Effects of Upper Atmospheric Chemistry Uncertainties on Ozone and Circulation
Ozone (O3) is produced naturally in the upper atmosphere (stratosphere). It protects life at Earth’s surface acting as a shield from harmful solar radiation. As climate changes, and as humans have emitted O3-destroying substances over recent decades, the amount of stratospheric O3 has changed. Since O3 absorbs sunlight heating the local air, changes in O3 have important effects on stratospheric temperatures and circulation. However, climate scientists are limited in their ability to simulate stratospheric O3 and dynamics due to basic uncertainties about the stratospheric chemical reactions that lead to O3 formation. In a recent Department of Energy study, researchers varied the O3 formation rate based on uncertainties in the observed oxygen amount, using the Community Atmosphere Model. The result was dramatic changes in O3 in the lower stratosphere and in the temperature. However, the key question is whether this changes overall stratospheric circulation and the net flux of stratospheric O3 into the lower atmosphere (troposphere). The team found that reducing the oxygen counterintuitively resulted in increased O3 in the lowermost stratosphere (less shielding of sunlight from above) and increased temperatures by up to 2 °C, creating greater stratification near the tropopause (the thermal boundary between the lower and upper atmosphere). As a consequence, the dynamical coupling between stratosphere and troposphere changes, affecting the tropical annual cycle of temperature and O3 in the lower stratosphere, but the overall circulation of the stratosphere is hardly altered. They also found that this warming in the lower-middle stratosphere due to increased O3 production has an impact on tropospheric climate as well. Uncertainties in the transport of certain chemical species that affect O3 production from the lower atmosphere to the stratosphere may also be important and should be investigated in future studies.
Hsu, J., M. Prather, D. Bergmann, and P. Cameron-Smith. 2013. “Sensitivity of Stratospheric Dynamics to O3 Production,” Journal of Geophysical Research Atmospheres 118, 8984-99. DOI:10.1002/jgrd.50689.