Interannual and long-term variability in the North Atlantic Oscillation and Indian Summer monsoon rainfall

The interannual and decadal scale variability in the North Atlantic Oscillation (NAG) and its relationship with Indian Summer monsoon rainfall has been investigated using 108 years (1881-1988) of data. The analysis is carried out for two homogeneous regions in India, (Peninsular India and Northwest India) and the whole of India. The analysis reveals that the NAO of the preceding year in January has a statistically significant inverse relationship with the summer monsoon rainfall for the whole of India and Peninsular India, but not with the rainfall of Northwest India. The decadal scale analysis reveals that the NAO during winter (December-January-February) and spring (March-April-May) has a statistically significant inverse relationship with the summer monsoon rainfall of Northwest India, Peninsular India and the whole of India. The highest correlation is observed with the winter NAG. The NAO and Northwest India rainfall relationship is stronger than that for the Peninsular and whole of India rainfall on climatological and sub-climatological scales. Trend analysis of summer monsoon rainfall over the three regions has also been carried out. From the early 1930s the Peninsular India and whole of India rainfall show a significant decreasing trend (1% level) whereas the Northwest India rainfall shows an increasing trend from 1896 onwards, Interestingly, the NAO on both climatological and subclimatological scales during winter, reveals periods of trends very similar to that of Northwest Indian summer monsoon rainfall but with opposite phases. The decadal scale variability in ridge position at 500 hPa over India in April at 75 degrees E (an important parameter used for the long-range forecast of monsoon) and NAO is also investigated.