ALBERT

Description: Using a state-of-the-art chemistry–climate model we investigate the future change in stratosphere–troposphere exchange (STE) of ozone, the drivers of this change, as well as the future distribution of stratospheric ozone in the troposphere. Supplementary to previous work, our focus is on changes on the monthly scale. The global mean annual influx of stratospheric ozone into the troposphere is projected to increase by 53 % between the years 2000 and 2100 under the RCP8.5 greenhouse gas scenario. The change in ozone mass flux (OMF) into the troposphere is positive throughout the year with maximal increase in the summer months of the respective hemispheres. In the Northern Hemisphere (NH) this summer maximum STE increase is a result of increasing greenhouse gas (GHG) concentrations, whilst in the Southern Hemisphere(SH) it is due to equal contributions from decreasing levels of ozone depleting substances (ODS) and increasing GHG concentrations. In the SH the GHG effect is dominating in the winter months. A large ODS-related ozone increase in the SH stratosphere leads to a change in the seasonal breathing term which results in a future decrease of the OMF into the troposphere in the SH in September and October. The resulting distributions of stratospheric ozone in the troposphere differ for the GHG and ODS changes due to the following: (a) ozone input occurs at different regions for GHG- (midlatitudes) and ODS-changes (high latitudes); and (b) stratospheric ozone is more efficiently mixed towards lower tropospheric levels in the case of ODS changes, whereas tropospheric ozone loss rates grow when GHG concentrations rise. The comparison between the moderate RCP6.0 and the extreme RCP8.5 emission scenarios reveals that the annual global OMF trend is smaller in the moderate scenario, but the resulting change in the contribution of ozone with stratospheric origin (O3s) to ozone in the troposphere is of comparable magnitude in both scenarios. This is due to the larger tropospheric ozone precursor emissions and hence ozone production in the RCP8.5 scenario.

Description: Model simulations consistently project an increase in the stratosphere-troposphere exchange (STE) of ozone in the future. Both, a strengthened circulation and ozone recovery in the stratosphere contribute to the increased mass flux. In our study, we investigate with a state-of-the-art chemistry-climate model the drivers of future STE change as well as the change in the distribution of stratospheric ozone in the troposphere. Our focus is on the investigation of the changes on the monthly scale. The global mean influx of stratospheric ozone into the troposphere is projected to increase between the years 2000 and 2100 by 53 % under the RCP8.5 greenhouse gas scenario. We find the largest increase of STE in the NH in June due to increasing greenhouse gas (GHG) concentrations. In the SH the GHG effect is dominating in the winter months, while decreasing levels of ozone depleting substances (ODS) and increasing GHG concentrations contribute nearly equally to the increase in SH summer. A large ODS-related ozone increase in the southern hemisphere (SH) stratosphere leads to a change in the seasonal breathing term which results in a future decrease of the ozone mass flux into the troposphere in the SH in September and October. We find that the GHG effect on the STE change is due to circulation and stratospheric ozone changes, whereas the ODS effect is dominated by the increased ozone abundance in the stratosphere. The resulting distributions of stratospheric ozone in the troposphere for the GHG and ODS changes differ because of the different regions of ozone input (GHG: midlatitudes; ODS: high latitudes) and the larger increase of tropospheric ozone loss rates due to GHG increase. Thus, the model simulations indicate that stratospheric ozone is more efficiently mixed to lower levels if only ODS levels are changed. The increase of the stratospheric ozone column in the troposphere explains more than 80 % of the tropospheric ozone trend in NH spring and in the SH except for the summer months. The importance of the future stratospheric ozone contribution to tropospheric ozone burdens therefore depends on the season.

Description: Three types of reference simulations, as recommended by the Chemistry–Climate Model Initiative (CCMI), have been performed with version 2.51 of the European Centre for Medium-Range Weather Forecasts – Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model: hindcast simulations (1950–2011), hindcast simulations with specified dynamics (1979–2013), i.e. nudged towards ERA-Interim reanalysis data, and combined hindcast and projection simulations (1950–2100). The manuscript summarizes the updates of the model system and details the different model set-ups used, including the on-line calculated diagnostics. Simulations have been performed with two different nudging set-ups, with and without interactive tropospheric aerosol, and with and without a coupled ocean model. Two different vertical resolutions have been applied. The on-line calculated sources and sinks of reactive species are quantified and a first evaluation of the simulation results from a global perspective is provided as a quality check of the data. The focus is on the intercomparison of the different model set-ups. The simulation data will become publicly available via CCMI and the Climate and Environmental Retrieval and Archive (CERA) database of the German Climate Computing Centre (DKRZ). This manuscript is intended to serve as an extensive reference for further analyses of the Earth System Chemistry integrated Modelling (ESCiMo) simulations.