Simulated Precipitation Changes In Western Central Asia Since The Last Glacial Maximum

Western Central Asia is one of the arid regions in the world,with limited freshwater source and very fragile ecological environment.Understanding the changing features and possible driving mechanisms of precipitation in the arid western Asia since the Last Glacial Maximum(LGM)will lay a solid foundation for predicting the climate change and evolution of the environment in the region.In this study,we explore precipitation changes in western Central Asia(35 °-50 ° N,50 °-70 ° E)since the Last Glacial Maximum and their response to external forcings based on model results from the ?Simulation of Transient Climate Evolution over the last 21,000 years?(Tra CE-21ka).The primary conclusions are as follows:(1)Transient climate simulations with Tra CE-FULL reveal that a wet climate occurred during the LGM,the B?lling-Aller?d warm period,and the middle Holocene,and a dry climate occurred during Heinrich event 1,the Younger Dryas,and the early Holocene in western Central Asia.Comparing the Tra CE-FULL simulation results with the proxy records,it is found that the Tra CE-FULL simulation results are basically the same as the proxy records except for the LGM period.(2)By comparing and analyzing the results of Tra CE-FULL and four single forcing simulations,it is found that the continental ice sheet and the discharge of meltwater were the main causes of the high rainfall during the LGM.From H1 to the YD,meltwater fluxes dominated precipitation changes in western Central Asia.During the Holocene,the main factors affecting precipitation changes were meltwater forcing and orbital forcing;Since the LGM,the contribution of GHG concentrations to precipitation changes was minor.In addition,on the basis of above analysis,this paper also mainly analyzes the possible physical mechanisms that affect the precipitation change under different forcing tests.When considering only orbital forcing(Tra CE-ORB),the result indicates that the annual precipitation in the western Central Asia depends mainly on the amount of spring precipitation.Spring precipitation is positively correlated with the mid-low latitudes(20 °-30 ° N,20W-30E)zonal wind at 850 h Pa.The negative phase of NAO can strengthen the westerly intensity in the mid-low latitudes of the North Atlantic Ocean,thus increasing the water vapor transported by the tropical Atlantic Ocean to the western Central Asia,leading to the high precipitation in the western Central Asia.So the precipitation is negatively correlated with the NAO index.In addition,orbitally-induced anticyclonic over the Arabian Sea further brought moisture from the Arabian Sea towards western Central Asia,also increasing the precipitation in western Central Asia.The results of the Tra CE-MWF simulation show that the discharge of meltwater can reduce the salinity of sea surface water and then weakened the strength of the AMOC,thereby lowering the temperature of the sea surface water,resulting in a positive phase NAO,while the positive phase NAO is not conducive to precipitation in the western Central Asia.On the other hand,the weakend AMOC also caused cooling over western Central Asia.The cooling further induced an increase in the SLP associating with more descending motions over western Central Asia,leading to a reduction of precipitation in this area.In addition,the decrease in temperature has weakened the evaporation in the upstream of the westerly winds,reducing the amount of water vapor transported to the west of Central Asia,thus reducing precipitation in western Central Asia.Precipitation changes in the Tra CE-ICE simulation were mainly related to the huge icesheets.The existence of the Northern Hemisphere huge ice sheets played an important role in the strength of the Icelandic low via the orographic effect.Icelandic low can strengthen the strength of the westerlies in the North Atlantic,which significantly increased the water vapor transport towards Central Asia and ultimately strengthened precipitation changes in this area.The orographic effect would have remarkably decreased when the ice sheets reduced to a certain elevation(e.g.,1500 m).Hence,precipitation only experienced significant changes at 14 ka BP because the North European ice sheet was basically disappeared after 14 ka BP.