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The efficiency of transport into the stratosphere via the Asian and
North American summer monsoon circulations

Abstract. Transport of pollutants into the stratosphere via the Asian summer monsoon (ASM) or North American summer monsoon (NASM) may affect the atmospheric composition and climate both locally and globally. We identify and study the robust characteristics of transport from the ASM and NASM regions to the stratosphere using the Lagrangian chemistry transport model CLaMS as driven by the ERA-Interim and MERRA-2 reanalyses. In particular, we investigate the relative influences of the ASM and NASM on stratospheric composition, the transport pathways by which these influences are effected, and the quantitative contributions and efficiencies of transport from different altitudes in these two monsoon regions to the stratosphere. We release artificial tracers in several vertical layers from the middle troposphere to the lower stratosphere in both ASM and NASM source regions during July and August 2010–2013 and track their evolution until the following summer. We find that the magnitude of transport from the ASM and NASM regions to the tropical stratosphere, and even to the Southern Hemispheric stratosphere, is higher when the tracers are released at the 350–360 K level. For tracers released close to the tropopause (370–380 K), transport is primarily into the Northern Hemispheric stratosphere. Results for different vertical layers or air origin reveal two transport pathways from the upper troposphere over the ASM and NASM regions to the tropical pipe: (i) quasi-horizontal transport to the tropics below the tropopause followed by ascent to the stratosphere via tropical upwelling, and (ii) ascent into the stratosphere inside the ASM/NASM followed by quasi-horizontal transport to the tropical lower stratosphere and tropical pipe. The tropical pathway (i) is faster than the monsoon pathway (ii), particularly in the ascending branch. Ultimately, the abundance of air in the tropical pipe that originates in the ASM upper troposphere (350–360 K, ~ 5 %) is comparable to that of air ascending directly from the tropics ten months after the release of the source tracers. By contrast, the air mass contributions from the ASM to the tropical pipe are about three times larger than the corresponding contribution from the NASM (~ 1.5 %). The transport efficiency into the tropical pipe, normalized by the mass of the domain, is greatest from the ASM region at 370–380 K. Transport from the ASM to the tropical pipe is almost twice as efficient as transport from the NASM or tropics to the tropical pipe. Although the contribution from the NASM to the stratosphere is less than that from either the ASM or the tropics, the transport efficiency from the NASM is comparable to that from the tropics.
