Variabilité des régimes pluviométriques dans le nord-ouest de l’Argentine : problèmes posés et analyse durant la deuxième moitié du vingtième siècle

Variability of the precipitation regimes in the North-West of Argentina : some questions and analyses during the second part of the twentieth century

¹ COSTEL LETG, UMR 6554 / FR 2116 CAREN-CNRS, Université Rennes-2
² DYMSET UMR 5064-CNRS, Université Bordeaux-3

^* olivier.planchon@uhb.fr
^** karine_rosier@yahoo.fr

Résumé

La variabilité climatique, notamment pluviométrique, a été étudiée dans le Nord-Ouest de l’Argentine durant la période 1951-2000. À une variabilité interannuelle d’autant plus forte que l’on se rapproche du piémont andin semi-aride et semi-aride (Mendoza, San Juán), s’ajoutent des oscillations de plus longue durée, en particulier une succession d’années anormalement arrosées à partir des années 1970. L’étude comparée de la période la plus arrosée (1971-1990) et de la période précédente, plus sèche (1951-1970), montre que l’augmentation des précipitations a été très variable d’une station à une autre, et l’analyse sur l’ensemble du vingtième siècle ne montre pas de lien significatif avec l’oscillation australe. Cependant, l’apparition plus fréquente de types de climats annuels plus humides, en particulier pour certaines stations situées en limite du domaine marqué par l’aridité (Santiago del estero), soulève le problème des conséquences économiques, humaines et environnementales d’un retour éventuel à une période moins arrosée.

Abstract

The climatic variability was studied in north-western Argentina for the period 1951-2000. The rainfall variability was especially analysed. North-western Argentina is a warm-temperate and subtropical area, which supplies agricultural produces not cultivated elsewhere in the country. In the subtropical latitudes of South America, the annual precipitation decreases between the wet Atlantic coast (1000-2000 mm / year, without dry period) and the Pacific coast (Chile) almost without precipitation. The regional climatic divisions are essentially defined by the summer rainfall amount and by the duration of the summer rainy period. The Andes cordillera, the sub-Andean ranges and the Sierras Pampeanas cause an more complicated spatial distribution of precipitation. The sub-Andean ranges are exposed to the humid deviated trade winds blowing from the north-east and locally receive more than 1000 mm / year. The Province of Tucumán is called the “garden of the Argentine Republic”, because of its varied agricultural produces. Further to the south, the Cuyo area (Provinces of Mendoza and San Juán) is subjected to an arid climate (100-200 mm / year), but vineyards and orchards have been developed with irrigation.

The inter-annual rainfall variability increases between the Pampa plain and the sub-Andean arid basins, but the northern humid subtropical sub-Andean slopes have more regular rainfall amounts. The climatic characteristics of north-western Argentina and the climatic annual characteristics of selected reference meteorological stations (precipitation regimes and frequency of the “annual climatic types”) were shown by using the W. Köppen’s classification. A period of abnormally rainy years was recorded in north-western Argentina between 1971 and 1990. The comparison between the rainy period 1971-1990 and the drier period 1951-1970 showed that the increase in precipitation was subjected to strong differences between the available meteorological stations. The analysis of the precipitation anomalies between 1901 and 2000 did not show significant correlations with the southern oscillation index. An other serie of rainy years occurred between 1917 and 1922, but the seventies - eighties rainy period was the longest of the twentieth century. Since the year 1991-1992, this rainy period is perhaps already ended in the area of Salta. The occurrence of several years with high positive precipitation anomalies during the period 1971-1990 was well shown for stations located on the spatial fringe of the arid and semi-arid areas. In these areas, the expansion and intensification of the agriculture benefited from a period of several years with higher rainfall amounts. Therefore, a return of years with drier climatic conditions could cause serious economic, human and environmental problems.