Even small variations in the distributions of trace gases, like water vapor and ozone, and thin cirrus clouds in the UTLS strongly impact radiative forcing of the Earth’s climate and surface temperatures, and are of key importance for understanding climate change (e.g. Riese et al., 2012, Solomon et al., 2010). There is also large uncertainty in future changes of ozone and water vapour in the UTLS, limiting the ability to predict the radiative forcing due to stratospheric ozone recovery and related processes during the 21st century: e.g., Bekki et al. (2013) have shown that different chemistry climate models not even agree on the sign of radiative forcing due to ozone recovery because of large uncertainties in modelled ozone changes in the vicinity of the tropopause. The uncertainty in the future development of ozone is further obvious from the observationthat ozone in the lower stratosphere outside the polar regions has declined since 1998 in spite of the changing halogen loading due to the Montreal protocol (Ball et al., 2018). However, a significant part of this decline is caused by dynamics of the UTLS (Chipperfield et al., 2018).

Most of the recent studies on the upper troposphere and lower stratosphere (UTLS) focused on the northern hemisphere (NH) or the tropics (e.g. Gettelman et al., 2011). However, satellite observations and model data indicate significant differences of transport processes and composition between the northern and the southern hemisphere (Hegglin et al., 2009; Konopka et al., 2015, Ploeger et al., 2013, Stiller et al., 2012). In the stratosphere the overturning BrewerDobson circulation (BDC) is weaker in the southern hemisphere (SH), whereas the polar vortex is stronger and more persistent in the SH. Therefore, satellite based observations show asymmetries of e.g. SF6 and thus stratospheric mean age in the corresponding seasons in the UTLS (Stiller et al., 2012). The southern hemisphere UTLS (SH-UTLS) further differs from its northern hemispheric counterpart due to the asymmetry in the monsoon circulations with the Asian summer monsoon being the strongest monsoon circulation on Earth (e.g. Park et al., 2009; Pan et al., 2016). As a consequence hemispheric differences of water vapour have been observed (Randel and Jensen, 2013) which are further related to differences in cross tropopause exchange (Hegglin et al., 2009, Grise et al., 2010, Ploeger et al., 2015). Exchange and coupling processes at the midlatitude tropopause are largely determined by strengths and locations of the jet streams, which are different in both hemispheres, due to the different distribution of wave breaking regions. In addition the hemispheric differences of anthropogenic emission sources such as biomass and fossil fuel burning have an impact on the SH-UTLS which is chemically different from the northern hemisphere.

Another important objective with major differences between the northern and southern hemisphere is gravity wave dynamics. Comparisons of stratospheric zonal-mean absolute GW momentum fluxes (GWMFs) from GCM predictions and global satellite data reveal a large variability between currently used GW parametrizations. Some features are reproduced reasonably well in terms of magnitudes and the latitudinal dependence, but there are still large deviations. Particular deviations are the ”gap“ in simulated zonal mean GWMFs near 60°S in winter (Geller et al. 2013, their Fig. 8), and the large simulated maxima above mountainous regions which are not observed by satellites such as SABER and HIRDLS (Geller et al. 2013, their Fig. 5).

Most GCMs exhibit large systematic biases in the simulation of the SH polar stratospheric circulation. These biases are known as the ”cold pole bias” occurring, e.g., in the Canadian Middle Atmosphere Model (CMAM) or the Whole Atmosphere Community Climate Model (WACCM) (SPARC CCMVal 2010, Butchart et al. 2011) as they underestimate the polar stratospheric temperatures. Due to isolation of the cold stratospheric air and the weak PW activity, there are significant late biases in the simulated spring-time breakups of the SH vortex. Most likely, the simplified representations of GWs is a major cause of the biases in the simulation of SH stratospheric climate.

