Abstract
Abstract. We have performed backward trajectory calculations and simulations with the three-dimensional Chemical Lagrangian Model of the Stratosphere (CLaMS) for two succeeding monsoon seasons using artificial tracers of air mass origin. With these tracers we trace back the origin of young air masses (age <6 months) at the top of the Asian monsoon anticyclone and of air masses within the tropical pipe (6 months < age <18 months) during summer 2008. The occurrence of young air masses (<6 months) at the top of the Asian monsoon anticyclone up to ∼460 K is in agreement with satellite measurements of chlorodifluoromethane (HCFC-22) by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument. HCFC-22 can be considered as a regional tracer for continental eastern Asia and the Middle East as it is mainly emitted in this region. Our findings show that the transport of air masses from boundary layer sources in the region of the Asian monsoon into the tropical pipe occurs in three distinct steps. First, very fast uplift in “a convective range” transports air masses up to 360 K potential temperature within a few days. Second, air masses are uplifted from about 360 K up to 460 K within “an upward spiralling range” within a few months. The large-scale upward spiral extends from northern Africa to the western Pacific. The air masses are transported upwards by diabatic heating with a rate of up to 1–1.5 K per day, implying strong vertical transport above the Asian monsoon anticyclone. Third, transport of air masses occurs within the tropical pipe up to 550 K associated with the large-scale Brewer–Dobson circulation within ∼1 year. In the upward spiralling range, air masses are uplifted by diabatic heating across the (lapse rate) tropopause, which does not act as a transport barrier, in contrast to the extratropical tropopause. Further, in the upward spiralling range air masses from inside the Asian monsoon anticyclone are mixed with air masses convectively uplifted outside the core of the Asian monsoon anticyclone in the tropical adjacent regions. Moreover, the vertical transport of air masses from the Asian monsoon anticyclone into the tropical pipe is weak in terms of transported air masses compared to the transport from the monsoon anticyclone into the northern extratropical lower stratosphere. Air masses from the Asian monsoon anticyclone (India/China) contribute a minor fraction to the composition of air within the tropical pipe at 550 K (6 %), and the major fractions are from Southeast Asia (16 %) and the tropical Pacific (15 %).
Highlights
The Asian summer monsoon is associated with deep convection over the Indian subcontinent and with an anticyclonic flow that extends from the upper troposphere into the lower stratosphere (UTLS) region, which is the most pronounced circulation pattern in these altitudes during boreal summer (e.g. Mason and Anderson, 1963; Li et al, 2005; Randel and Park, 2006; Park et al, 2007)
Vogel et al (2015) showed that the emission tracer for India/China is a good proxy for the location and shape of the Asian monsoon anticyclone using pattern correlations with potential vorticity (PV), and Microwave Limb Sounder (MLS) O3 and carbon monoxide (CO) satellite measurements between 360 and 400 K
It has been found in several satellite measurements (Griessbach et al, 2013; Fromm et al, 2014; Fairlie et al, 2014) that polluted air masses from the Nabro eruption in Eritrea, northeastern Africa, in June 2011 are transported around the anticyclone and at higher levels are found above the anticyclone, consistent with the Chemical Lagrangian Model of the Stratosphere (CLaMS) tracer for the tropical adjacent regions
Summary
The Asian summer monsoon is associated with deep convection over the Indian subcontinent and with an anticyclonic flow that extends from the upper troposphere into the lower stratosphere (UTLS) region, which is the most pronounced circulation pattern in these altitudes during boreal summer (e.g. Mason and Anderson, 1963; Li et al, 2005; Randel and Park, 2006; Park et al, 2007). The Asian summer monsoon is associated with deep convection over the Indian subcontinent and with an anticyclonic flow that extends from the upper troposphere into the lower stratosphere (UTLS) region, which is the most pronounced circulation pattern in these altitudes during boreal summer B. Vogel et al.: Transport at the top of Asian monsoon anticyclone trace gases in the anticyclone, isolating them from the surrounding air (stratospheric background), as shown by a variety of satellite measurements (e.g. Rosenlof et al, 1997; Li et al, 2005; Park et al, 2007; Fadnavis et al, 2014; Glatthor et al, 2015; Chirkov et al, 2016; Santee et al, 2017). The transport of tropospheric trace gases by the Asian monsoon anticyclone into the lower stratosphere changes the chemical composition in this part of the Earth’s atmosphere. Active species transported into the lowermost extratropical stratosphere have a significant impact on surface climate (e.g. Solomon et al, 2010; Riese et al, 2012; Hossaini et al, 2015) or can cause regional radiative forcing like that caused by the Asian tropopause aerosol layer (ATAL) (e.g. Vernier et al, 2015)
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