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Coupling processes at the Southern Hemisphere tropopause: UTLS composition and dynamics in the southern hemisphere from observations and models
In general, very little is known about the chemical composition of the SH UTLS, due to a lack of observations in this region. In particular water vapor shows maximum values in both hemispheres in September/October, but a different hemispheric behavior compared to the underlying mixing processes (see Fig. DYN1-1).
The stratospheric BDC is a major factor controlling the trace gas composition of the UTLS with tropospheric air entering the stratosphere in the tropics and circulating to high latitudes where it subsides back into the tropopause region. The BDC shows a clear seasonal cycle and interhemispheric differences (e.g., Konopka et al., 2015). The circulation is driven by breaking of atmospheric waves of different wavelengths. This forcing is stronger during periods of westerly stratospheric winds, thus during hemispheric winter and spring. Stronger planetary wave activity leads to a stronger forcing of the BDC on the NH causing hemispheric differences in trace gas transport in the UTLS.
The different transport pathways and related transit time scales lead to differences in the chemical composition. These differences are further evident in age spectra, the probability density function for transit times. Age spectra can be constrained from measurements of long-lived trace gases with different atmospheric lifetimes. The fractions of young and old air masses, as deduced from age spectra, allow to better understand the relative roles of BDC and extratropical STE in determining the chemical composition (Ploeger and Birner, 2016; Krause et al., 2018). In particular the young air mass fraction indicates clear hemispheric transport differences in the summertime “flushing” of the mid-latitude lowermost stratosphere with young air (see Fig. DYN1-2).
Large-scale transport by the BDC can conceptually be divided into a slow residual mean mass circulation and eddy mixing which operates on much shorter timescales (days to weeks). Both transport components lead to different age structures in both hemispheres (e.g., Ploeger et al., 2015). The fast quasi-isentropic eddy mixing causes a fast exchange between tropics and high latitudes as well as cross tropopause exchange and is largely related to breaking Rossby-waves (Haynes and Shuckburgh, 2000). These processes also determine the composition and vertical extent of the ExTL (Extratropical Transition Layer) (Hoor et al.2004, 2010), which is indicated to be smaller compared to the northern hemisphere on the basis of ACE-FTS observations (Hegglin et al., 2009). The related largescale stirring and mixing transport is different in both hemispheres with maxima during November and April in the SH lowermost stratosphere (Fig. DYN1-3). In the NH such transport events have been investigated during recent measurement campaigns (e.g., WISE, PGS, TACTS, see Müller et al., 2015, Krause et al., 2018), but in the SH a detailed process understanding based on in-situ measurements is still missing.
In-situ measurements in the mid-latitudes of the SH are very rare above the 360 K potential temperature level. Even though, the SH provides particularly favorable conditions for isentropic exchange studies due to the proximity of young subtropical air and extremely old air masses within the wintertime polar vortex. Mixing of aged and extremely dry vortex air into the UTLS, even below the tropopause, has recently been shown to occur from HALO observations (Rolf et al., 2015). However, the representativeness of such events is under debate. Here, we will particularly investigate processes transporting air masses low in ozone and water vapor and perturbed in chlorine chemistry (Müller et al., 1996) from the Antarctic lowermost stratosphere into the high latitude troposphere and mid-latitude stratosphere.
Reanalysis data indicate preferred regions of Rossby-wave breaking in the SH around Southern South America over the Atlantic and Central to Eastern Pacific (e.g., Homeyer et al., 2013), related to regions of westerly tropical zonal winds („westerly ducts“), which prevail from around November to March (Waugh and Polvani, 2000). These regions allow Rossby-waves to propagate deep into the tropics (see Fig. DYN1-3b). Potentially, a link exists between the Rossby-wave breaking and the South American anticyclonic circulation („Bolivian high“, Postel and Hitchman, 1999). Since the strength of the westerly ducts is modulated by the El Nino Southern oscillation (e.g., Konopka et al, 2016), a related inter-annual modulation in isentropic transport is expected. The westerly ducts extend from the NH to the SH and likely represent regions of preferred interhemispheric transport, for instance of NH pollution into the SH. However, no observational evidence for such transport exists, hitherto.
The composition of the UTLS is also affected by transport in the troposphere, especially within mesoscale convective systems and extratropical cyclones which can lift air masses from the surface into the UTLS on time scales of hours to several days. While convective systems are most prominent in the tropics and subtropics, extratropical cyclones have a maximum appearance around Antarctica but also between 30 and 50 degrees S, with a local maximum of cyclogenesis at the east coast of South America (Simmons and Keay, 2000). However, due to the lack of observations their impact is not well known, which is also reflected in the varying representation of these systems in different reanalysis data sets (Grieger et al., 2014). Using balloon-borne measurements Khaykin et al. (2016) showed that convection over South America may moisten the SH lower stratosphere. Most importantly, cyclones and convective systems affect the structure of the tropopause and thus STE: (1) the tropopause inversion layer (TIL) (Birner et al., 2002; Birner 2006) shows a strong asymmetry between NH and SH in spring (Grise et al., 2010), and (2) tropopause folds differ between SH and NH (e.g., Skerlak et al., 2014a,b).
Notably, factors controlling the structure and dynamics affecting the TIL and the ExTL differ from the northern hemisphere, such as diabatic downwelling (Birner 2010), baroclinic wave activity (Kunkel et al., 2016) and the effect of gravity waves on the TIL (Kunkel et al., 2014). Water vapour transport and gradients at the tropopause are essential for the TIL formation (Kunkel et al., 2016), but are different between both hemispheres (e.g. due to water vapour export from the Asian summer monsoon, Vogel et al., 2016; Rolf et al., 2018).
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