Glacier Mass Balance and Climate Research in the Tropical Cordillera Blanca, Peru.

Most recent discussions propose humidity changes as major reasons for sudden climate jumps. Tropical high mountain environments, and their glaciers in particular, are considered to be very sensitive indicators. Against this background, and based on the applicants theoretical considerations, mass budget investigations on tropical glaciers are proposed. The negligible thermal seasonality leads to a glacier mass budget regime which is markedly different from that in the mid-latitudes. In the Tropics, ablation occurs during the entire year, whereas accumulation is linked to the position of the Inter Tropical Convergence Zone (ITCZ) and the respective precipitation seasons. The persistently humid Inner Tropics can be distinguished from the Outer Tropics that are characterized by one humid and one dry season. There, the mass balance during the humid season is dominated by accumulation, sensible heat flux and long-wave radiation, whereas the short wave radiation and the latent heat flux become leading variables during the dry season. Thus, eventual climate changes affect the glaciers differently in each season. The Cordillera Blanca in Peru is assigned to the Outer Tropics and is by far the most ice covered Tropical mountain range with 722km 2 of glacier surfaces. It reaches elevations far above 6000m a.s.l. and stretches from north to south, being a massive barrier to the predominant easterly currents of moist air. Thus, in addition to the seasonal variations, a marked spatial difference between the windward, and therefore humid eastern slopes, and the pronounced dry western side is offered there within a short horizontal distance. Therefore, glacier mass balance investigations in the Cordillera Blanca promise to be a highly interesting tool for understanding Tropical and global climate and its fluctuations. In addition to temperature related interpretations, detailed information on variations of air humidity and related parameters can be expected from glaciological investigations. Glacier fluctuations derived from palaeoglaciological evidence can be evaluated and the results from the proposed project can contribute to the calibration of climate models. The mass balance of the Cordillera Blanca glaciers will be determined on 2 to 4 index glaciers by use of a method which is adapted to the prevailing circumstances. The usually preferred direct glaciological method is unsuitable for the determination of the accumulation due to the absence of annual impermeable layers in the accumulation areas (no thermal seasonality) and, in addition, the inaccessibility of the accumulation areas (steepness, crevasses, ice falls). Therefore, the mean accumulation will be determined from the mass flux through the cross section below the equilibrium line. The necessary balance velocity on the surface will be measured from stake observations, the cross section with geodetic techniques and ablation with an adequate stake network. In addition to the field measurements, the suitability of interferometric analyses of SAR data from tropical glaciers will be tested. These analyses will give fields of motion, which can be used for both the calculation of the mass flux of the index glaciers and the extrapolation of the results to other glaciers. Four automatic weather stations will provide the climatological data necessary for the investigation.

The mass balance of a glacier is the link between climate, the glacier extent, and the glaciers contribution to runoff. Understanding the processes of gain and loss of snow and ice masses on a glacier opens possibilities to reconstruct climate from glacier history and to provide tools for planning regional water management. Between 1999 and 2003, members of the Innsbruck Tropical Glaciology Group, Institute of Geography, Innsbruck University have investigated the effects of climate on glaciers and melt water runoff in the tropical Cordillera Blanca, Peru. Contrary to the mid latitudes, glacier regimes in the tropics are not dominated by seasonal temperature variations but by a more or less pronounced seasonality of precipitation, cloudiness and several other variables related to atmospheric moisture. A dry summer day on an Alpine glacier is like any day during the dry season on a tropical glacier and an Alpine wet day corresponds to tropical humid conditions. Glaciers in the tropics are highly sensitive to both changes in atmospheric temperature and moisture content. In the Cordillera Blanca the hygric seasonality is strongly related to the large scale atmospheric circulation which carries moisture from different source regions in the tropical Atlantic. These circulation patterns show a general seasonal pattern, but vary in their intensity as well as in their duration from year to year. We have developed a model which allows us to analyse glacier history in terms of both temperature and humidity changes. Applied to several glaciers and over longer time scales we expect findings and understanding of variations and changes of the tropical atmospheric circulation. First results show that the strongest modern time glacier retreat in the Cordillera Blanca, which occurred during the 1930s and 1940s, was due to markedly dry conditions. The missing of temperature seasons gives also reason to the missing of seasonal snow cover outside the glaciers. Thus, runoff and therefore water availability is almost only from glacier melt during the dry seasons and, of course, the amounts of water depend on the degree of glacierization. Obviously, water supply is essentially affected by glacier fluctuations. Modelling glacier runoff will help to support water management policy in regions where water demand reaches or exceeds availability during dry seasons in many cases. Because of this last aspect, the project was considered appropriate to be refunded by the Jubiläumsfonds of the Oestereichische Nationalbank (OeNB) for its potential to contribute to a sustainable development of the study region, socially and economically.