Abstract

Summer monsoon rainfall variability from June to September over Nepal (SMRN) was statistically investigated with a focus on the large-scale atmospheric patterns such as El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), as well as examination of moisture fluxes toward Nepal. A main large-scale pattern influential on the SMRN variability was explained by ENSO as a significant correlation with Southern Oscillation Index (SOI): i.e., when La Niña occurs, the eastward moisture flux is blocked over Bay of Bengal (BOB) by the anomalous circulation extending from the Pacific. The local-scale condition for higher SMRN is implied by two moisture routes, a main one of which is moisture carried along the eastern arm of the low pressure in northeastern India. A minor one is the route from the Arabian Sea (AS). This aspect was proved by a correlation between SMRN anomalies and the moisture fluxes. As implied by a high cloud core over northern India, moisture accumulates and tends to flow toward Nepal producing high SMRN. The significant correlation between SMRN and moisture in June suggested that an early onset of monsoon is an additional indication of high SMRN. Since the correlation between SMRN and moisture fluxes was not as high as that between SMRN and SOI, the additional cause has to be sought, this could be related with local re-evaporation.

Keywords: SMRN; IOD; SOI; Moisture flux