Thermal regime of the Grigoriev ice cap and the Sary-Tor glacier in the Inner Tien Shan, Kyrgyzstan.

The thermal regime of glaciers and ice caps represents the internal distribution of ice temperatures. Accurate knowledge of the thermal regime is crucial to understand the dynamics and response of ice masses to climate change, and to model their evolution. The ice temperature for example strongly controls the plasticity and the deformation rate of the ice with higher temperatures encouraging movement, and whether a glacier can slide over its base. Although the assumption is that most ice masses in the Inner 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. A 3D thermo-mechanical higher-order model is applied. Input data and boundary conditions are inferred from a surface energy mass balance model, a historical air temperature and precipitation series, ice thickness reconstructions, and digital elevation models. Calibration and validation of the englacial temperatures is performed using historical borehole measurements on the Grigoriev ice cap, radar measurements for the Sary-Tor glacier and temperature measurements on other glaciers in the area. 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 surface temperatures, especially in the accumulation area, which are subsequently advected downstream. Further, ice velocities are much lower for the Grigoriev ice cap compared to the Sary-Tor glacier with consequent lower advection rates. Since the selected ice masses are representative examples of the (Inner) Tien Shan glaciers and ice caps, our findings can be generalised allowing this to improve the understanding of the dynamics and future evolution of the studied ice masses as well as other glaciers and ice caps in the region.