Theme DYN-1

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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).

Figure DYN1-1: Satellite measurements of water vapor at the 390 K potential temperature level observed by the AURA-MLS instrument. The data show transport of anomalies from the tropics to high latitudes (Figure adapted from Randel and Jensen, 2013).

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).

Figure DYN1-2: Fraction of young air mass with stratospheric transit time below 6 months, as deduced from CLaMS model age spectra (adapted from Ploeger and Birner, 2016).

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.

Figure DYN1-3: (a) Annual cycle of Rossby wave-breaking transport at 420K (top), 380K (middle) and 350K (bottom). Anticyclonically sheared events are shown in black and cyclonically sheared events in gray. Dashed lines are equatorward transport and dotted lines poleward. The thick black line is the net transport (stratosphere to troposphere at lower levels). (Figure adapted from Homeyer and Bowman, 2013). (b) Climatological zonal wind U at 390 K from ERA-Interim for October (top), November (middle) and December (bottom).

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).


Birner, T. (2006). “Fine-scale structure of the extratropical tropopause region”. In: J. Geophys. Res. 111.D4. DOI: 10.1029/2005jd006301.

Birner, Thomas (2010). “Residual Circulation and Tropopause Structure”. In: J. Atmos. Sci. 67.8, pp. 2582–2600. DOI: 10.1175/2010jas3287.1.

Birner, Thomas, Andreas Dörnbrack, and Ulrich Schumann (2002). “How sharp is the tropopause at midlatitudes?” In: Geophys. Res. Lett. 29.14, pp. 45–1–45–4. DOI: 10.1029/2002gl015142.

Grieger, J., G.C. Leckebusch, M.G. Donat, M. Schuster, and U. Ulbrich (2014). “Southern Hemisphere winter cyclone activity under recent and future climate conditions in multi-model AOGCM simulations”. In: Int. J. Climatol. 34.12, pp. 3400–3416. DOI: 10.1002/joc.3917.

Grise, Kevin M., David W. J. Thompson, and Thomas Birner (2010). “A Global Survey of Static Stability in the Stratosphere and Upper Troposphere”. In: J. Clim. 23.9, pp. 2275–2292. DOI: 10.1175/2009jcli3369.1.

Haynes, Peter and Emily Shuckburgh (2000). “Effective diffusivity as a diagnostic of atmospheric transport: 1. Stratosphere”. In: J. Geophys. Res. Atmos. 105.D18, pp. 22777–22794. DOI: 10.1029/2000jd900093.

Hegglin, Michaela I. and Theodore G. Shepherd (2009). “Large climate-induced changes in ultraviolet index and stratosphere-to-troposphere ozone flux”. In: Nat. Geosci. 2.10, pp. 687–691. DOI: 10.1038/ngeo604.

Homeyer, Cameron R. and Kenneth P. Bowman (2013). “Rossby Wave Breaking and Transport between the Tropics and Extratropics above the Subtropical Jet”. In: J. Atmos. Sci. 70.2, pp. 607–626. DOI: 10.1175/jas-d-12-0198.1.

Hoor, P., C. Gurk, D. Brunner, M. I. Hegglin, H. Wernli, and H. Fischer (2004). “Seasonality and extent of extratropical TST derived from in-situ CO measurements during SPURT”. In: Atmos. Chem. Phys. 4.5, pp. 1427–1442. DOI: 10.5194/acp-4-1427-2004.

Hoor, P., H. Wernli, M. I. Hegglin, and H. Bönisch (2010). “Transport timescales and tracer properties in the extratropical UTLS”. In: Atmos. Chem. Phys. 10.16, pp. 7929–7944. DOI: 10.5194/acp-10-7929-2010.

Khaykin, S. M., S. Godin-Beekmann, A. Hauchecorne, J. Pelon, F. Ravetta, and P. Keckhut (2018). “Stratospheric Smoke With Unprecedentedly High Backscatter Observed by Lidars Above Southern France”. In: Geophys. Res. Lett. 45.3, pp. 1639–1646. DOI: 10.1002/2017gl076763.

Konopka, Paul, Felix Ploeger, Mengchu Tao, Thomas Birner, and Martin Riese (2015). “Hemispheric asymmetries and seasonality of mean age of air in the lower stratosphere: Deep versus shallow branch of the Brewer-Dobson circulation”. In: J. Geophys. Res. Atmos. 120.5, pp. 2053–2066. DOI: 10.1002/2014jd022429.

Konopka, Paul, Felix Ploeger, Mengchu Tao, and Martin Riese (2016). “Zonally resolved impact of ENSO on the stratospheric circulation and water vapor entry values”. In: J. Geophys. Res. Atmos. 121.19, pp. 11, 486–11, 501. DOI: 10.1002/2015jd024698.

Krause, Jens, Peter Hoor, Andreas Engel, Felix Plöger, Jens-Uwe Grooß, Harald Bönisch, Timo Keber, Björn-Martin Sinnhuber, Wolfgang Woiwode, and Hermann Oelhaf (2018). “Mixing and ageing in the polar lower stratosphere in winter 2015–2016”. In: Atmos. Chem. Phys. 18.8, pp. 6057–6073. DOI: 10.5194/acp-18-6057-2018.

Kunkel, Daniel, Peter Hoor, and Volkmar Wirth (2014). “Can inertia-gravity waves persistently alter the tropopause inversion layer?” In: Geophys. Res. Lett. 41.22, pp. 7822–7829. DOI: 10.1002/2014gl061970.

