Stratosphere-Troposphere Exchange

Important greenhouse gases such as water vapor and ozone, with steep gradients between their tropospheric and stratospheric mixing ratio values, exhibit large spatial and temporal variability in the UTLS resulting from stratosphere-troposphere exchange [1-2]. Prominent underlying processes are the Brewer-Dobson circulation, quasi-horizontal isentropic transport between the tropical tropopause layer (TTL), the extratropical lowermost stratosphere (LMS) and vertical transport from below by convection. Transport of air from the troposphere deep into the stratosphere occurs mainly in the tropics and is associated with the ascending branch of the large-scale Brewer-Dobson circulation, which connects the tropical tropopause layer (TTL) with the deep stratosphere. In this section, we focus on our results concerning the role of the Asian monsoon in stratosphere-troposphere exchange and on quasi-horizontal transport processes influencing the water vapor budget in the extratropical UTLS.

The Asian monsoon forms a high-pressure area at an altitude of approx. 10–18 km that is almost stationary for three months and covers almost all of Asia in the form of an anticyclone. This anticyclone, with its core located at approx. 30° N, very effectively traps emissions from Asia transported to this altitude by a strong convection current, particularly above India, southern China and Indonesia. As a direct consequence, elevated levels of tropospheric trace gases such as water vapour, CO or HCN can be found in the anticyclone, as well as reduced amounts of stratospheric trace gases such as ozone or HCl. This finding, which has been the subject of much discussion in the last few years, was derived mainly from satellite observation data. However, the extent to which emissions from Asia influence the composition of the tropical tropopause layer (TTL) is still uncertain. The TTL is a region of the atmosphere stretching across the equator which has a decisive impact on mass transport into the stratosphere and is therefore referred to as the “gateway to the stratosphere”. Meridional transport from the anticyclone into the TTL in the direction of the equator is intermittent and strongly influenced by planetary waves and mixing processes. Current research aims to make qualitative statements regarding the amounts of emissions transported.
IEK-7 has studied the impact of this circulation pattern on the annual cycle of ozone in depth as part of model simulations [3-5]. It was also discussed how the representation of vertical transport in models influences the composition of the TTL [6].
We investigated the Asian monsoon anticyclone in terms of its barrier function and transport pathways of the air masses contained in it. The analysis highlights a barrier to horizontal transport along the 380 K isentrope in the anticyclone, which can be determined from a local maximum in the gradient of potential vorticity (PV), following methods developed for the polar vortex [7]. The monsoon anticyclone is dynamically highly variable and the maximum in the PV gradient is weak, such that additional constraints are needed (e.g. time averaging). Nevertheless, PV contours in the monsoon anticyclone agree well with contours of trace gas mixing ratios (CO, O₃) and mean age from model simulations with ClaMS and satellite observations from the Microwave Limb Sounder (MLS) instrument. It was further shown that two main pollution transport pathways from the anticyclone emerge into (i) the tropical stratosphere (tropical pipe) and (ii) into the northern hemisphere extratropical lower stratosphere [8]. Anticyclone air mass fractions correlate well with satellite observations of hydrogen cyanide, reaching around 5 % in the tropical pipe and 15 % in the extratropical lowermost stratosphere over the course of a year. Cross-tropopause transport occurs in a vertical chimney, but with the emissions transported quasi-horizontally along isentropes above the tropopause into the tropics and NH.
The Asian monsoon provides a link between near-surface pollution and global circulation because it rapidly transports anthropogenic emissions from the near-surface boundary layer into the TTL. We analyzed the impact of different boundary layer source regions in Asia on the chemical composition of the Asian monsoon anticyclone in 2012 based on simulations of the Chemical Lagrangian Model of the Stratosphere (CLaMS) using artificial emission tracers [9]. The contribution of different boundary source regions to the composition of the Asian monsoon anticyclone in the upper troposphere strongly depends on its intraseasonal variability and is therefore more complex than hitherto believed. Our findings also show that the temporal variability of the contribution of different convective regions is imprinted in the chemical composition of the Asian monsoon anticyclone. In the early (mid-June to mid-July) and late (September) period of the 2012 monsoon season, contributions of emissions from Southeast Asia are highest; in the intervening period (early August), emissions from northern India have the largest impact.

Our analysis further indicates that the interplay between the Asian monsoon anticyclone and tropical cyclones has an impact on the chemical composition of the UTLS. We found for the first time [10] that the combination of rapid uplift by a typhoon and eastward eddy shedding from the Asian monsoon anticyclone is a novel fast transport pathway that carries boundary emissions from Southeast Asia/West Pacific within approximately 5 weeks to the lowermost stratosphere in northern Europe. Analyses of balloon-borne sensors launched from Lhasa, China, in August 2013 show that nearly half of the measured ozone profiles in the upper troposphere are influenced by tropical cyclones occurring over the west Pacific [11]. This indicates that the influence of tropical cyclones on air masses in the Asian monsoon anticyclone is a regularly occurring feature.
Long-range transport of air masses from the Asian monsoon anticyclone into the extra-tropical lower stratosphere is the subject of two further studies [12-13]. An important feature is the separation of air masses at the northeast flank of the anticyclone caused by disturbances of the subtropical jet by strong Rossby waves and subsequent eastward transport within the tropics along the subtropical jet. Eastward-migrating anticyclones break off from the main anticyclone, referred to as “eddy shedding” events, a few times each summer and have the potential to carry air with high amounts of tropospheric trace gases from the Asian monsoon anticyclone to middle and high latitudes in the northern hemisphere. We demonstrated [13] that spatially highly resolved measurements of peroxyacetyl nitrate (PAN) and O₃ by the CRISTA infrared limb-sounder taken in August 1997 allow a detailed analysis of an eastward eddy-shedding event of the ASM anticyclone. In addition to this main transport pathway, a second westward horizontal transport pathway out of the anticyclone influences composition in the TTL and subsequently in the extratropics in agreement with MIPAS HCFC-22 measurements [12].
Transport influencing water vapor in the lowermost stratosphere: Our analyses also underline the importance of transport by the Asian monsoon for the water vapor budget in the extratropical lower stratosphere. The Asian and American monsoons emerge as regions of particularly high water vapor mixing ratios in the lower stratosphere during boreal summer [14]. CLaMS simulations with transport barriers confirm that almost the entire annual cycle of water vapor in northern mid-latitudes above about 360 K, with maximum mixing ratios during summer and fall, is caused by horizontal transport from the subtropics. This is consistent with simulation results showing that young, moist air masses from Asia and from the tropical Pacific flood the extra-tropical lower stratosphere in the northern hemisphere in autumn with contributions of up to ≈ 30 % at 380 K [12]. Observational evidence for this process is given by HALO observations during the TACTS/ESMVal campaign in September 2012, including FISH (Fast In-situ Stratospheric Hygrometer) observations of enhanced water vapor values in filamentary structures originating from the Asian monsoon [10,12,15]. GLORIA and FISH observations during the same campaign in the Antarctic also highlight the transport of strongly dehydrated stratospheric air into the troposphere [16].

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