Interannual variation of summer monsoon rainfall over Nepal and its relation to Southern Oscillation Index

Nepal, lying in the southern periphery of the Tibetan Plateau receives about 80% of the total annual rainfall during summer monsoon (June-September). Rainfall analysis shows that summer monsoon is more active in the southern part of Nepal but in the high Himalayas and Trans-Himalayan region other weather systems like western disturbances are also as effective as monsoon in giving rainfall. The influence of Southern Oscillation (SO) in Nepal monsoon rainfall is found to be very significant. The years with significant negative (positive) Southern Oscillation Index (SOI) have less (more) rainfall in most of the cases during the 32-year period. This relationship is also found to vary with time. The years with deficient rainfall are associated most of the times with negative departure of SOI and the composite chart during these occasions shows about 95% area of Nepal experiencing below normal rainfall. Likewise at the time of positive departure of SOI, most of the region (94%) experienced above normal rainfall. There is a good relation between SOI and rainfall over Nepal during monsoon. The correlation coefficient between Nepal monsoon rainfall and monthly SOI shows a statistically significant in-phase relationship during and after monsoon but poor relation during the months prior to monsoon season. These results suggest that monsoon plays an active and effective role on the interannual variability including SOI.