| Mineral dust,mainly from desert areas,is an important component of tropospheric aerosols.Asia is one of the major source regions of global mineral dust.The Asian atmospheric circulation system is important for the dust transport,while dust aerosols influence the global and regional climate system through changing the energy balance of the ground-air system.Previous studies on the interaction between dust aerosols and atmospheric circulation have mostly focused on North Africa and South Asia,but there is a lack of studies on East Asia.In this study,we use Regional Climate Model(Reg CM4)to simulate the changes in the East Asian dust cycle under present conditions,assessing the effects of East Asia Monsoon and Westerly jet on dust transport.In addition,we use Column Atmosphere Model(CAM4)to evaluate the influence of the Present Day and Last Glacial Maximum snow-darkening effect and direct effects of dust on seasonal changes in coupled monsoon westerlies over East Asia.The main conclusions obtained are as follows:(1)The dust at the upper level is mainly transported by the westerly circulation,while that of the lower layer is mainly transported by the East Asia Winter Monsoon.In March,the westerly jet is located on the south side of the Tibet Plateau and the highlevel dust aerosol is transmitted eastward to the northern Pacific.Low-level dust is transmitted to the southeastern China with the influence of East Asia Winter Monsoon.With the northward shift of the westerly jet,the control range of the westerly winds increases in May and their correlations are weakened.In contrary,the impact of East Asia Winter Monsoon on the lower layer dust is enhanced.The effect of East Asia Winter Monsoon is sensitive to the dust particle sizes.Under the action of East Asia Winter Monsoon,fine-grained dust is transmitted far away,while coarse-grained dust is limited to the vicinity of the source area.Once the dust is carried to the westerly layer,the influence of westerly winds on the transmission of different particle sizes dust is similar.(2)The snow-darkening forcing of dust causes anomalies in temperature,precipitation,and atmospheric circulation over East Asia.In December and January(DJ),the temperature increase over the Tibetan Plateau is significant,and the anomalous anticyclonic circulation causes a weakening of the westerly circulation during this period.In February,March,and April(FMA),the snow-darkening forcing of dust causes an increase in temperature over the plateau and its eastern flank and widespread anomalous cooling to the north,which can be attributed to diabatic cooling of the upper atmosphere.These changes in thermal conditions lead to enhanced westerly winds and weakened East Asian winter monsoons during FMA.The enhanced westerly winds bring more precipitation to the East Asian continent.In May,June,and July(MJJ),only the Tibetan Plateau is significantly warmed in East Asia.At this time,the westerly circulation weakens,and the East Asian summer monsoon strengthens.The low-level atmospheric circulation shows anomalous westerly winds over South Asia.The anomalous westerly winds from Central Asia bring dry air and result in reduced precipitation over India.(3)Compared to present day,more dust and snow cover in LGM allows heavier dust deposition and snowmelt.This dust-in-snow forcing leads to an annually averaged northern hemisphere warming of 0.31°C,with the strongest in boreal spring and summer.Spatially,the prominent warming centers are found around LGM ice sheets and Tibetan Plateau,up to 2-3°C annually,which significantly contributes to the melt of ice and triggers the termination of glaciation.Over Asia,the present dust-in-snow forcing weakens the Indian summer monsoon although not significant.However,strong LGM forcing does not amplify the weakening but intensifies the Indian and East Asian monsoon and precipitation.The reverse effect is mainly ascribed to newly emerged source areas and dust deposition in snow over northern Central Asia during LGM.Thus,the potential role of dust and its interaction with snow cover should be mentioned in the glacial-scale climate change.(4)The direct radiative effect of dust at present day leads to lower temperatures in East Asia during the DJ.In FMA,the temperature decreases in the Tibetan Plateau and Central Asia and increases in the rest of East Asia.During the MJJ,the temperature decreases in the Tibetan Plateau and India and increases in the Siberian Plain.In addition,the direct radiative effect of dust enhanced westerly circulation in DJ and weakened East Asia winter monsoon.The direct radiative effect during the LGM period causes an increase in westerly circulation in spring.This forcing on the monsoon system is consistent with the modern dust direct radiative effect of modern dust,causing a weakening of the East Asian summer monsoon.The surface temperature decreases during spring and summer,mainly due to the decrease of short-wave radiation reaching the Tibetan Plateau surface.In contrast to spring and summer,surface temperature of the Tibetan Plateau abnormally increases in autumn and winter. |
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