Anomalous heat and mass budget of Glaciar Zongo, Bolivia, during the 1997/98 El Nino year

During El Nino-Southern Oscillation (ENSO) warm events, outer tropics glaciers usually experience a deficit of precipitation, an increase of air temperature and a strongly negative mass balance. At Glaciar Zongo, Bolivia, this was particularly striking during the vigorous 1997/98 El Nino event, one of the strongest of the century, and which resulted in an annual depth of runoff two-thirds higher than normal. We compare the energy balance on the glacier between two contrasting cycles, 1996/97 (La Nina year) and 1997/98 (El Nino year). Due to a 1.3 degreesC increase of annual mean air temperature, the sensible-heat flux slightly increases from 6.1 to 9.8 W m(-2). During the El Nino year, sublimation is reduced, leaving more energy for melting (LE = -18.1 W m(-2) in 1996/97 and LE = -11.6 W m(-2) in 1997/98). The main factor responsible for the dramatic increase in melting is the net all-wave radiation, which is three times higher in 1997198 than in 1996/97 (48.7 and 15.8 W m(-2), respectively). This sharp increase of net all-wave radiation is related to the decrease of albedo due to the precipitation deficit.