Transport of trace gases via eddy shedding from the Asian summer monsoon anticyclone and associated impacts on ozone heating rates

Abstract

Abstract. The highly vibrant Asian summer monsoon (ASM) anticyclone plays an important role in efficient transport of Asian tropospheric air masses to the extratropical upper troposphere and lower stratosphere (UTLS). In this paper, we demonstrate long-range transport of Asian trace gases via eddy-shedding events using MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) satellite observations, ERA-Interim reanalysis data and the ECHAM5–HAMMOZ global chemistry-climate model. Model simulations and observations consistently show that Asian boundary layer trace gases are lifted to UTLS altitudes in the monsoon anticyclone and are further transported horizontally eastward and westward by eddies detached from the anticyclone. We present an event of eddy shedding during 1–8 July 2003 and discuss a 1995–2016 climatology of eddy-shedding events. Our analysis indicates that eddies detached from the anticyclone contribute to the transport of Asian trace gases away from the Asian region to the western Pacific (20–30∘ N, 120–150∘ E) and western Africa (20–30∘ N, 0–30∘ E). Over the last two decades, the estimated frequency of occurrence of eddy-shedding events is ∼68 % towards western Africa and ∼25 % towards the western Pacific. Model sensitivity experiments considering a 10 % reduction in Asian emissions of non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NOx) were performed with ECHAM5–HAMMOZ to understand the impact of Asian emissions on the UTLS. The model simulations show that transport of Asian emissions due to eddy shedding significantly affects the chemical composition of the upper troposphere (∼100–400 hPa) and lower stratosphere (∼100–80 hPa) over western Africa and the western Pacific. The 10 % reduction of NMVOCs and NOx Asian emissions leads to decreases in peroxyacetyl nitrate (PAN) (2 %–10 % near 200–80 hPa), ozone (1 %–4.5 % near ∼150 hPa) and ozone heating rates (0.001–0.004 K day−1 near 300–150 hPa) in the upper troposphere over western Africa and the western Pacific.