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\subsection{Background}  %Stratospheric air is relatively rich in ozone, which provides us with the ozone layer and protects us from much UV radiation.   %In the troposphere ozone is a trace gas mostly generated by photochemistry, due to both pollution and natural processes.   %Stratosphere to Troposphere Transport of ozone events(STTs) are a relatively small source of tropospheric ozone, however the regional %effect can be quite large and the dynamics which cause these events are not perfectly understood \cite{Mihalikova2012,Zhang_2014}.  Ozone is present in the troposphere due to a variety of dynamical and photochemical processes, including downward transport from the ozone-rich stratosphere and anthropogenic pollution. Ozone-rich air mixes irreversibly down from the stratosphere during meteorologically conducive conditions \cite{Sprenger2003, Mihalikova2012}; these are referred to as Statosphere - Troposphere Transport (STT) events. In the extra-tropics, STT most commonly occurs during synoptic-scale tropopause folds \cite{Sprenger2003} and are characterised by tongues of high PV air descending to low altitudes. These tongues become elongated and filaments separate from the tongue which mix into tropospheric air. Stratospheric ozone brought deeper (lower) into the troposphere is more likely to affect the surface ozone budget and tropospheric chemistry \cite{Zanis2003}.  While the amount of tropospheric ozone is small compared with that found in the stratosphere, it is an important constituent \textbf{XXXX - Robyn please add some chemistry stuff here...}. constituent.  The relative contributions to the tropospheric ozone budget of photochemistry and STT (dynamical transport) is still uncertain \cite{Zanis2003}. A high correlation is found between lower stratospheric and tropospheric ozone \cite{Terao_2008} with the highest STT associated with the jet-streams over the oceans in winter.  In a future climate, a warmer, wetter troposphere will change the chemical processing of ozone, and also dynamical processes such as STT, boundary layer ventilation and convection changes will alter tropospheric ozone distributions.  \cite{Hegglin_2009} estimate that climate change will lead to increased STT of the order of 30 (121) Tg yr-1 relative to 1965 in the southern (northern) hemisphere due to an acceleration in the Brewer Dobson circulation.   %Hegglin, M. I., and T. G. Shepherd (2009), Large climate-induced changes in ultraviolet index and stratosphere-to-troposphere ozone flux, Nature Geosci, 2(10), 687–691, doi:10.1038/NGEO604.  % AIMs paragraph  Using several years of ozonesonde flights from three mid-latitude locations in the Southern Hemisphere, we will characterise the seasonal cycle of STT events and determine their contribution to the total amount of tropospheric ozone. We will examine the depth of the intrusions and using case studies, relate these STT to meteorological events.  %\textbf{Jesse: I don't think this needs a separate subsection, usually it's the last paragraph of the preceding section}  %We provide one method of quantitatively characterising STTs and examine in detail a few events over Davis (78E, 68.6S), Macquarie island station (158.9E, 54.6S), and Melbourne (145E, 37.7S).  %Examine the coincidental weather and determine possible causes or necessary conditions of an event over the three southern hemisphere sites.  %We also examine seasonality and look at the distributions of ozone intrusions classified by intrusion depth.