Summer heat waves

Recent summer heat waves had strong socio-economic impacts in different parts of Europe. The record-breaking 2003 heatwave increased mortality by around 35,000 heat-related deaths across Europe and resulted in large financial losses due to crop shortfall and forest fires. These recent heat waves highlight the importance of a detailed understanding of the key processes and feedbacks contributing to such extreme events. Heat waves are generally associated with quasi-stationary anticyclonic circulation anomalies. These anticyclonic anomalies produce subsidence, clear skies, warm-air advection, and prolonged hot conditions at the surface.

However, several recent studies have highlighted the role of land–atmosphere interactions in addition to circulation patterns for the occurrence of heat waves. Most of the recent European summer heat waves have been preceded by a pronounced precipitation deficit in the antecedent spring. The lack of precipitation and the associated depletion of soil moisture result in reduced latent cooling and thereby amplify the summer temperature extremes (Fig. 1). In order to quantify the contribution of land-atmosphere feedbacks, we conducted regional climate simulations with and without land-atmosphere coupling for four selected major summer heat waves in 1976, 1994, 2003, and 2005.

For each event, two simulations were performed using the regional climate model CHRM: One simulation with interactively coupled land-atmosphere processes and another simulation where the soil moisture evolution at each time-step was prescribed by the climatological mean field. To ensure an optimal representation of the continental-scale atmospheric circulation, all simulations were driven with lateral boundary conditions and sea-surface temperatures from the ERA-40 reanalysis (for 1976 and 1994) and from ECMWF operational analysis (2003 and 2005).

The evaluation of the experiments reveals that land-atmosphere interactions play an important role for the evolution of the investigated heat waves. During all simulated events soil moisture-temperature feedbacks increase the heat wave duration and account for typically 50-80% of the number of hot summer days (Fig. 2). The largest impact is found for daily maximum temperatures during heat wave episodes. Furthermore, drought conditions are revealed to influence the tropospheric circulation by producing a surface heat low and enhanced ridging in the mid-troposphere, suggesting a positive feedback mechanism between land surface, continental-scale circulation and surface temperature.

Fischer, E. M., S. I. Seneviratne, D. Lüthi, and C. Schär (2007), Contribution of land-atmosphere coupling to recent European summer heat waves, Geophys. Res. Lett., 34, L06707, doi:10.1029/2006GL029068. PDF

Fischer E.M., S.I. Seneviratne, P.L. Vidale, D. Lüthi and C. Schär, 2007: Soil moisture - atmosphere interactions during the 2003 European summer heatwave. J. Climate , in press.