Characteristics Of Variation Of Air Transport From The Troposphere To The Stratosphere

Chemical species in the stratosphere,affected by the troposphere-to-stratosphere transport（TST）,play a critical role in protecting the lives on Earth and regulating the tropspheric weather and climate.Therefore,investigations on the long-term trends of troposphere-to-stratosphere transport（TST）are of great significance for understanding the historical variations of the stratospheric composition and atmospheric chemistry and projecting their trends in future.Using observational data,reanalysis data and numerical simulations,this paper investigated the variations of deep convection,Asian summer monsoon anticyclone（ASMA）,and the troposphere-to-stratosphere transport over the tropical western Pacific and their underlying physical mechanisms.The main results are as follows:Firstly,using observations and reanalysis data,this paper studies the long-term trend of lightning activity,a proxy for the intensity of deep convection.The results show that the mean lightning activity in the tropics and subtropics increased insignificantly during 1996-2013,while the lightning activity over the southeast North America,central South America and eastern Australia decreased significantly at rates of-0.26 fl km^-2a^-1,-0.17 fl km^-2a^-1and-0.14 fl km^-2a^-1,respectively.The lightning activity over the northwest South Asia shows a significantly increasing trend at a value of 0.24 fl km^-2a^-1.Further investigations based on the multiple linear regression showed that the decreased lightning activity over the southeast North America,central South America,and eastern Australia is mainly caused by the decreased local convective available potential energy and the increased aerosol loading is responsible for the increased lightning activity over the northwest South Asia.The above results put insights into the understanding of the variations of deep convection and its impact factors and have important implications for the future projection of the deep convection.The overshooting convection in the tropics and subtropics showed weak trends,while the overshooting convection over the northwest South Asia increased significantly during 1998-2013,which suggests an enhanced direct transport from the troposphere to the lower stratosphere by the overshooting convection in the northwest South Asia.The long-term trend of the ASMA and the inner atmospheric composition are also studied.The geopotential height anomalies derived from zonal mean and the stream function showed significantly decreasing trends in the upper troposphere and lower stratosphere over Asia during 1958-2020,while the potential vorticity increased significantly,which suggests a weakened ASMA.At meantime,the CO and water vapor derived from Microwave Limb Sounder（MLS）decreased significantly inside the ASMA,while the MLS observed O₃increased significantly,which provide further evidence for the weakening trend of ASMA in terms of the changes in the atmospheric composition.Results based on the model output from Phase 6 of the Coupled Model Intercomparison（CMIP6）suggested that the increased anthropogenic aerosol loading over the South and East Asia and the decreased anthropogenic aerosol loading over the Europe and Central Asia lead to a decreased meridional gradient of tropospheric air temperature in the low latitudes over Asia and an enhanced meridional gradient of tropospheric air temperature in the mid and high latitudes over Asia.The changed meridional gradient of tropospheric air temperature over Asia further induced a weakened ASMA.The increased greenhouse gases mainly result in a decreased meridional gradient of tropospheric air temperature in the Asia and lead to westerly trends over the whole ASMA region.The above results suggest that the change in the anthropogenic aerosol is a critical impact factor of the change in the ASMA and has important implications for predicting the future changes in ASMA and stratospheric composition.This paper further investigated the trends of TST over the tropical western Pacific in the boreal winter.The results from ERA5,JRA55,and MERRA2 reanalysis data sets show that the upward motion in the upper troposphere over the tropical western Pacific increased significantly.Furthermore,the observed CO from MLS and Measurements of Pollution in the Troposphere（MOPITT）shows significantly increasing trends in the upper troposphere over the tropical western Pacific,which provides further evidence for the increased upward motion in the upper troposphere over the tropical western Pacific.At meantime,the tropical upwelling of the Brewer-Dobson（BD）circulation increased significantly in the past decades in the boreal winter,which makes the air in the tropical tropopause layer enter the stratosphere more easily.Hence,the TST over the tropical western Pacific increased significantly.The simulated CO based on the WACCM4 model shows significantly increasing trends through the troposphere over the tropical western Pacific and in the upper troposphere and lower stratosphere,which further suggests an enhanced TST over the tropical western Pacific.The simulations also suggest that the warmed global sea surface temperature（SST）could lead to an enhanced Pacific Walker circulation and deep convection over the tropical western Pacific and a strengthened upward branch of the BD circulation in the lower stratosphere,implying that the warmed global SST is a critical impact factor of the enhanced TST over the tropical western Pacific.Finally,the impact of the changes in the SST over the different regions on the ASMA and the TST over the tropical western Pacific in the boreal winter is investigated based on the CMIP6 and CESM-LE output data as well as the WACCM4simulations.Results showed that the warmed SSTs over the tropical western Pacific could lead to the increased convective precipitation over the South Asia,southeast Asia,and western Pacific,which results in the changed meridional gradient of tropospheric temperature and then the increased ASMA.The changes in the global SST,especially the SST over the tropical western Pacific in the boreal winter could lead to the strengthened Pacific Walker circulation and deep convection over the tropical western Pacific,which results in the enhanced upward motion in the upper troposphere over the tropical western Pacific,while the warmed SST over the tropical Indian Ocean and tropical eastern Pacific has an opposite effect.At meantime,the warmed SST over the tropical western Pacific could lead to enhanced wave activity in the upper troposphere and lower stratosphere in the subtropics and hence a strengthened tropical upwelling of the BD circulation.The warmed SST over the tropical Indian Ocean could lead to strengthened tropical upwelling of the BD circulation and the warmed SST over the tropical eastern Pacific only result in weak changes in the BD circulation.Therefore,the warming trend of SST over the tropical western Pacific is responsible for the enhanced TST over the tropical western Pacific.The warming trends of SST over the tropical Indian Ocean and eastern Pacific exert a limited impact on the TST over the tropical western Pacific.The above results suggest that the SST over the tropical western Pacific could not only strongly impact the strength of ASMA,but also exert an important impact on the TST over the tropical western Pacific,which suggests an important implication for the predictions for the stratospheric composition.