On the impact of future climate change on tropopause folds and tropospheric ozone

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 are identified in both Hemispheres, occasionally exceeding 3 %, which are associated with the projected changes in the position and intensity of the subtropical jet streams. A strengthening of ozone STT is projected for future at both Hemispheres, with an induced increase of transported stratospheric ozone tracer throughout the whole troposphere, reaching up to 10 nmol/mol in the upper troposphere, 8 nmol/mol in the middle troposphere and 3 nmol/mol near the surface. Notably, the regions exhibiting the maxima changes of ozone STT at 400 hPa, coincide with that of the highest fold frequencies, highlighting the role of tropopause folding mechanism in STT process under a changing climate. For both the eastern Mediterranean and Middle East (EMME), and the 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 of ozone at 150 hPa and tropopause folds 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) at the Northern Hemisphere, and during DJF (December, January and February) at the Southern Hemisphere, due to the decline of ozone precursors emissions, while in the rest of the troposphere ozone shows a remarkable increase owing to the STT strengthening.