Abstract
<p>Inter-hemispheric transport may strongly affect the trace gas composition of the atmosphere, especially in relation to anthropogenic emissions which originate mainly in the Northern Hemisphere. This study investigates the transport from the boundary surface layer of the Northern Hemispheric (NH) extratropics (30-90<sup>o</sup>N), Southern Hemispheric (SH) extratropics (30-90<sup>o</sup>S), and tropics (30<sup>o</sup>S-30<sup>o</sup>N) into the global upper troposphere and lower stratosphere (UTLS) using simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS). In particular, we diagnose inter-hemispheric transport in terms of the air mass fractions (AMF), age spectra, and the mean age of air (AoA) calculated for these three source regions. We find that the AMFs from the NH extratropics to the UTLS are about five times larger than the corresponding contributions from the SH extratropics and almost twenty times smaller than those from the tropics. The amplitude of the AMF seasonal variability originating from the NH extratropics is comparable to that from the tropics. The NH and SH extratropics age spectra show much stronger seasonality compared to the seasonality of the tropical age spectra. The transit time of NH extratropical origin air to the SH extratropics is longer than vice versa. The asymmetry of the inter-hemispheric transport is mainly driven by the Asian summer monsoon (ASM). We confirm the important role of ASM and westerly ducts in the inter-hemispheric transport from the NH extratropics to the SH. However, we find that it is an interplay between the ASM and westerly ducts which triggers such cross-equator transport from boreal summer to fall, mainly westerly ducts over the eastern Atlantic.</p>
Highlights
The transport from the troposphere to the stratosphere plays an important role in determining the chemical composition of the atmosphere and radiative features, which can impact atmospheric chemistry and global climate
We investigate the transport from the Northern Hemisphere (NH) to the Southern Hemisphere (SH) and vice versa using the simulations from the three-dimensional Chemical Lagrangian Model of the Stratosphere (CLaMS) with the atmospheric source regions divided into three domains
Significant differences are found between Orbe et al (2015) and our study. Their results show that the air mass fractions (AMFs) from the NH to the global lower stratosphere are comparable to the corresponding contributions from the SH, while we find that the contributions from the NH extratropics to the global lower stratosphere are about 5 times larger than those from the SH extratropics
Summary
The transport from the troposphere to the stratosphere plays an important role in determining the chemical composition of the atmosphere and radiative features, which can impact atmospheric chemistry and global climate. The source distributions of many tracers are different in the SH and NH, the observed trends of the tracers are almost homogeneous in the global upper troposphere and lower stratosphere (UTLS), which suggests the key role of inter-hemispheric transport in regulating the distribution of atmospheric trace gases and maintaining the mass balance (e.g., Müller and Brasseur, 1995; Wang and Shallcross, 2000; Liang et al, 2014; Patra et al, 2014; Chen et al, 2017). Most previous studies have focused on the interhemispheric transport from the NH to the SH in the troposphere using a two-box model based on zonal mean results, with relatively few analyses investigating the crossequatorial transport in the stratosphere (e.g., Lintner, 2003; Patra et al, 2009, 2011; Holzer, 2009).
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