Abstract
Abstract. Global simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) using artificial tracers of air mass origin are used to analyze transport mechanisms from the Asian monsoon region into the lower stratosphere. In a case study, the transport of air masses from the Asian monsoon anticyclone originating in India/China by an eastward-migrating anticyclone which broke off from the main anticyclone on 20 September 2012 and filaments separated at the northeastern flank of the anticyclone are analyzed. Enhanced contributions of young air masses (younger than 5 months) are found within the separated anticyclone confined at the top by the thermal tropopause. Further, these air masses are confined by the anticyclonic circulation and, on the polar side, by the subtropical jet such that the vertical structure resembles a bubble within the upper troposphere. Subsequently, these air masses are transported eastwards along the subtropical jet and enter the lower stratosphere by quasi-horizontal transport in a region of double tropopauses most likely associated with Rossby wave breaking events. As a result, thin filaments with enhanced signatures of tropospheric trace gases were measured in the lower stratosphere over Europe during the TACTS/ESMVal campaign in September 2012 in very good agreement with CLaMS simulations. Our simulations demonstrate that source regions in Asia and in the Pacific Ocean have a significant impact on the chemical composition of the lower stratosphere of the Northern Hemisphere. Young, moist air masses, in particular at the end of the monsoon season in September/October 2012, flooded the extratropical lower stratosphere in the Northern Hemisphere with contributions of up to ≈ 30 % at 380 K (with the remaining fraction being aged air). In contrast, the contribution of young air masses to the Southern Hemisphere is much lower. At the end of October 2012, approximately 1.5 ppmv H2O is found in the lower Northern Hemisphere stratosphere (at 380 K) from source regions both in Asia and in the tropical Pacific compared to a mean water vapor content of ≈ 5 ppmv. In addition to this main transport pathway from the Asian monsoon anticyclone to the east along the subtropical jet and subsequent transport into the northern lower stratosphere, a second horizontal transport pathway out of the anticyclone to the west into the tropics (TTL) is found in agreement with MIPAS HCFC-22 measurements.
Highlights
The Asian summer monsoon is associated with strong upward transport of tropospheric source gases by deep convection
Backward trajectory calculations (Vogel et al, 2014; Müller et al, 2016) show that enhanced tropospheric trace gases which were measured over northern Europe in the extratropical lower stratosphere during the TACTS/ESMVal campaign in August and September 2012 are affected by air masses from the circulation of the Asian monsoon anticyclone or which originate in the Asian monsoon anticyclone itself
The combination of the separation of anticyclones or filaments at the northeastern flank of the Asian monsoon anticyclone and subsequent horizontal transport along the subtropical jet associated with Rossby wave breaking is put forward here as a longrange transport mechanism from the Asian monsoon anticyclone to the Northern Hemisphere lower stratosphere
Summary
The Asian summer monsoon is associated with strong upward transport of tropospheric source gases by deep convection. Backward trajectory calculations (Vogel et al, 2014; Müller et al, 2016) show that enhanced tropospheric trace gases which were measured over northern Europe in the extratropical lower stratosphere during the TACTS/ESMVal campaign in August and September 2012 are affected by air masses from the circulation of the Asian monsoon anticyclone or which originate in the Asian monsoon anticyclone itself. The same model simulation as in Vogel et al (2015) with the CLaMS model (e.g., Pommrich et al, 2014, and references therein) is used, including artificial emission tracers that mark source regions in the boundary layer of the Earth’s atmosphere This allows the origin of the air masses and the detailed transport pathways from Asian source regions into the northern lower stratosphere to be quantified. – Potential temperature ( ) was deduced from the Basic HALO Measurement and Sensor System (BAHAMAS) that yields basic meteorological and avionic data for all TACTS/ESMVal flights
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