A comparison of glacier melt on debris-covered glaciers in the northern and southern Caucasus

摘要

The glacier coverage in the Caucasus Mountains underwent considerablechanges during the last decades. In some regions, the observed reduction inglacier area is comparable to those in the European Alps and the extent ofsupra-glacial debris increased on many glaciers. Only a few glaciers in theCaucasus are monitored on a regular basis, while for most areas nocontinuous field measurements are available. In this study, regionaldifferences of the conditions for glacier melt with a special focus ondebris covered glacier tongues in the well-studied Adyl-su basin on thenorthern slope of the Caucasus Mountains (Russia) is compared with theZopkhito basin which has similar characteristics but is located on thesouthern slope in Georgia. The paper focuses on the effect of supra-glacialdebris cover on glacier summer melt. There are systematic differences in thedistribution and increase of the debris cover on the glaciers of the twobasins. In the Adyl-su basin an extensive debris cover on the glaciertongues is common, however, only those glacier tongues that are positionedat the lowest elevations in the Zopkhito basin show a considerable extent ofsupra-glacial debris. The observed increase in debris cover is considerablystronger in the north. Field experiments show that thermal resistance of thedebris cover in both basins is somewhat higher than in other glaciatedregions of the world, but there is also a significant difference between thetwo regions. A simple ablation model accounting for the effect of debriscover on ice melt shows that melt rates are considerably higher in thenorthern basin despite a wider debris distribution. This difference betweenthe two regions can be attributed to different meteorological conditionswhich are characterised by more frequent cloud cover and precipitation inthe south. Furthermore ablation is strongly influenced by the occurrence ofsupra-glacial debris cover in both basins, reducing the total amount of melton the studied glaciers by about 25 %. This effect mitigates glacierretreat in the lower sectors of the ablation zones considerably. Thesensitivity to moderate changes in the debris cover, however, is rathersmall which implies only gradual changes of the melt regime due to debriscover dynamics during the near future.