Rapport quadriennal 1995-1998

The high sensibility and the short lag time response of glaciers to climate change make them relevant climatic indicators in the Tropics. Climatic signals lead to variation of the glacier mass balance calculated by the difference between the accumulation terms (precipitation) and the ablation terms (melting and sublimation). The discharge of the pro-glacial stream integrates most of the reaction of the glacier in response to climate variability. Here we present the main results of a research program developed since 1991 in the Andes. This paper is based on the data obtained on Zongo Glacier, Cordillera Real of Bolivia (16°S, 68°W). The 2.1 km2 glacier is part of a 3 km2 basin above the main hydrometric station. The hydrological regime is analysed at different time scales and compared to the mass balance measurements and to the energetical fluxes close to the Equilibrium Line Altitude. The accumulation season, between October and March, coincides with the period of higher ablation. During this humid season, melt rates at the glacier surface lead to high discharges of the stream escaping from the glacier snout. Net all-wave radiation is the main source of energy at the glacier surface and shows strong fluctuations in relation to the highly variable albedo. An important peculiarity of tropical glaciers is the negative latent heat flux indicating strong sublimation, particularly during the dry season. The El Nino-Southern Oscillation (ENSO) events have a great influence on tropical glaciers, as shown during the last 1997-98 ENSO event. The main factor responsible for the dramatic melting of the glacier during the last ENSO warm event is the deficit of precipitation which reduces the accumulation and causes a low albedo on a large part of the glacier. Since 1973, monthly discharges from Zongo Glacier have been calculated, on the base of two daily readings at a staff gauge downstream the main hydrometric station... (D'après résumé d'auteur