Computation of the space and time evolution of equilibrium-line altitudes on Andean glaciers (10 degrees N-55 degrees S)

International audienceA previous study of Fox [Fox, A.N. 1993. Snowline altitude and climate at present and during the Last Pleistocene Glacial Maximum in the Central Andes (5 degrees-28 degrees S). Ph.D. Thesis. Cornell University.] showed that for a fixed 0 degrees C isotherm altitude, the equilibrium-line altitude (ELA) of the Peruvian and Bolivian glaciers from 5 to 20 degrees S can be expressed based on a log-normal expression of local mid-annual rainfall amount (P). In order to extrapolate the function to the whole Andes (10 degrees N to 55 degrees S) a local 0 degrees C isotherm altitude is introduced. Two applications of this generalised function are presented. One concerns the space evolution of mean inter-annual ELA for three decades (1961-1990) over the whole South American continent. A high-resolution data set (grid data: 10' for latitude/longitude) of mean monthly air surface temperature and precipitation is used. Mean annual values over the 1961-1990 period were calculated. On each grid element, the mean annual 0 degrees C isotherm altitude is determined from an altitudinal temperature gradient and mean annual temperature (T) at ground level. The 0 'C isotherm altitude is then associated with the annual precipitation amount to compute the ELA. Using computed ELA and the digital terrain elevation model GTOPO30, we determine the extent of the glacierised area in Andean regions under modern climatic conditions. The other application concerns the ELA time evolution on Zongo Glacier (Bolivia), where inter-annual ELA variations are computed from 1995 to 1999. For both applications, the computed values of ELA are in good agreement with those derived from glacier mass balance measurements