Abstract
Abstract. In this study, we have investigated the interannual variability and the decadal trend of carbon monoxide (CO), carbonaceous aerosols (CA) and mineral dust in the Asian Tropopause Aerosol Layer (ATAL) in relation to varying strengths of the South Asian summer monsoon (SASM) using MERRA-2 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 Asian monsoon anticyclone (AMA) is more expansive and shifted northward compared to weak years. In spite of the 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 the mid-troposphere in monsoon regions, all three constituents are found to be more abundant in an elongated accumulation zone in the ATAL, on the southern flank of the expanded AMA. Enhanced transport to the ATAL by overshooting deep convection is found over preferred pathways in the Himalayan-Gangetic Plain (HGP) and the Sichuan Basin (SB). The long-term positive trends of ATAL CO and CA are robust, while the 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 ATAL constituents by enhanced overshooting convection over the HGP and SB regions, outweighing the strong reduction of CA and dust from the surface to the mid-troposphere.
Highlights
This trend has been validated by observational data (Fig. S1 in the Supplement), and a similar increasing decadal trend of the South Asian summer monsoon (SASM) has been reported in previous studies (Jin and Wang, 2017)
We have investigated the roles of monsoon physical processes in the interannual variability and longterm change of Asian Tropopause Aerosol Layer (ATAL) gaseous and aerosol species, i.e., carbon monoxide (CO), carbonaceous aerosol (CA) and dust using 15 years (2001– 2015) of NASA MERRA-2 reanalysis data
A monsoon index based on areal mean rainfall over the South Asia summer monsoon (SASM) region shows strong interannual variability and a robust long-term trend
Summary
The discovery from satellite lidar observations of the Asian Tropopause Aerosol Layer (ATAL) – a planetary-scale aerosol layer situated 13–18 km above sea level, spanning vast regions from the Middle East, south and east Asia to the western Pacific during the Asian summer monsoon (ASM) – has spurred active research on the composition (H2O, chemical gaseous and aerosol species) and the relationship between the ATAL and the Asian monsoon anticyclone (AMA), and climate change (Fadnavis et al, 2013; Lelieveld et al, 2018; Li et al, 2005; Randel and Park, 2006; Randel et al, 2010; Thomason and Vernier, 2013; Vernier et al, 2011, 2015, 2017; Yu et al, 2015). Other studies reported that the ASM system can act as a conduit for these chemicals and aerosols convectively transported to the UTLS region (Bergman et al, 2013, 2015; Bourassa et al, 2012; Garny and Randel, 2016). Recent results from lidar observations, high-altitude balloon sounding data and model simulations have shown a relatively higher concentration of chemicals and aerosols in the UTLS during the boreal summer, indicating effective vertical transport by the ASM Pan et al (2016) found that CO can be lifted into the UTLS by deep convection over the southern flank of the Tibetan Plateau (TP) during boreal summer, and suggested that the dynamics of monsoon subseasonal variability may play an important role in this. Physical processes linking the ATAL and ASM on interannual and longer timescales are still unknown
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
