Le bioclimat mediterraneen: Caracteres generaux, modes de caracterisation

Abstract

This first paper on the Mediterranean bioclimate is devoted to the definition of the mediterranean climate and to its territorial extension. It is noted that all climatologists agree on the essential characteristic of this type of climate: the relative dryness of the summer. The author maintains the definition given by Emberger, implying an estival pluviometric minimum and a biologically dry summer. The problem of the delimitation of the seasons and the presence of a late summer in coastal regions are discussed. A territory around the Mediterranean Sea is delimited in which the summer is the least wet season, separated by a narrow transition zone from another territory in which summer is the wettest season. The degree of dryness of the summer is maintained: Koppen's criterion for a month is dry if it receives less than 30 mm of precipitation. Gaussen's dryness criteria (P+2T), retained by Walter, and those of Emberger-Giacobbe (P/M=S), are equivalent if the maximum value of the latter is taken as 5. These criteria provide a delimitation of the extension of climates exhibiting a dry summer that is closely similar to that based on Koppen's criterion. The simultaneous application of both types of criterion enables a characterization of the extension of the climates in which the summer is both the least wet and a biologically dry season. It appears that the Mediterranean Isoclimatic Area territory thus considered extends far wider than found by western elimatologists. However, the author stresses some agreement with eastern and Arab climatologists. This divergence may be explained by supplementary constraints added to the general definition of the Mediterranean climate: on the whole a moderate dryness and temperate conditions. It is noted how some biologically routine delimitations are erroneous, e.g. climate of the olive tree,Olea europaea, andQuercus ilex, and sclerophyllous evergreen formations. Some authors seem to reduce the mediterranean climate to a coastal azonality, whereas it has been shown that this is one of the great types of climate resulting from general atomospheric circulation. The passage from a typical Mediterranean regime to a summer rain regime is more progressive than the study of seasonal regimes implies. An examination of the types of Mediterranean climate in the A.I.M. follows. Monthly precipitation regimes of part of the stations of the A.I.M. are compared and classified by application of a classification algorithm to the matrix of Kendall's rank correlation coefficients. Two large units are determined, the first comprising four groups and the second five. Global continentality is dealt with by combination of pluvial continentalityC, expressed by Angot's ratio, which is the ratio of precipitation during the six longest months to the precipitation during the six shortest months, together with thermal continentalityK' expressed by a coefficient derived from that of Gorczinski. It is emphasized that a Mediterranean climate is marked by a pluvial continentality < 1, and a thermal continentality withK'/25 ≈C. Thermal forms are defined in accordance with the principles of Trewartha. Aridity is considered successively from several viewpoints. The value of the pluviometric module is discussed without being adopted, as it is quite inadequate. The module of the driest month, combined with the number of dry months, gives good overall results. Characterization by “Klimadiagramm” is analysed, compared with Martonne's index, and reviewed. The method of Thornthwaite is also examined. Finally, Giacobbe's study of arido-humidity is presented in detail; standards for interpretation are given as well as an example. An application of the analysis of the duration of the vegetative pause is given, as well as a measure of the intensity of aridity. Various applications of this method are discussed.