Impacts Of Tibetan Plateau On Extreme Weather And Climate In Surrounding Regions

As the highest plateau in the world,the Tibetan Plateau(TP)has significant impacts on the weather and climate in Asia and globally through thermal and dynamical effects.The TP can change the vertical circulation through thermal effects,extending the anomalous signal of TP to a wider region.Simultaneously,due to the dynamic effects of the TP,the westerly and monsoon circulation will be affected,which in turn affects the regional weather and climate.The TP is located in the arid belt at mid-latitudes of the Northern Hemisphere and at the crossroads of several dust sources.The thermal and dynamical effects of TP play a key role in the dust weather occurrence in arid regions.Meanwhile,the dusts transported from surrounding sources to TP can change the energy budget on the TP by generating radiation effects,which in turn affects the TP heat source.Moreover,the TP is a sensitive area in the global warming,the warming rate of TP is significantly higher than the global average,inducing the changes in the TP heat source consequently.Therefore,understanding the role of TP is a key scientific issue in understanding the mechanism of extreme weather and climate in China and Asia.To address the above scientific issues,this paper combines multi-sourced data and numerical simulations to investigate the impacts of TP on two types of extreme events,including dust weather and heat wave,in the surrounding areas,and reveals the mechanisms of TP influencing dust weather and heat waves in surrounding regions.This study can provide a basis for the attribution and prediction of extreme weather and climate changes in the surrounding regions.The main conclusions are as follows:(1)The spatial and temporal distributions of heat source over the TP,dust and heat waves in surrounding regions are clarified.The atmospheric heat source(AHS)over the TP is positive from May to September and negative from October to the following April.The AHS over the TP shows an increasing trend in summer during 1980～2020,with a distribution feather showing stronger over the eastern TP than that over the western TP and stronger over the southern TP than that over the northern TP;while a weakening trend in winter.The sensible heat and latent heat fluxes in summer on the TP are equivalent,the sensible heat fluxes on the western and northern TP are stronger than those on the eastern and southern TP,with decreasing trends during 1980～2020;while the latent heat fluxes are stronger on the southeastern TP,with an increasing trend.The highest dust AODs are found over the north of TP in spring and on the south of TP in summer,with increasing trends of dust AODs in all seasons around TP.The areas with higher variations of dust AODs over the north and south of TP are closer to TP in summer.A large amount of dust accumulates over the north and south slopes of TP in spring and summer,and some dust can be transported to the TP,and the transport of dust from Tarim Basin to TP increases in summer.The areas with more heat wave days during summer in China are mainly distributed in Northwest China,Eastern China,the Sichuan Basin and the Guanzhong Basin during1980～2020,with increasing trends in the number of heat wave days,especially in the middle and lower reaches of the Yangtze River,the eastern Tarim Basin and the Sichuan Basin.The western and central TP also become high value areas of heat wave days under the 90% quantile definition.(2)The impact of TP topography on dust weather and consequent effect on energy budget are revealed.Due to the blocking of TP topography,strong upslope winds form in the southwestern Taklamakan Desert on the north of TP,transporting dust to the upper atmosphere.Dust from the Thar Desert on the southwest of TP climbs along the southern slope of TP through the “heat pump effect” of TP,and then lifts up to the upper atmosphere.The existence of TP causes the “narrow pipe effect”in the Hexi Corridor on the northeast of TP,which enhances the dust activity in the Gobi Desert.The TP topography influences dust emission by adjusting atmospheric circulation to change surface wind speed,moving the dust emission area in the Taklamakan Desert closer to TP and enhancing the dust emission in the Thar and Gobi deserts.Dust aerosols can affect the surface net radiation on the north of TP,which in turn affects other components of the earth-atmosphere energy exchange.The area with stronger dust emission has stronger surface net radiation.The surface net radiation decreases during dust transport.The effects of dust aerosols on surface net radiation and sensible heat fluxes are consistent,while those on latent heat fluxes are relatively small and those on soil heat fluxes are opposite to those on sensible heat fluxes.(3)The mechanisms of AHS over TP affecting heat waves in China are found.The stronger AHS over the TP in summer causes anomalous ascending motions to trigger anomalous anticyclones over the northeast of TP,and the geopotential height anomalies are shown as a latitudinal wave train pattern at mid-latitudes,which makes the West Pacific Subtropical High and South Asian High stronger and closer through teleconnection,leading to descending motion dominating the Eastern China and making heat waves frequent in this region.On the other hand,the anomalies of AHS over TP can trigger northwest-southeast wave trains,the stronger vertical diabatic heating and weaker temperature advection over the TP lead to weaker westerlies at 200 h Pa.The weakened mid-and low-latitude westerlies favor the occurrence and maintenance of anticyclones which are located in the north of TP.The downslope topography of TP and the northern slope of Tianshan further intensifies the descending motions generated by anticyclones and triggers heat waves in the Northwest China.(4)The impact of AHS over TP on heat waves in China is quantified and the future changes are projected.After turning off the vertical diabatic heating over the large topography of the TP and adjacent regions,the summer AHS over TP is weakened by 136.62%;the number of heat wave days in Eastern China is reduced by63.44%;and the number of heat wave days changes from the increasing trend to the decreasing trend during 1979～2014;the average decrease of heat wave temperature reaches 1.2°C;no heat wave is simulated in Northwest China.The simulations of CMIP6 models for heat wave days in Northwest China are better than those in Eastern China.The future changes in the number of heat wave days in Northwest China during 2015～2100 are smaller than those in Eastern China,but the changes in the average temperature of heat waves are larger than those in Eastern China.In the future scenarios,with the enhanced emissions,the summer AHS over the TP will strengthens,and the number of heat wave days and heat wave temperatures in China will further increase,the relationship between the two will be closer.