Glacier-Climate Interaction in the Low Latitude Andes.

The particular sensitivity of low latitude glaciers is, in addition to air temperature variations, due to processes related to air humidity. The decoding of the glacier-climate interaction will result in a highly informative tool for the understanding of tropical and subtropical climate and its fluctuations as well as for the understanding of past, present, and future glacier runoff scenarios. A glacier mass balance model, in first parts presented by Kaser (2001), meets the requirements of low latitude glacier and climate regimes. In these climate regimes, duration and pronouncement of humid and dry periods dominate the seasonality while atmospheric thermal conditions are homogenous. The aim of the proposed research project is the calibration and validation of the mentioned low latitude glacier mass balance model as well as its application for climatic and hydrological studies. The test side chosen is the Shallap Glacier in the Peruvian Cordillera Blanca (835 - 10 S) with 4.5 km2 spanning from 4,450 to 5,990 m a.s.l. Data to be recorded by an automatic weather station beside the glacier as well as high frequency detailed information taken from the glacier itself are the basis for scaling procedures, aiming to adapt several variables of reanalysed atmospheric data (NCEP-NCAR) as parameters for the model input. For validation the seasonal mass balance of the entire glacier will be determined by applying the glaciological method. Model results will be compared to known surface area and length variations of several glaciers, and to mass balance series reconstructed from hydrological data. It is further intended to apply the tested model for the climatological interpretation of former ice extents of Shallap Glacier and other glaciers in the Cordillera Blanca, and to examine the models ability to calculate the seasonal variation of glacier runoff.

Glaciers in the low latitudes are very sensitive to variations in duration and intensity of wet seasons. These variations, in turn, are linked to large scale oceanic and atmospheric patterns. If properly "calibrated", tropical glaciers can tell of changes in climate in its full complexity and in a variety of scales. Particularly in the tropical Andes, glaciers provide the only notable storage in the regional seasonal water cycle since no seasonal snow cover develops outside the glaciers. During the dry season when almost no or no precipitation occurs for months, glacier runoff is crucial for the availability of water for social and ecosystems in the mountain valleys but also in the big cities along the Pacific coast. When glaciers shrink under changing climate, serious shortcomings in freshwater availability can lead to ecological changes, economic problems and social conflicts. Both the climate change indication role of tropical glaciers and their role as water suppliers have been addressed in the FWF Project. Model tools were developed, calibrated and applied to study the climate processes that govern the mass balance of the glacier and that allow to simulate runoff scenarios. Necessary input was obtained from field measurements that we carried out mainly on Glaciar Artesonraju in the Cordillera Blanca, Peru. Besides perturbations in air temperature it is mainly the atmospheric moisture content that drives mass gain and losses on the glacier surface. The moisture content and its variations impact on a variety of energy and mass balance terms such as the cloudiness that shields the ice from solar radiation, the sublimation that consumes high amounts of energy but removes only little mass when atmospheric conditions are dry, the reflectivity of the ice against solar radiation, and the occurrence of precipitation. Driven by the atmospheric moisture content glaciers accumulate during the wet season when also melting is strongest. In turn, the mass turnover is reduced during the dry season. Intermediate months show a considerable inter-annual variety and can be crucial to the annual mass balance. The variability of seasonal moisture content is in high correlation with sea surface temperature distribution in the tropical Pacific. Our model tools allow for connecting large scale atmospheric conditions - such as provided by Global Climate Models - with processes that control glacier mass balance and melt water production. Modelled runoff from glacierized catchments is in good agreement with measured runoff. Consequently, we performed model runs with future climate scenarios that show a decrease in dry season runoff up to 40 % from present day conditions until 2080. Additionally, runoff will increase up to 48 % during the wet season, seriously increasing the risk of floods and landslides.