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 Os cillation (NAG) and its relationship with Indian Summer monsoon rainfa ll has been investigated using 108 years (1881-1988) of data. The anal ysis is carried out for two homogeneous regions in India, (Peninsular India and Northwest India) and the whole of India. The analysis reveal s that the NAO of the preceding year in January has a statistically si gnificant inverse relationship with the summer monsoon rainfall for th e whole of India and Peninsular India, but not with the rainfall of No rthwest 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 Pe ninsular and whole of India rainfall on climatological and sub-climato logical scales. Trend analysis of summer monsoon rainfall over the thr ee regions has also been carried out. From the early 1930s the Peninsu lar India and whole of India rainfall show a significant decreasing tr end (1% level) whereas the Northwest India rainfall shows an increasin g trend from 1896 onwards, Interestingly, the NAO on both climatologic al and subclimatological scales during winter, reveals periods of tren ds very similar to that of Northwest Indian summer monsoon rainfall bu t with opposite phases. The decadal scale variability in ridge positio n at 500 hPa over India in April at 75 degrees E (an important paramet er used for the long-range forecast of monsoon) and NAO is also invest igated.