Variability of summer monsoon rainfall over Indo-Gangetic plains in relation to El-Nino/La-Nina

Abstract

The Indo-Gangetic plains (IGPs) incorporate seven meteorological subdivisions, viz. Punjab, West Uttar Pradesh and East Uttar Pradesh, Bihar, Jharkhand, and Gangetic West Bengal. IGPs are generally characterised by fertile soils, favourable climate and an abundant supply of water. They are considered as the ‘breadbasket’ for much of South Asia. The main source of rainfall is the southwest monsoon which is normally sufficient for agriculture. Based on observed available rainfall data, a time series of summer monsoon rainfall (SMR) (1871–2012) over the IGPs has been developed. Decadal and 30-year variability in SMR reveal alternate epochs of above and below normal rainfall. These epochs tend to last for a decade or two. Although small in magnitude, the long-term trend (1871–2012) in SMR shows a decreasing nature. On the other hand, the decadal variability in the SMR reveal a dominance of a positive phase in the past (during the period 1913–1922) and a negative phase during the recent decade (1993–2002). Recent climatology (1983–2012) shows a downward trend of the rainfall anomalies with the turning point around 1997 (associated with century’s strongest El-Nino, in 1997). The period before 1997 depicts subdued above normal rainfall, while the period after 1997 depicts below normal rainfall activity. Furthermore, the study reveals that SMR variability is enhanced during the El-Nino years and is suppressed during the La-Nina years, but the presence of El-Nino (La-Nina) does not guarantee droughts (floods) over the Gangetic plains. The spatiotemporal changes in rainfall activity are attributed to global warming and associated changes in the Indian summer monsoon circulations and the general atmospheric circulation.