Ozone budget and trends in AerChemMIP integrations using UKESM-1

CMIP6 offers the opportunity for the first time to connect changes in emissions to climate forcing via the underpinning chemical processes using Earth system models. Of particular interest are the radiatively active gases tropospheric ozone and methane, which form part of a coupled chemical system, mediated by interactions between NOx, OH, CO and other VOCs.

For CCMI, we undertook a detailed analysis of trends in the tropospheric and stratospheric ozone budget. We diagnosed physical and chemical tendencies in both regions, and showed that the recent historical trend in tropospheric ozone is controlled by a delicate balance of large chemical and physical terms, driven partly by changes in stratospheric composition [Griffiths et al., AerChemMIP, Reading 2018].

For CMIP6 and AerChemMIP, we will lead analyses across the multi-model dataset to address the chemical ozone budget in the troposphere, with a focus on chemical production and loss, transport across the tropopause and dry deposition. We aim to extend our earlier study to address the origin of the frequently observed low bias in methane lifetime in chemistry-climate models via analysis of ozone budget, biases in tropospheric ozone with respect to observations and the connection of these to the variation in OH and methane levels.

At the CMIP6 meeting we will show a preliminary analysis, based on the UKESM1 PiControl transient integrations covering the period 1850-2010. We will show model trends in ozone, OH and impacts on tropospheric methane lifetime. We aim to show how the variation in these quantities across the historical period can be related to the variation in OH source and sink terms and to the ozone production efficiency over this time. Finally, we connect these changes in CH4 and O3 to the radiative impact, and we will use our participation to invite discussion with interested colleagues on the above.