Impact of the Asian monsoon on the extratropical lower stratosphere: trace gas observations during TACTS over Europe 2012

Abstract

The transport of air masses originating from the Asian monsoon anticyclone into the extratropical upper troposphere and lower stratosphere (Ex-UTLS) above potential temperatures Theta = 380K was identified during the HALO aircraft mission TACTS in August and September 2012. In situ measurements of CO, O-3 and N2O during TACTS flight 2 on 30 August 2012 show the irreversible mixing of aged stratospheric air masses with younger (recently transported from the troposphere) ones within the Ex-UTLS. Backward trajectories calculated with the trajectory module of CLaMS indicate that these tropospherically affected air masses originate from the Asian monsoon anticyclone. These air masses are subsequently transported above potential temperatures Theta = 380K from the monsoon circulation region into the Ex-UTLS, where they subsequently mix with stratospheric air masses. The overall trace gas distribution measured during TACTS shows that this transport pathway had affected the chemical composition of the Ex-UTLS during boreal summer and autumn 2012. This leads to an intensification of the tropospheric influence on the extratropical lower stratosphere with PV > 8 pvu within 3 weeks during the TACTS mission. During the same time period a weakening of the tropospheric influence on the lowermost stratosphere (LMS) is determined. The study shows that the transport of air masses originating from the Asian summer monsoon region within the lower stratosphere affects the change in the chemical composition of the Ex-UTLS over Europe and thus contributes to the flushing of the LMS during summer 2012

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

“…-Carbon monoxide (CO) and methane (CH 4 ) measurements are used from TRIHOP, a three-channel quantum cascade laser infrared absorption spectrometer (for more details, see Müller et al, 2016), which is an updated version of the three-channel tunable diode laser instrument for atmospheric research (TRISTAR) used in previous aircraft campaigns (e.g., Hoor et al, 2004). …”

“…Backward trajectory calculations 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 composition of the young air masses is dominated by air masses primarily originating in India, China, and southeast Asia, in particular at the end of the monsoon season in September/October 2012. Thus, the Asian monsoon anticyclone and boundary emissions from Asia contribute to the maximum of tropospheric signatures found in the northern extratropical lower stratosphere in boreal summer and autumn (e.g., Hoor et al, 2005;Hegglin and Shepherd, 2007;Bönisch et al, 2009;Zahn et al, 2014;Müller et al, 2016), sometimes also referred to as "flushing" of the lower stratosphere (Hegglin and Shepherd, 2007;Bönisch et al, 2009).…”

“…In summer 2012, enhanced tropospheric trace gases were found in the upper troposphere over Asia from mid-June to late October both during the existence of the Asian monsoon anticyclone and after its breakup . Studies for summer 2012 show that air masses from the Asian monsoon impact the chemical composition of the lower stratosphere over northern Europe, in particular at the end of the monsoon season in September and October 2012 Müller et al, 2016). An increase in the concentrations of tropospheric source gases such as CO, H 2 O, and N 2 O and a decrease of the stratospheric tracer O 3 was identified by in situ aircraft measurements in the lower stratosphere over Europe from summer to autumn 2012, associated with transport from the Asian monsoon anticyclone .…”

Water vapor increase in the northern lower stratosphere by the Asian monsoon anticyclone observed during TACTS/ESMVal campaigns

“…TRIHOP methane data are available every eight seconds with an integration time of 1.5 seconds, a precision of 9.5 ppbv (2 σ) and an uncertainty of 15 13.5 ppbv relative to the standard. For more details see Müller et al (2016).…”