered by direct impact of GHG forcing.
Whilst each forcing factor causes summer mean surface warming and associated temperature extreme changes over Western Europe, the spatial patterns of responses and physical processes are distinct in each case (Fig. 13). For example, SST/SIE changes lead to more or less uniform summer mean warming at the surface. In contrast, changes in AAer lead to a band of surface warming and temperature extreme changes in latitude of 40°N–55°N. AAer changes also lead to an increase in summer blocking frequency by 18 % (Figure not shown), which in turn may have an impact on surface warming and increases in temperature extremes. The results in this study illustrate the important role of the direct impact of changes in AAer not only on summer mean temperature but also on temperature extremes. Reduction of AAer precursor emissions not only induces increased downward solar radiation through aerosol-radiation and aerosol-cloud interactions, but also induces local positive feedbacks between surface warming and reduced cloud cover, increased blocking frequency, reduced precipitation, soil moisture, and evaporation (Fig. 13). These changes occur on very short time scales of weeks (e.g., Dong et al. 2015) and are important processes leading to the local increases in DTR, TXx, and frequency of summer days (Fig. 13). The change in cloud, in turn, is closely related to the change in the hydrological cycle.
In the next few decades, greenhouse gas concentrations will continue to rise and anthropogenic aerosol precursor emissions over Europe and North America will continue to decline. Our results suggest that the changes in seasonal mean SAT and temperature extremes over Western Europe since the mid-1990s are most likely to be sustained or amplified in the near term, unless other factors intervene.
This work was supported by EUCLEIA project funded by the European Union’s Seventh Framework Programme [FP7/2007-2013] under Grant Agreement No. 607085. BD, RTS, and LS are supported by the U.K. National Centre for Atmospheric Science-Climate (NCAS-Climate) at the University of Reading. UD, GPCP, GISTEMP, NCEP/NCAR reanalysis is provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.esrl.noaa.gov/psd/. CRUTS3.21 data are available from the British Atmospheric Data Centre from the site http://badc.nerc.ac.uk/browse/badc/cru/data/cru_ts/cru_ts_3.21/data/. HadISST, HadSLP2, and HadCRUT4 data sets are available from http://www.metoffice.gov.uk/hadobs/. E-OBS gridded data set is available from http://www.ecad.eu/download/ensembles/ensembles.php. Authors would like to thank Laura Wilcox for comments and suggestions on the early version of the paper. The authors would also like to thank two anonymous reviewers for their constructive comments and suggestions on the paper.
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