Stratosphere-troposphere exchanges of air mass and ozone concentration in the Last Glacial Maximum

Abstract

Stratosphere-troposphere exchange (STE) of ozone represents a significant source term in the tropospheric ozone budget and can impact surface ozone concentrations, tropospheric oxidation capacity, and methane lifetime. Using the Whole Atmosphere Community Climate Model 6, changes in the air mass and ozone STEs in the Last Glacial Maximum (LGM) as compared with preindustrial (PI) climate are investigated. We deploy the dynamic isentropic surfaces that are determined by fitting to the tropical tropopauses as the upper boundary of the lowermost stratosphere in a mass budget approach, which are suitable to estimate air mass and ozone STEs and their changes in different climates. It is shown that the magnitude of ozone STE is decreased by 14-19%, 18-24%, 19-20%, 16-21%, 15-21% over the Northern hemisphere extratropics, Southern hemisphere extratropics, the tropics, the extratropics, and the globe, respectively, in the LGM versus PI. The extratropical and global decreases are mainly caused by decreased ozone in the extratropical lower stratosphere associated with a weakening of Brewer-Dobson circulation, while changes in air mass fluxes play a minor role because the effects of weakening Brewer-Dobson circulation and increased isentropic density partly cancel each other. Analysis of the modelled tropospheric ozone budget indicates that the ozone STE in the LGM is 28% of the tropospheric ozone production rate, as compared to about 9% in the modern climate (year 2000) and 19% in the PI.