Long-term changes in total ozone column content and its association with stratospheric temperature over two neighbouring tropical Asian stations

This paper focuses on the long-term declining change in total ozone column TOC derived from satellite measurements over a 25 year period over two neighbouring tropical Asian sites, Karachi, and Mt Abu. A strong declining trend was observed in TOC at both sites, with a significance level of over 95% and a higher magnitude of 4 to 10 DU per decade in September to December and a weak statistical significance level of below 85% and a lower magnitude of 2 to 4 DU per decade in pre-monsoon months. However, during the monsoon months, a small declining trend of about 1 DU per decade was observed, but this variation is statistically insignificant. Further, the long-term changes in TOC exhibit seasonal dependence with a more negative change of 10 DU per decade in winter over Karachi and 7 DU per decade over Mt Abu. The consequence of such a net long-term declining change in TOC as high as 10 DU per decade is expected to have serious environmental implications due to an overall increase in ground-level solar UV radiation of 18% over the normal values in the tropics. In order to identify some plausible causes of this depletion trend in ozone concentration with stratospheric temperature and solar activity, it is clear that there is a strong relationship between the seasonal dependence of the long-term declining trend of TOC on air temperature at 10 mb or stratospheric cooling. At the same time, there is a less significant long-term variation in TOC due to altered solar activity levels.