CLIMATIC MEDIATION OF MOISTURE SOURCES ON THE SOUTHERN EDGE OF SIBERIA

Mountain glaciations in arid Central Asia advance and retreat in spatially variable and temporally idiosyncratic patterns relative to those in the Alps or Sierra Nevada. This variability has been attributed to changes in paleo-precipitation causing local shifts in the balance between melting and sublimation, which control glacier ablation: sublimation may dominate in cold, hyperarid (<~200 mm/yr) settings such as occur today in parts of High Asia. This variability is superimposed on regional changes in accumulation and variations in melting related to temperature fluctuations in warmer and less arid areas. Paradoxically, during the intensely cold global Last Glacial Maximum (LGM), some mountain ranges in Central Asia were only lightly glaciated because they were precipitation-starved, whereas nearby regions may have had large glaciers and much lower (>500 m) Equilibrium-Line Altitudes. These ELA patterns are especially noticeable in northern Central Asia and on the southern border of Siberia.

But what controls the paleo-precipitation there? We propose that the Fenno-Scandian Ice Sheet (FSIS) exerted strong control over moisture delivery to Siberia and Central Asia via two mechanisms, and changes in duration of sea ice in the Arctic Ocean north of Siberia together with seasonal melting of Siberian permafrost also exerted control. Today, westerlies split around the 8000-m Pamir, some delivering moisture to the southern front of the Himalaya and others to the Tien Shan and northern Central Asia. At the maximum southern extent of the FSIS the jet stream may have moved south, reducing the moisture delivered to northern Central Asia. Additionally, the FSIS blocked and reorganized drainages, increasing LGM flow into the Black and Caspian seas and the area of water available for advection east. NOAA’s HYSPLIT model shows that today summer precipitation can be delivered to northern Central Asia from the Arctic Ocean, a pathway that would be reduced or eliminated with lower summer temperatures in the LGM. Similarly, summer melting of ground ice in Siberia today may also have enhanced summer precipitation to the south and would not have during the LGM. Understanding the response of mountain glaciers in Central Asia to these forcing mechanisms helps to characterize paleo conditions in Europe and NW Asia.