CLIMATIC INTERPRETATION FROM MOUNTAIN GLACIATIONS IN CENTRAL ASIA

Glaciers advance or retreat in response to changes in precipitation, summer temperatures, insolation and cloudiness. Therefore, the extent of ancient glaciers in Central Asia can be used to understand some aspects of its paleoclimate. The timing of the local maximum advances and their equilibrium-line altitudes exhibit strong spatial structure. We recognize three main zones of glaciers in Central Asia: 1) a western zone, from the Karakoram north to the Kyrgyz Tien Shan, and east to central Tibet; 2) a northern zone, from the Xinjiang Tien Shan to central Mongolia, and north to southern Siberia; and 3) eastern Tibet. In zone 1, the maximum glacier advances occurred early in the ice-age cycle (~110-14 ka) as determined by ¹⁰Be dating: ~110-90, 63-45, and 30 ka. "Last Glacial Maximum" (~20 ka) glaciers were little larger than modern glaciers. In zone 2, the LGM glaciers were as large or larger than the older glaciers. In zone 3, Holocene glaciers were nearly as large as the Pleistocene glaciers. We interpret these patterns in terms of the climatic factors affecting glacier growth.

These regional variations in glacial history must reflect regional differences in the evolution of the controlling climate factors. Owing to the location of Central Asia, the climate there is relatively straightforward, and represents a continental `end member' of the system. (Modern precipitation is mainly from spring and fall low-pressure systems guided by the jet stream. Strong thermal high-pressure cells over Siberia during the winter limit the number of storms in the winter months. Summertime temperatures are largely radiatively determined, and the extreme orography of the region plays an important role in shielding the area from the monsoons). Despite its simplicity, the interannual and decadal climate variability there has received scant attention. Using the relevant regional climate indicators for glaciers (e.g., positive degree days, annual precipitation), we employ composite analysis of atmospheric reanalysis data to explore how circulation regimes in the current climate give rise to regional variability. Using these results and the available suite of global climate model simulations for glacial climates, we aim to construct self-consistent climate scenarios which can be reconciled with the glacial history of the region.