Comparative Study On Spatiotemporal Variations Of Dust Aerosols And Their Effects On The Regional Monsoons In Asia And Africa

Dust aerosol sources are widely distributed in the northern hemisphere,mainly in arid and semi-arid regions in north Africa,west Asia,central Asia,south Asia and east Asia.Dust aerosols can affect the local and even global climate through direct and indirect effects.Although there have been many studies on the change of dust aerosols in specific source regions,there is still a lack of systematic analysis on the spatiotemporal variation characteristics and their causes,furthermore,the climate effects of dust aerosol in different source regions.Using the MERRA-2 reanalysis dataset from1980 to 2017,this study compared and analyzed the seasonal and interannual variations of dust aerosols over the middle and low latitudes of Asia and Africa,and explored the reasons for these variations.The direct radiative effects(DRE)of dust aerosols on west African summer monsoon,East Asian summer monsoon and South Asian summer monsoon were studied by using the ocean-atmosphere coupled and uncoupled simulation results of the fourth version Community Atmospheric Model(CAM4-BAM)with dust aerosol parameterization module.The main conclusions are as follows:(1)The seasonal variation of dust aerosols in the mid-latitude source region is different from that in the low-latitude source region.The deserts in African and Asian can be divided into mid-latitude and low-latitude sources.The mid-latitude sources include the east Asia deserts(EAD)and the central Asia deserts(CAD),which the dust emission fluxes usually reach their peak values in spring,accounting for 37% and 33%of the annual dust emissions,respectively.The low latitude sources include the north African deserts(NAD),the west Asian deserts(WAD)and the south Asian deserts(SAD),which the dust aerosol fluxes reach their peak values in summer,accounting for33%,37% and 41% of the annual dust emissions,respectively.The seasonal and interannual variations of dust aerosols in different sources are mainly affected by the increases of vertical wind shear in the lower troposphere.In addition,the seasonal variation of aerosol column density in the central Asian desert is also affected by soil moisture and snow depth.The interannual variations are mainly negatively correlated with soil moisture in central Asian deserts(R=-0.65).(2)Analyses of the model simulation results show that the influence of dust aerosols on the summer monsoon are vary from place to place.The DRE of dust aerosols over north African deserts can enhance the general circulation and precipitation of west African summer monsoon(WASM),while the DRE of dust aerosols over east Asian deserts can weaken the circulation and precipitation of east Asian summer monsoon(EASM).The dust DRE of the west Asian deserts on the early season of the south Asian summer monsoon(SASM)is different from that in the late monsoon: the dust DRE of west Asian deserts strengthens SASM in the early monsoon season(MayJune),but weakens it in the late monsoon season(July-August).(3)The responses of WASM,EASM and SASM to dust DRE are not exactly the same.The dust DRE can modulate the summer monsoon systems through by directly heating the atmosphere(fast response)and cooling the sea surface(slow response).the responses of the WASM and EASM to the dust DRE are more prominent when the ocean-atmosphere coupled processes are added,the distinct effects of dust aerosols on the WASM and EASM are resulted from the relative locations of the cooling ocean regions compared to the corresponding monsoon regions: the cooling ocean is upwind of the WASM region but downwind of the EASM region.This cooling ocean causes a south-north pressure gradient in North Africa,which enhances the westerly winds of the WASM.For the EASM,the cooling region generates an east-west pressure gradient,which strengthens the northerly winds and weakens the EASM.The fast response of SASM to the WAD dust DRE dominates its enhancement during the early monsoon season,while the slow response dominates its weakening during the late monsoon season: In the early monsoon season,the fast response of the SASM due to the dust DRE heats the lower troposphere over the WAD,strengthens the southwesterly flows over the Arabian Sea and provides abundant water vapor supply for the land thus increases the precipitation in the monsoon region.However,for the late monsoon season,more dust aerosols are transported to the Arabian Sea and the slow response of the SASM to the dust DRE is enhanced,which cools the sea surface and the lower troposphere over the Arabian Sea,then creates an anomaly high with the easterly winds,inhibits the moisture transport from ocean to land and weakens the precipitation.