Dynamique spatio-temporelle d’un événement météo-climatique extrême

La canicule de l’été 2003 en Europe de l’ouest

Spatio-temporal dynamic of an extreme meteorological-climatic event The scorching heat of the summer 2003 in western Europe

Laboratoire Géophen, U.M.R 6554 C.N.R.S., LETG, U.F.R. de Géographie, Université de Caen Basse-Normandie

^* olivier.cantat@unicaen.fr

Résumé

Durant l’été 2003, l’Europe occidentale a connu une période de canicule sans précédent. L’article démontre l’ampleur de cet événement météo-climatique, tant dans son intensité que dans sa durée et son extension spatiale. Les cartes d’altitude et les radiosondages verticaux permettent ensuite d’appréhender la genèse et la dynamique spatio-temporelle tridimensionnelle du phénomène. Dans le cadre d’une approche systémique, la prise en compte simultanée des événements se déroulant en altitude et dans les basses couches de l’atmosphère démontre la complexité de processus en constante interaction. L’analyse prouve que la canicule de 2003 trouve son origine en altitude mais que son caractère exceptionnel est imputable à des phénomènes d’interface air-sol. Des éléments sur l’amélioration attendue de la prévision des phénomènes météorologiques extrêmes concluent cet article.

Abstract

During summer 2003, western Europe knew a period of unprecedented scorching heat since reliable meteorological statements exist (end of the 19^th century). From the analysis of the temperatures in three European capitals (Lisbon, Paris and London) and of a regional study more deepened on France, the article demonstrates the dimension of this meteorological-climatic event, both in its intensity and in its duration and its spatial extension. The charts of altitude and the vertical radiosonde investigations of the air allow to understand the genesis of this phenomenon and to consider its spatio-temporal dynamic in the three dimensions of the atmosphere. Within the framework of a systemic approach, the simultaneous study of the events proceeding in altitude and in the low layers of the atmosphere shows the complexity of process in constant interaction. In our case, the analysis proves that the heat wave finds its origin in altitude but that its intensity is ascribable with phenomena of air-to-ground interface. In the end of July 2003, a phenomenon of atmospheric divergence appeared on Europe, what induced the development of the scorching heat on the ground by a vertical movement falling of the air (compression) and a horizontal movement (tropical advection). In altitude, the rough rise of the temperatures and the frank dilation of the air mass preceded by two days the forming of the scorching heat in the low layers of the atmosphere. This enchainement of facts demonstrates the dynamic origin of the phenomenon and not the simple consequence of clear and quiet weather on the temperatures to the interface in summer. Then, the high dynamic pressures settled on Europe constituted a durable dam in passages of the Atlantic atmospheric disturbances, what allowed the scorching heat to become more marked in a very unusual way with the middle latitudes, particularly for the western frontage of the continent usually under maritime influence. The discussion gives some elements on the awaited improvement of the forecasts concerning the extreme phenomena.