Long-term trends and decadal solar variability in ozone near the tropopause over the Indian region

Abstract

To investigate the long-term trends and effects of decadal solar variability in the upper tropospheric ozone, data obtained from the Stratospheric Aerosol and Gas Experiment II (SAGE II) aboard the Earth Radiation Budget Satellite (ERBS) during the period 1985–2005 were analysed using a multifunctional regression model over the Indian region (8–40° N; 65–100° E). Analysis of time series spanning these years shows statistically insignificant trends (at the two-sigma level (95% confidence level)) at upper tropospheric pressure levels (10−16 km). This period covers two solar cycles, one lasting from 1985 to 1995 and the other from 1996 to 2005; these are referred to as decade I and decade II, respectively. Since temporal variation in ozone number density indicates 11 year periodicity, trends are statistically significant when calculated separately during each solar cycle. Trend analysis indicates statistically significant positive trends (0.7 ± 1.7% to 3.9 ± 2.9% year⁻¹ during decade I, and 2.2 ± 1.6% to 4.5 ± 3.0% year⁻¹ during decade II). In general, higher ozone trends are observed during decade II. Seasonal variation in trends during decade II shows increasing trends during the pre-monsoon (0.8−3.8% year⁻¹), monsoon (0.8−7.1% year⁻¹), and post-monsoon (2.8−8.0% year⁻¹) seasons. The annually averaged solar signal in ozone is found to be of the order of around −5 ± 4.3% to −13.8 ± 6.7%/(100 sfu). Results obtained in the present study are also compared with those obtained by other researchers.

Acknowledgements

We acknowledge with thanks Director IITM for his encouragement during the course of this study. We also thank the SAGE II team for providing data and necessary information on data quality. The authors are grateful to anonymous reviewers for their critical comments.