Pierre Ribstein

Coherent isotope history of Andean ice cores over the last century

[1]xa0Isotope records from Andean ice cores provide detailed and high-resolution climate information on various time scales. However, the relationship between these valuable isotope records and local or regional climate remains poorly understood. Here we present results from two new drillings in Bolivia, from the Illimani and the Sajama ice caps. All four high altitude isotope signals in the Andes now available (Huascaran, Quelccaya, Illimani and Sajama) show near identical decadal variability in the 20th century. Comparison with general circulation model results and meteorological data suggest that the Andean high altitude records are primarily controlled by precipitation variability over the Amazon basin.

A new Andean deep ice core from Nevado Illimani (6350 m), Bolivia

Abstract A new ice core record from the Nevado Illimani (16°S), Bolivia, covers approximately the last 18u2008000 years BP. A comparison with two published ice records, from Sajama (18°S), Bolivia [Thompson et al., Science 282 (1998) 1858–1864] and Huascaran (9°S), Peru [Thompson et al., Science 269 (1996) 46–50], documents a regionally coherent transition from glacial to modern climate conditions in South America north of 20°S. The strong resemblance between the Illimani and Huascaran water isotope records and their differences from the Sajama record, in particular during the period from 9000 years BP to 14u2008000 years BP, suggest that local water recycling or local circulation changes played a major role for Sajama. We interpret the common Illimani/Huascaran water isotope history in terms of a common change from wetter/cooler conditions during glacial times to drier/warmer conditions in the Early Holocene.

Solid precipitation on a tropical glacier in Bolivia measured with an ultrasonic depth gauge

[1]xa0An ultrasonic depth gauge was used to measure snowfall over a 2-year period near the equilibrium line of the Zongo glacier (2.4 km2), Bolivia (16°S). Study of the influence of wind, air temperature, and air moisture on the measurements gives a quantification of snowfall at a 3-hour time step, with a sensitivity of 1 cm of snow. The density of fresh snow is estimated by comparison with rain gauge measurements. The year is marked by a dry season from May to August and a wet season from December to April, during which accumulation and melting coincide on the glacier. Snowfall events are associated with a wind of moderate speed from the valley (less than 4 m s−1). Masses of moist air originate in the Amazon basin. The orographic effect produces precipitation at midday in the Andean valleys and in the afternoon in the high mountains. Nighttime snowfall events occur during periods of bad weather related to the regional atmospheric circulation and last several days. The density of fresh snow is high, about 250 kg m−3, because of the high air temperature during snowfall events (over −3°C). The high snow density and the moderate wind speeds prevent snow drifting conditions, which results in low spatial variability of the accumulation on tropical glaciers. Accurate recording of snowfall at a short time step is important for the study of energy fluxes at the glacier surface because snowfall events greatly increase the albedo and solar radiation is generally the main source of melting energy.

Glaciers of the Tropical Andes: Indicators of Global Climate Variability

Over the last decade, mass balance has been monitored on several glaciers of the tropical Andes by the Institute of Research for Development (IRD, France) in collaboration with South American partners. This network includes glaciers in the Cordillera Real of Bolivia, Zongo and Chacaltaya (16°S), glaciers in the Cordillera Blanca of Peru, Yanamarey and Artezonraju (9°S), and glaciers in the eastern and western cordilleras of Ecuador, Antizana (0°28’S) and Carihuayrazo (1°S) (Fig. 1). Some of these have been listed as benchmark glaciers by the Word Glacier Monitoring Service (WGMS 2001), and the data are accessible to the scientific community. This network is designed to capture the effects of climate change, and especially ENSO variability, both in the outer (Bolivia, Peru) and the inner (Ecuador) tropical Andes. Glaciers have been selected to be representative of the regional glacierization. Each monitoring programme includes two glaciers, a large one (1 km2 or more) with a substantial accumulation zone, and a small one that is more directly sensitive to ablation processes. Information about the long-term evolution of some of these glaciers has been extracted from aerial photographs, available for the last five decades (Francou et al. 2000; Ramirez et al. 2001). The particular nature of climate in the Tropics allows ablation to occur at anytime throughout the year in the lowest part of glaciers. Thus, the ablation zone has been surveyed in monthly intervals at several sites, providing interesting details about the seasonal response of tropical glaciers (Francou et al. 2003).