Track changes as supplement

Abstract

The thermal regime of glaciers and ice caps represents the internal distribution of ice temperatures. Accurate knowledge of the thermal regime is important to understand the dynamics and response of ice masses to climate change, and to model their evolution. Although the assumption is that most ice masses in the Tien Shan are polythermal, this has not been examined in appropriate detail so far. In this research, we investigate the thermal regime of the Grigoriev ice cap and the Sary-Tor glacier, both located in the Inner Tien Shan in Kyrgyzstan, using a 3D higher-order thermomechanical ice flow model. Input data and boundary conditions are inferred from a surface energy mass balance model, a historical air temperature and precipitation series, ice thickness measurements and reconstructions, and digital elevation models. Calibration and validation of the englacial temperatures is performed using historical borehole measurements on the Grigoriev ice cap and radar measurements for the Sary-Tor glacier. The results of this study reveal a polythermal structure of the Sary-Tor glacier and a cold structure of the Grigoriev ice cap. The difference is related to the larger amount of snow (insulation) and superimposed ice (release of latent heat) for the Sary-Tor glacier resulting in higher ice surface temperatures, especially in the accumulation area, which are subsequently advected downstream. Further, ice velocities are much lower for the Grigoriev ice cap with consequent lower horizontal advection rates. A detailed analysis concerning the influence of temperature and precipitation changes at the surface reveals that the thermal structure of both ice bodies is not a constant over time, with recent climate change causing more temperate ice in higher areas. The selected ice masses are representative examples of the (Inner) Tien Shan glaciers and ice caps. Therefore, our findings and the calibrated parameters can be generalised allowing to improve the understanding of the dynamics and future evolution of other glaciers and ice caps in the region.