Kunkel, D., P. Hoor, and V. Wirth (2016). “The tropopause inversion layer in baroclinic life-cycle experiments: the role of diabatic processes”. In: Atmos. Chem. Phys. 16.2, pp. 541–560. DOI: 10.5194/acp-16-541-2016.

Müller, Rolf, Paul J. Crutzen, Jens-Uwe Grooß, Christoph Brühl, James M. Russell, and Adrian F. Tuck (1996). “Chlorine activation and ozone depletion in the Arctic vortex: Observations by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite”. In: J. Geophys. Res. Atmos. 101.D7, pp. 12531–12554. DOI: 10.1029/95jd00220.

Müller, Stefan, Peter Hoor, Heiko Bozem, Ellen Gute, Bärbel Vogel, Andreas Zahn, Harald Bönisch, Timo Keber, Martina Krämer, Christian Rolf, Martin Riese, Hans Schlager, and Andreas Engel (2016). “Impact of the Asian monsoon on the extratropical lower stratosphere: trace gas observations during TACTS over Europe 2012”. In: Atmos. Chem. Phys. 16.16, pp. 10573–10589. DOI: 10.5194/acp-16-10573-2016.

Ploeger, F., M. Abalos, T. Birner, P. Konopka, B. Legras, R. Müller, and M. Riese (2015). “Quantifying the effects of mixing and residual circulation on trends of stratospheric mean age of air”. In: Geophys. Res. Lett. 42.6, pp. 2047– 2054. DOI: 10.1002/2014gl062927.

Ploeger, Felix and Thomas Birner (2016). “Seasonal and inter-annual variability of lower stratospheric age of air spectra”. In: Atmos. Chem. Phys. 16.15, pp. 10195–10213. DOI: 10.5194/acp-16-10195-2016.

Postel, Gregory A. and Matthew H. Hitchman (1999). “A Climatology of Rossby Wave Breaking along the Subtropical Tropopause”. In: J. Atmos. Sci. 56.3, pp. 359–373. DOI: 10.1175/1520-0469(1999)056<0359:acorwb>;2.

Randel, William J. and Eric J. Jensen (2013). “Physical processes in the tropical tropopause layer and their roles in a changing climate”. In: Nat. Geosci. 6.3, pp. 169–176. DOI: 10.1038/ngeo1733.

Rolf, C., A. Afchine, H. Bozem, B. Buchholz, V. Ebert, T. Guggenmoser, P. Hoor, P. Konopka, E. Kretschmer, S. Müller, H. Schlager, N. Spelten, O. Sumin´ska-Ebersoldt, J. Ungermann, A. Zahn, and M. Krämer (2015). “Trans- port of Antarctic stratospheric strongly dehydrated air into the troposphere observed during the HALO-ESMVal campaign 2012”. In: Atmos. Chem. Phys. 15.16, pp. 9143–9158. DOI: 10.5194/acp-15-9143-2015.

Rolf, Christian, Bärbel Vogel, Peter Hoor, Armin Afchine, Gebhard Günther, Martina Krämer, Rolf Müller, Stefan Müller, Nicole Spelten, and Martin Riese (2018). “Water vapor increase in the lower stratosphere of the Northern Hemisphere due to the Asian monsoon anticyclone observed during the TACTS/ESMVal campaigns”. In: Atmos. Chem. Phys. 18.4, pp. 2973–2983. DOI: 10.5194/acp-18-2973-2018.

Simmonds, Ian and Kevin Keay (2000). “Mean Southern Hemisphere Extratropical Cyclone Behavior in the 40- Year NCEP–NCAR Reanalysis”. In: J. Clim. 13.5, pp. 873–885. DOI: 10.1175/1520-0442(2000)013<0873:mshecb>;2.

Škerlak, B., M. Sprenger, and H. Wernli (2014). “A global climatology of stratosphere–troposphere exchange using the ERA-Interim data set from 1979 to 2011”. In: Atmos. Chem. Phys. 14.2, pp. 913–937. DOI: 10.5194/acp-14-913-2014.

Škerlak, Bojan, Michael Sprenger, Stephan Pfahl, Evangelos Tyrlis, and Heini Wernli (2015). “Tropopause folds in ERA-Interim: Global climatology and relation to extreme weather events”. In: J. Geophys. Res. Atmos. 120.10, pp. 4860–4877. DOI: 10.1002/2014jd022787.

Vogel, Bärbel, Gebhard Günther, Rolf Müller, Jens-Uwe Grooß, Armin Afchine, Heiko Bozem, Peter Hoor, Martina Krämer, Stefan Müller, Martin Riese, Christian Rolf, Nicole Spelten, Gabriele P. Stiller, Jörn Ungermann, and Andreas Zahn (2016). “Long-range transport pathways of tropospheric source gases originating in Asia into the northern lower stratosphere during the Asian monsoon season 2012”. In: Atmos. Chem. Phys. 16.23, pp. 15301– 15325. DOI: 10.5194/acp-16-15301-2016.

Waugh, Darryn W. and Lorenzo M. Polvani (2000). “Climatology of intrusions into the tropical upper troposphere”. In: Geophys. Res. Lett. 27.23, pp. 3857–3860. DOI: 10.1029/2000gl012250.

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