Trends in Precipitation Extremes over India - (IMD), Pune

1. IntroductionAbout 60-90% of the annual rainfall over India is received during thesouthwest monsoon season (June to September), which is vital for the economy ofthe country. Inter-annual variation of seasonal and annual rainfall is a subject formore serious research work in India. However, information about the long termtrends of rainfall is also important. Previous studies have addressed the issue ofchanges in the mean rainfall. For example, Guhathakurta and Rajeevan (2006) haveshown that there is no long term trend in the southwest monsoon seasonal rainfallover the country as a whole, but there are significant regional variations. However,changes in extreme precipitation are also equally important to investigate. Impact ofclimate changes are felt most strongly through changes in climate extremes. Anypositive or increasing trend in the extreme rainfall events is also a serious concern.The recent extreme heavy rainfall event occurred over Mumbai on 26 th July 2005prompts us to think whether there is any significant trend in extreme rainfall eventsover different parts of India.One of the most significant consequences of global warming due to increasein greenhouse gases would be an increase in magnitude and frequency of extremeprecipitation events. These increased extreme precipitation events can be attributedto increase in moisture levels, thunderstorm activities and large scale storm activity.In the global warming scenario, climate models generally predict an increase in largeprecipitation events (Houghton et al 2001). The numerical modelling community anddata analysts have shown interest on the issue of extreme events occurring aroundthe world.The recent studies have shown that there is an increasing trend of extremeprecipitation events in USA and Australia (Easterling et al. 2000, Haylock andNicholls 2000, Groisman et al. 2001; Kunkel 2003), western New Zealand(Salingerand Griffiths 2001), the UK in winter (Osborn et al. 2000), and south Africa(Fauchereau et al. 2003). Extreme rainfall events in Canada show no trend (Zhang2