Abstract

Historic mudflows in Antofagasta, Chile, and their relationship to the El Nino/Southern Oscillation events. The coastal zone of the Atacama Desert is submitted to an extremely arid climate, characterized, in Antofagasta, by mean annual rainfall of 4 mm (1904-1998). However, the sporadic occurrence of heavy rainfall, together with the geomorphologic situation of the city, may produce debris and mud flows (' aluviones '). The occurrence of alluvial flows during the 20(th) century was investigated through the study of newspapers (available from 1916), instrumental data of precipitation (from 1904) and the observation of alluvial deposits which include anthropic remains (principally after 1900). The relationship between these evidences of alluvial activity with the occurrence of El Nino events was examined th rough a comparison between historical data, SOI (Southern Oscillation Index) data, and rainfall data in Antofagasta. Between 1916 and 1999, the city was affected by alluvial events in seven oportunities: 1925, 1930, twice in 1940, 1982, 1987 and 1991, with the most important episodes in 1940 and 1991. in ail the cases, the rains occurred during the winter period of the developpement phase of El Nino (ENOS) events (very strong to moderate intensity), and were associated to frontal systems coming from higher latitudes, which also struck a major part of northen Chile. The convective character of the rainstorms is the cause of the great spatial zonification in the precipitations, within the same rainfall event. The comparison of annual rainfall data in Antofagasta with tendencies, at both regional and global scales, of the air temperature and the sea surface temperature, shows a coincidence between periods with more important precipitations and particular conditions, at an interdecadal scale, of the ocean-atmosphere global system. During the 20(th) century, alluvial episodes in the coastal area of northen Chile were coeval with periods during which a systematic increase of regional and global anomalies of the temperature of the air, and positive anomalies of the sea surface temperature were observed (between 1925 and 1942 -or 1947-, and from 1977 onwards).