Impact of soil moisture on extreme maximum temperatures in Europe

Abstract

Land-atmosphere interactions play an important role for hot temperature extremes in Europe. Dry soils may amplify such extremes through feedbacks with evapotranspiration. While previous observational studies generally focused on the relationship between precipitation deficits and the number of hot days, we investigate here the influence of soil moisture (SM) on summer monthly maximum temperatures (TXx) using water balance model-based SM estimates (driven with observations) and temperature observations. Generalized extreme value distributions are fitted to TXx using SM as a covariate. We identify a negative relationship between SM and TXx, whereby a 100 mm decrease in model-based SM is associated with a 1.6 °C increase in TXx in southern-central and southeastern Europe. Dry SM conditions result in a 2-4 °C increase in the 20-year return value of TXx compared to wet conditions in these two regions. In contrast with SM impacts on the number of hot days (NHD), where low and high surface-moisture conditions lead to different variability, we find a mostly linear dependency of the 20-year return value on surface moisture conditions. We attribute this difference to the non-linear relationship between TXx and NHD that stems from the threshold-based calculation of NHD. Furthermore the employed SM data and the standardized precipitation index (SPI) are only weakly correlated in the investigated regions, highlighting the importance of evapotranspiration and runoff for resulting SM. Finally, in a case study for the hot 2003 summer we illustrate that if 2003 spring conditions in southern-central Europe had been as dry as in the more recent 2011 event, temperature extremes in summer would have been higher by about 1 °C, further enhancing the already extreme conditions which prevailed in that year

Understanding the rapid summer warming and changes in temperature extremes since the mid-1990s over Western Europe

“…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).…”

“…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 T max , TX x , and the frequency in summer days as well as an enhancement of DTR (Fig. 9b).…”

“…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 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.…”

“…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 landatmosphere 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;DellaMarta et al 2007;Carril et al 2008;Feudale and Shukla 2011a, b;Kamae et al 2014).…”

Drought stress impact on vegetable crop yields in the Elbe River lowland between 1961 and 2014

“…The combination of high temperatures and droughts initiate a positive regional feedback mechanism: the precipitation deficits and enhanced evaporative demand generally associated with warm spells (e.g. atmospheric blockings) triggers soil moisture deficit, thus suppressing evaporative cooling (Teuling et al, 2013) and leading to hotter and drier conditions if soil moisture becomes limiting for evapotranspiration (Whan et al, 2015). Trnka et al (2015b) concluded that increased global radiation and air temperature together with decreased relative humidity led to increase of reference evapotranspiration in all month of the growing season (particularly in April, May and August) over the CR.…”