El Nino-Southern Oscillation (ENSO) influence on a Sajama volcano glacier (Bolivia) from 1963 to 1998 as seen from Landsat data and aerial photography

Sajama volcano, located in the Bolivian Altiplano, is the southernmost trop ical glacier and, owing to its situation, approximately 100 km east of the Pacific coast, is well suited to study the El Nino-Southern Oscillation phe nomenon. Landsat data from 1972 to 1998 and a 1963 aerial photograph are us ed to monitor the snow line fluctuations on a selected part of Sajama volca no. We assume that a few months after the rainy season, the snow line is re presentative of the previous rainy season, if no recent snowfall has occurr ed. By observing precipitation from the stations surrounding Sajama volcano and by verifying snow presence on surrounding summits, we detect images wi th recent snowfall likely to disturb the climatic significance of the snow line. A snow line evolution model takes into account the different image ac quisition dates and adjusts the snow line elevation accordingly for the mid dle of the dry season. A progressive rise of the snow line elevation is obs erved from 1963 to 1998 with a sustained rise from 1984 to 1990. The snow l ine altitude is related to the Southern Oscillation Index. Even after the h igh precipitation of the 1996-1997 wet season, the following El Nino 1997-1 998 leads to a substantial rise of the snow line. The snow line elevation i s related primarily to the total rainy season precipitation and to a lesser degree to the maximum monthly mean temperature of the warmest month, thus confirming a greater snow line sensitivity to precipitation than to tempera ture.