Long-range transport pathways of tropospheric source gases originating in Asia into the northern lower stratosphere during the Asian monsoon season 2012

Abstract

<p><strong>Abstract.</strong> Global simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) using artificial tracers of air mass origin are used to analyze transport pathways 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 breaking 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 at the polar side by the subtropical jet such as the vertical structure looks like 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 are 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 by flooding the extratropical lower stratosphere with young moist air masses in particular at end of the monsoon season in September/October 2012 (up to ~30 % at 380 K) in contrast to the southern hemisphere. End of October 2012, approximately 1.5 ppmv H<sub>2</sub>O is found in the lower northern hemisphere stratosphere (at 380 K) from source regions in Asia and 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.</p>

Importance of seasonally resolved oceanic emissions for bromoform delivery from the tropical Indian Ocean and west Pacific to the stratosphere

“…The lifetime of bromoform in the TTL is around 25 to 30 days (Hossaini et al, 2010), which could be long enough for it to accumulate in the Asian monsoon anticyclone and then be distributed across the Northern Hemisphere over Asia in SON. This spreading of air masses from the anticyclone across the Northern Hemisphere and also into the Southern Hemisphere during the breakup of the anticyclone in September has been observed for trace gases like CO, H 2 O, and O 3 with satellites (Santee et al, 2017) and also simulated with a chemistry transport model (e.g., Vogel et al, 2016). Thus, the negative anomalies in the anticyclone in JJA (Fig.…”

Water vapor increase in the lower stratosphere of the Northern Hemisphere due to the Asian monsoon anticyclone observed during the TACTS/ESMVal campaigns

“…Figure 1 shows the map of all considered flight paths and the respective phases they are attributed to. Vogel et al (2016) provided grounds for this selection, showing that the strongest transport of young air masses from the AMA into the Ex-LS occurred in the period between August and the end of September, mainly associated with a pronounced eddyshedding event on 20 September 2012 . Thus, the selection of these dates reveals a clear difference in the water vapor distribution in the Ex-LS, as shown in Sect.…”

“…Thus, the air in the Ex-LS constitutes a mixture of young air masses originating from quasi-horizontal transport out of the TTL and old stratospheric air in the downwelling branch of the BDC (e.g., Bönisch et al, 2009;Ploeger et al, 2013;Vogel et al, 2016). As a rule, increasing inflow of tropospheric air masses from the tropics in combination with higher temperatures in the tropopause region causes a moistening of the Ex-LS in summer compared to winter months (e.g., Hoor et al, 2005;Krebsbach et al, 2006;Ploeger et al, 2013;Randel and Jensen, 2013;Zahn et al, 2014).…”

“…A horizontal transport pathway out of the anticyclone is directed westwards into the tropical tropopause layer (TTL; Popovic and Plumb, 2001;Randel and Park, 2006). A second pathway of moist and polluted air masses exists, where air from the AMA is transported eastwards along the subtropical jet and subsequently into the northern lower stratosphere via eddy shedding followed by tropopause crossing (e.g., Dethof et al, 1999;Hsu and Plumb, 2000;Vogel et al, 2014Vogel et al, , 2016. These frequently occurring eddy-shedding events triggered by Rossby wave activity flood the northern lower stratosphere with tropospheric air masses from the Asian subcontinent and cause a moistening of the Ex-LS (e.g., Müller et al, 2016;Vogel et al, 2014Vogel et al, , 2016.…”

“…A second pathway of moist and polluted air masses exists, where air from the AMA is transported eastwards along the subtropical jet and subsequently into the northern lower stratosphere via eddy shedding followed by tropopause crossing (e.g., Dethof et al, 1999;Hsu and Plumb, 2000;Vogel et al, 2014Vogel et al, , 2016. These frequently occurring eddy-shedding events triggered by Rossby wave activity flood the northern lower stratosphere with tropospheric air masses from the Asian subcontinent and cause a moistening of the Ex-LS (e.g., Müller et al, 2016;Vogel et al, 2014Vogel et al, , 2016. In addition, Aura Microwave Limb Sounder (MLS) satellite observations showed that the Asian monsoon is hydrating the northern stratosphere (Uma et al, 2014).…”