Climate warming and decreasing total column ozone over the Tibetan Plateau during winter and spring

The long-term trends of the total column ozone (TCO) over the Tibetan Plateau (TP) and factors responsible for the trends are analysed in this study using various observations and a chemistry–climate model (CCM). The results indicate that the total column ozone low (TOL) over the TP during winter and spring is deepening over the recent decade, which is opposite to the recovery signal in annual mean TCO over the TP after mid-1990s. The TOL intensity is increasing at a rate of 1.4 DU/decade and the TOL area is extending with 50,000 km2/decade during winter for the period 1979–2009. The enhanced transport of ozone-poor air into the stratosphere and elevated tropopause due to the rapid and significant warming over the TP during winter reduce ozone concentrations in the upper troposphere and lower stratosphere and hence lead to the deepening of the TOL. Based on the analysis of the multiple regression model, the thermal dynamical processes associated with the TP warming accounts for more than 50% of TCO decline during winter for the period 1979–2009. The solar variations during 1995–2009 further enlarge ozone decreases over the TP in the past decade. According to the CCM simulations, the increases in NOx emissions in East Asia and global tropospheric N2O mixing ratio for the period 1979–2009 contribute to no more than 20% reductions in TCO during this period

Tropopause Folds Over the Eastern Mediterranean and the Middle East in EMAC Simulations: Implications for Summertime Tropospheric Ozone

A 35 year climatological analysis (1979–2013) of tropopause fold frequency has been performed over the eastern Mediterranean and the Middle East, focusing on the implications for tropospheric ozone. The analysis is based on simulations with the EMAC (ECHAM5-MESSy) atmospheric chemistry climate model using a T42L90MA resolution, nudged tropospheric meteorology towards ERA-Interim reanalysis data and a tracer for stratospheric ozone (O3s). We implement a 3-D labeling algorithm, in order to detect tropopause folding events in EMAC simulations as areas of multiple crossings of the dynamical tropopause. A clear hot spot of tropopause folds occurs over the broader eastern Mediterranean and Middle East region during the summer season. A significant contribution of tropopause folds to the summertime pool of high tropospheric ozone over the eastern Mediterranean is identified during the selected fold events. Finally, a year-to-year analysis indicates a significant positive correlation between the observed near surface ozone and EMAC simulated middle/lower tropospheric ozone over the eastern Mediterranean