The seasonal variations of ozone in the stratosphere and their hemispheric asymmetries

The ozone seasonal variations and ozone trends in the upper stratosphere were investigated by using SBUV/SBUV2, MLS, and SAGE I/II observations, and numerical experiments, with the focus on the asymmetries between the Southern Hemisphere and the Northern Hemisphere. Both the EOF analyses and the calculated annual amplitudes from SBUV ozone measurements show that the maximum annual variations occur at mid-high latitudes around 1–2 mb. There is a different intensity in ozone annual cycle between two hemispheres, with the annual amplitude about 15% larger in the Southern Hemisphere. The trend calculations indicate that ozone annual cycle in the upper stratosphere has decreased with time in 1980s, with the more negative trend occurring in the Southern Hemisphere. The SAGE I/II version 6.0 ozone data show a similar result in ozone annual trends in 1980s. Furthermore, the comparisons of SAGE ozone trends show that ozone trends in the upper stratosphere are more negative from 1979–1988 than from 1979–1999. The hemispheric asymmetries of ozone trends are also larger in the 1979–1988 years period.; The annual variations of ozone and its hemispheric asymmetries are simulated reasonably well at 45° latitude around 2mb with a chemical box model. The temperature differences between two hemispheres are demonstrated to be responsible for the hemispheric asymmetries in the annual variations of ozone. The simulated CIO variations are similar to the UARS observations. The ozone trend experiments show that ozone decreases by about 1%/year from 1979 to 1999, with a maximum loss occurring in winter. Differences in ozone loss between two hemispheres are also simulated with about 2% more ozone decrease being indicated to occur in the Southern Hemisphere from winter to early spring. These hemispheric asymmetries in ozone are shown to be caused by temperature and CH₄ asymmetries. In addition, the numerical experiments also show that the long term trends in CH ₄, H₂O, and temperature can contribute to the changes of ozone trend.