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    We develop a quantitative method for determining Stratosphere to Troposphere Transport events (STTs) and a minimum bound for this transported ozone quantity using ozonesondes over Melbourne, Macquarie Island, and Davis.



    Tropospheric ozone is important for both air quality and climate change. Over the industrial period, tropospheric ozone, the third most potent greenhouse gas has been estimated to exert a radiative forcing equivalent to a quarter of the CO2 forcing. The primary sources of tropospheric ozone are chemical creation and stratospheric input, estimated using a model ensemble to be \(5100\pm600\) Tg/yr and \(550\pm170\) Tg/yr, respectively. The primary sinks are chemical destruction and dry deposition, estimated to be \(4700\pm700\) Tg/yr and \(1000\pm200\) Tg/yr, respectively (Stevenson 2006).

    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 (Sprenger 2003, Mihalikova 2012); these are referred to as Statosphere - Troposphere Transport events (STTs). In the extra-tropics, STTs most commonly occur during synoptic-scale tropopause folds (Sprenger 2003) and are characterised by tongues of high Potential Vorticity (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 (Zanis 2003, Zhang 2014).

    While the amount of tropospheric ozone is small compared with that found in the stratosphere, it is an important constituent. The relative contributions to the tropospheric ozone budget of photochemistry and STT (dynamical transport) is still uncertain (Zanis 2003, (citation not found: paper in open tab). A high correlation is found between lower stratospheric and tropospheric ozone (Terao 2008) with the highest STT associated with the jet-streams over the oceans in winter. Irreversible STT of ozone have been shown to be important for explaining tropospheric ozone variability (Tang 2011).

    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. Hegglin et al. (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.

    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.