Variation of total column ozone along the monsoon trough region over north India

This study examined the total column ozone (TCO) variations over New Delhi (28.65 degrees N, 77.217 degrees E) and Varanasi (25.32 degrees N, 83.03 degrees E), which lie along the monsoon trough region, and over the tropical station Kodaikanal (10.23 degrees N, 77.46 degrees E), which lies outside the monsoon trough. Monthly, seasonal and annual TCO variations were determined using data from ground-based Dobson spectrophotometers during 2000-2008, Brewer spectrophotometers during 2000-2005 and the satellite-based Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) during 2002-2008. We found that Dobson, Brewer and SCIAMACHY TCO variations showed negative trends, indicating a decreasing tendency during the period studied at all three stations. Over Varanasi, the trend decreased further by about 3 DU year-1. Quasi-Biennial Oscillation (QBO) influences were seen in the time series of TCO over New Delhi and Varanasi, and weaker QBO signals over Kodaikanal. Comparisons were made between ground-based Dobson and Brewer spectrophotometer and SCIAMACHY satellite monthly mean TCO values. The differences between SCIAMACHY and Dobson TCO were 0.4-4.2% for New Delhi and 2.3-6.2% for Varanasi. The differences between SCIAMACHY and Brewer TCO values were 2.0-6.4% for Kodaikanal. In the peak monsoon months (July and August), decreases in TCO values over New Delhi and Varanasi (the monsoon trough region) may be due to the deep convection present during the monsoon season. During the monsoon season, several intense cyclonic systems appear over the monsoon trough region and may cause lowering of the TCO. Kodaikanal shows opposite features, with high values being observed during the peak monsoon months. TCO values over New Delhi were found to be higher than those over Varanasi and Kodaikanal, and TCO values over Varanasi were higher than over Kodaikanal. It was concluded that TCO values increase with increasing latitude.