Interhemispheric transport into the southern hemisphere polar stratosphere from the Asian monsoon region

Abstract

The South-East Asian boundary layer has become one of the most polluted regions in recent years due to rapideconomic growth, which even affect the trace gas composition in the southern hemisphere by inter-hemispheric transport. Westudy the transport from the boundary layer of the Asian summer monsoon (ASM) region [15◦ N, 45◦ N, 30◦ E, 120◦ E] intothe global upper troposphere and lower stratosphere (UTLS) using the Lagrangian chemistry transport model CLaMS drivenby the ERA5 reanalysis during 2010-2014. In particular, we quantify the inter-hemispheric transport contribution from theASM region to the southern hemisphere polar region (SP) [60◦ S, 90◦ S] and investigate the influence on pollution. Despite thesmaller size of ASM area compared to the southern hemisphere (SH) subtropics [15◦ S, 45◦ S] and tropics [15◦ S, 15◦ N], wefind that the air mass fractions (AMF) from the ASM to the SP are about 1.5 times larger than the corresponding contributionsfrom the SH subtropics and about two times smaller than those from the tropics. Transport from the ASM boundary layer tothe Southern polar vortex occurs largely above about 450 K and on timescales longer than 2 years, while transport timescalesto the Antarctic region below the vortex are shorter than about 2 years. The transport contribution from the ASM region tothe SP presents distinct inter-annual variability, which is strongly related to the strength of polar vortex. The relatively young(less than two years) tracers originating from the ASM region show good correlations with CCl4, F12, and CH3Cl observationsfrom ACE-FTS in the antarctic UTLS. The reconstructed SF6 indicates that about 20% of SF6 in the SP stratosphere originatesfrom the ASM boundary layer, which is larger than the SF6 fraction of SH subtropical origin, while 50% of SF6 in the SPstratosphere originates from the tropical boundary layer.