Tropical glaciers are known to be very sensitive climatic indicators. To link the fluctuations of their mass baIance to climate changes, we have realised a surface energy balance on Zongo Glacier (16°S), Bolivia, between march 96 and august 98. This work presents firstly, the methods used to derive the local energy balance on this glacier, and secondly, the interest of such a study to better understand the functioning of the glacier and its relation with the local climate. In the first part, a detailed description of the local climate is made from the records of several automatic meteorological stations installed on Zongo Glacier. Tropical glaciers are submitted to air temperature and incident solar radiation very little variable throughout the year, although the discharge of the proglacial stream escaping from the glacier is highly seasonal. To understand which energy fluxes are responsible for the seasonality of the surface melting on the glacier, we have calculated the energy balance on a point of the ablation area, at 5150 m. The energy balance shows a special functioning of tropical glaciers. Like for the glaciers of higher latitudes, the albedo is a very important parameter controlling the amount of energy available for surface melting. But the main peculiarity of these glaciers comes from the strong consumption of energy as latent heat flux, which corresponds to high sublimation. Moreover, the latent heat flux is very variable with seasons which explains the seasonality of the proglacial stream discharge. Indeed, during the dry season, sublimation is the highest and therefore, very little energy remains for melting explaining why discharge is low. The reverse situation is observed during the humid season. On the time scale of the entire measuring period, the very negative mass balance of the 1997-98 El Nino year is mainly explained by a deficit of precipitation, responsible for a sharp decrease of the mean annuai albedo. The reduced snow cover of the 1997-98 accumulation season has disappeared faster than during the previous year, leaving dirty ice surfaces of very low albedo on larger areas than usually, and during a longer period. Moreover, higher sensible heat flux and lower sublimation have both contributed to increase the surface melting which also reduced glacier mass balance.
