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Large Scale Circulation (LSC) changes around Mediterranean and Europe: a worsening factor affecting hot season shortages of water supply through a regional intensification of global warming.

Plaut Guy

With Global Warming, Extreme Heatwaves will certainly worsen; however several mechanisms may play a role and LSC changes may, a priori, as well reinforce the regional consequences of Global Warming, or reduce them.

During the hottest 2003 and 2006 (except august) summers, characteristic LSC patterns actually repeatedly occured, with a relative Low off Portugal, and High over Spain and (or) western Mediterranean and continental Europe in the lower troposphere. Such patterns contributed to advect hot tropospheric air masses from Maghreb and Sahara, also providing anticyclonic subsidence over western Europe and Mediterranean. Intensified drought resulted, together with increased evapo-transpiration, hydrological stress, and reduced river discharges and water supply.

Here we focus on the recent and future worsening of extreme heatwaves conditions. We first use 60 year long series of daily observed TX temperatures from Météo-France stations in order to quantify the recent increase of TXQ90, the 90th percentile of each summer TX. By analogy with the so-called Weather Regimes, we call Heatwave Regime the LSC pattern most responsible for intense heatwaves. We show that the increase of TXQ90 originates both from Global Warming and from LSC changes.

GCM cannot directly simulate local station conditions. However a strong correlation exists, for each station, between any summer TXQ90, and the corresponding T850Q90, the 90th percentile of the reanalysed 850 hPa temperature above the station. GCM actually simulate 850 hPa temperatures; we characterize the end of the 21st century intensification of extreme heatwaves comparing maps of simulated T850Q90 for 1960-1990 and 2070-2100. With model simulations at our disposal, the future rise of TXQ90 also appears to receive a substantial contribution from the increasing frequency of Heatwave Regime-like LSC pattern occurence, the same mechanism already at work during last decades.