Investigation Of The Origin,Maintenance,Variability Of The Asian Tropopause Aerosol Layer(ATAL):The Roles Of Asian Summer Monsoon

The upper troposphere lower stratosphere(UTLS),situated between 200-50 hPa pressure levels,is an important region connecting the troposphere and the stratosphere.While the earth's climate change is mostly measured in terms of conditions of the troposphere and at the earth surface,water vapor,ozone and other chemical constituents in the UTLS are known to have important impacts on climate at the earth surface and human health.In recent years,Venier et al.discovered from NASA satellite observations the existence of an extensive Asian Tropopause Aerosol Layer(ATAL)between 13-18 km above the earth surface over a vast region spanning,from the Middle East to Eastern Asia during the boreal summer monsoon season.The discovery of the ATAL has sparked much interests in research on its composition,origin and relationships to the transport processes of atmospheric constituents in the upper troposphere and lower stratosphere(UTLS)and the variability of the Asian Monsoon Anticyclone(AMA).In this research,based on analysis of MERRA2 reanalysis data,as well as MLS and CALIPSO satellite data,we present results showing that the mechanism of AT AL formation and variation of transportation of aerosols(carbonaceous aerosols,dust)and chemical gas(CO)originated from the earth surface due to the seasonal variation,intra-seasonal oscillation and interannual variation of Asian summer monsoon.Case studies were carried out with GEOS5 in different scenarios.Results from simulations of UTLS transportation were also used for further investigating the interaction in "Aerosol-Monsoon system' and climate effect.This study will provide scientific basis for understanding the characteristic and formation mechanism of ATAL in Asian Monsoon area and have significant practical value in coming aerosol and monsoon research.Following are the main results of this study.(1)Using NASA MERRA2 daily data,we investigated the origin,maintenance and variability of the ATAL in relation to variations of the AMA during the summer of 2008.During May-June,abundant quantities of carbon monoxide(CO),carbonaceous aerosols(CA)and dusts are found in the mid-and upper troposphere over India and China,arising from enhanced biomass burning emissions,as well as westerly transport from the Middle East deserts.During July-August,large quantities of dusts transported from the deserts are trapped and accumulate over the southern and eastern foothills of the Tibetan Plateau.Despite strong precipitation washout,ambient CO,CA and dust are lofted by orographically forced deep convection to great elevations,12-16 km above sea level,via two key pathways over heavily polluted regions:a)the Himalayas-Gangetic Plain,and b)the Sichuan Basin.Upon entering the upper-troposphere-lower-stratosphere,the pollutants are capped by a stable layer near the tropopause,advected and dispersed by the anticyclonic circulation of AMA,forming the ATAL resembling a planetary-scale "double-stem chimney cloud".The development and variability of the ATAL are strongly linked to the seasonal march and intra-seasonal(20-30 days and higher frequency)oscillations of the Asian monsoon.The monsoon variabilities and process of UTLS transport experience 4-5 cycles in the whole ASM season.(2)We have investigated the interannual variability and the decadal trend of carbon monoxide(CO),carbonaceous aerosols(CA),and mineral dust in the ATAL in relation to varying strengths of the South Asian summer monsoon(SASM)using MERRA2 reanalysis data(2001-2015).Results show that during this period,the aforementioned ATAL constituents exhibit strong interannual variability and rising trends connected to the variations of the strength of SASM.During strong monsoon years,the AMA is more expansive and shifted northward compared to weak years.In spite of effect of quenching of biomass burning emissions of CO and CA by increased precipitation,as well as the removal of CA and dust by increased washout from the surface to mid-troposphere in monsoon regions,all three constituents are found to be more abundant in an elongated accumulation zone at ATAL,on the southern flank of the expanded AMA.Enhanced transport to the ATAL by overshooting deep convection is found over preferred pathways along the foothills of the Himalayan-Gangetic Plain,and the Sichuan Basin.The long-term positive trends of ATAL CO and CA are robust,while ATAL dust trend is weak due to its large interannual variability.The ATAL trends are associated with increasing strength of the AMA,with earlier and enhanced vertical transport of AT AT constituents by enhanced overshooting convection over the HGP and SB regions,out-weighing the strong reduction of CA and dust from surface to the mid-troposphere.(3)The responses of ASM climate and UTLS transport process to radiative forcing of total aerosol over the ASM region are studied using the updated NASA Goddard Earth Observing System(version 5)-Atmospheric General Circulation Model(GEOS5 AGCM).Simulations were carried out first with all aerosols as the control(CTRL),and second with all aerosol radiation disabled(NRF).It was found that the climate effect of aerosols has significant impact on both monsoon dynamics and formation of the ATAL in the boreal summer.We find evidence of an "elevated heat pump"(EHP)mechanism that underlines the responses of the UTLS transport to carbonaceous and dust aerosols forcing as follow.During the pre-monsoon season,Shortwave radiation absorption by aerosols warms the atmosphere and cools the surface,while longwave has the opposite response.The elevated aerosol layer warms the air over West Africa and the Middle East area.As the warm air rises,it spawns a large-scale onshore flow carrying the moist air from the Indian Ocean to the India Peninsula.As a result of large-scale atmospheric feedback triggered by absorbing aerosols,rainfall and cloudiness are enhanced over the central and northern India,and suppressed over the southeastern Asia and eastern China.During the peak monsoon season,a more vast and stronger AMA can be found in the UTLS region.Due to the enhancement of precipitation,convections over both of the Himalaya-Gangetic plain pathway and Sichuan basin pathway were strongthened.For carbonaceous aerosol,the ambient concentration is seen increased in the mid-and upper troposphere,but decreased near surface because of the washout effect from monsoon rainfall.For mineral dust,it is seen seriously suppressed in the transport pathways since dust is sensitive to the rainout effect and horizontal transportation decreases with the weakening of westlie during peak monsoon.