Abstract. Using a transient simulation for the period 1960–2100 with the state-of-the-art ECHAM5/MESSy Atmospheric Chemistry
(EMAC) global model and a tropopause fold identification algorithm, we explore the future projected changes in tropopause
folds, stratosphere-to-troposphere transport (STT) of ozone, and tropospheric ozone under the RCP6.0 scenario. Statistically
significant changes in tropopause fold frequencies from 1970–1999 to 2070–2099 are identified in both hemispheres, regionally
exceeding 3 %, and are associated with the projected changes in the position and intensity of the subtropical
jet streams. A strengthening of ozone STT is projected for the future in both hemispheres, with an induced increase
in transported stratospheric ozone tracer throughout the whole troposphere, reaching up to 10 nmol mol−1 in
the upper troposphere, 8 nmol mol−1 in the middle troposphere, and 3 nmol mol−1 near the surface. Notably, the
regions exhibiting the largest changes of ozone STT at 400 hPa coincide with those with the highest fold frequency
changes, highlighting the role of the tropopause folding mechanism in STT processes under a changing climate. For both the
eastern Mediterranean and Middle East (EMME) and Afghanistan (AFG) regions, which are known as hotspots
of fold activity and ozone STT during the summer period, the year-to-year variability of middle-tropospheric
ozone with stratospheric origin is largely explained by the short-term variations in ozone at 150 hPa and
tropopause fold frequency. Finally, ozone in the lower troposphere is projected to decrease under the RCP6.0
scenario during MAM (March, April, and May) and JJA (June, July, and August) in the Northern Hemisphere and during
DJF (December, January, and February) in the Southern Hemisphere, due to the decline of ozone precursor emissions
and the enhanced ozone loss from higher water vapour abundances, while in the rest of the troposphere ozone shows
a remarkable increase owing mainly to the STT strengthening and the stratospheric ozone recovery.
Model Animals for Space Experiments - Species Flown in the Past and Candidate Animals for the Future Experiments