Discrepancies in model climatologies can cause different circulation responses to anthropogenic forcing such as increasing GHG concentrations (Alexander et al. 2010, Sigmond and Scinocca 2010, Sigmond and Shepherd 2014) and ozone-depleting substances (ODSs) (Son et al. 2010, Garcia et al. 2017). For the latter, the inappropriate temperatures in the SH polar vortex influence the length of the period over which heterogeneous chemistry is active in austral spring and lead to unreliable predictions of ozone loss and recovery in a changing polar climate (Austin et al. 2003, McLandress and Shepherd 2009). Therefore, an unrealistic downward coupling of strong ozone depletion in austral spring to tropospheric SH summer climate is simulated. In addition, it was shown that details of vortex dynamics have impact on tropospheric responses (Arblaster and Meehl 2006, McLandress et al. 2011, Lin et al. 2016). Thus, changes in stratospheric and tropospheric circulations and chemical ozone depletion might not be correctly projected into a future climate, if the effects of GWs on the model climatology are not captured, that is, parametrized properly in GCMs.

McLandress et al. (2012) showed that in a GCM with data assimilation, large analysis increments of zonal winds are found around 60°S and between 3 and 1 hPa. These increments point to missing GWD in this altitude region. They performed numerical experiments with the free-running model, where GWD was added artificially by uniform subgrid-scale topography centred around 60°S and obtained more realistic stratospheric winds and higher polar temperatures. In the same spirit, Garcia et al. (2017) enhanced the OGW forcing in the SH by doubling the magnitude of sources. Again, more realistic climatologies of tropospheric and stratospheric winds and temperatures are found. However, Garcia et al. (2017) also showed that instead of modifying the OGWD, a modified parametrization for NOGWs by fronts leads to similar improvements of wind and temperature climatologies. Thus, OGWs likely play an important role, but also other sources could be the origin of the ”missing drag at 60°S“ (de la Camara et al. 2016). Observations show a zonal feature of enhanced GW activity that is often called the GW belt: In the stratospheric PNJ of the SH, satellite and balloon-borne observations indicate high values of GWMFs at around 60°S (e.g. Ern et al. 2006, Hertzog et al. 2008, Alexander et al. 2010).

Generally, the physical origin of the observed belt of enhanced GWMF around 60°S can be explained by different mechanisms:

  1. The downwind advection and meridional refraction of OGWD from the Southern Andes and the Antarctic Peninsula into the PNJ (Sato et al. 2009).
  2. Unresolved OGWs from small islands (Alexander et al. 2009, Alexander and Grimsdell 2013).
  3. The generation of secondary GWs generated locally in the breaking region of these primary orographic waves.
  4. NOGWs from sources associated with winter storm tracks over the southern oceans (Hendricks et al. 2014).
  5. A zonally uniform distribution of small amplitude waves from non-orographic mechanisms such as spontaneous adjustment and jet instability around the edge of the stratospheric PNJ (Hindley et al. 2015).

The horizontal propagation of OGWs redistributes the associated fluxes over a broader region and is one of the least studied processes so far. Supporting this idea, Amemiya and Sato (2016), allowing a 3D distribution of the GW forcing, found an enhancement of the westward GW forcing in the SH winter mesosphere above the core of the PNJ. In their study, this enhancement results from the significant latitudinal propagation of parameterized OGWs toward the jet axis which leads to a much smoother distribution of GWMFs. They showed that GWs located at 60° S at 1 hPa originate from a broad range of latitudes between 90°S and 30°S at the source level. Interestingly, the maximum in the source distribution is located around 60° S, though, emphasizing the role of a yet unidentified source.

Summarizing, the problem of parameterizing unresolved GWs and their sensitive interaction with large-scale dynamics is among the most uncertain aspects of climate modelling (Shepherd 2014, and references therein).

Despite the importance of the global UTLS (including the southern hemisphere) field campaigns investigating comprehensively the dynamical and chemical processes relevant for the structure of the SH-UTLS have not yet been performed. The DEEPWAVE project provided new data on specifically the dynamics aspects of gravity waves. The HIPPO and the ATom projects focused mainly on tropospheric composition and most other campaigns were dedicated to the Antarctic vortex mainly at higher altitudes. Measurements from commercial airliners (IAGOS) rarely extend to latitudes south of 35°S and cover only the lower part of the extratropical lowermost stratosphere. Therefore, we propose a HALO campaign, which addresses open key aspects regarding the SH-UTLS region.


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