Dominant pattern of year-to-year variability of summer precipitation in Nepal during 1987–2015

The study of inter-annual variability of summer precipitation is essential for Nepal as it manifests extreme events and thus impacts the socio-economy of the country. This study presents the dominant pattern of summer precipitation (JJAS) and its controlling phenomena during 1987–2015 over Nepal. Here, we used 0.25° × 0.25° APHRODITE and ERA5 reanalysis datasets to compute the empirical orthogonal function (EOF), wavelet, regression, and correlation analysis. The results show two dominant modes of EOF of summer precipitation over Nepal. EOF1 displays a single mode of variability with a 36.3% of variance, whereas EOF2 displays dipole mode of variability with 19.2% of variance, characterized by positive (negative) anomalies in the western (eastern) Nepal. The potential mechanism for the two leading modes is different. PC1 is highly negatively correlated with summer El Nino-Southern Oscillation (ENSO), whereas PC2 is the influence of the Indian Ocean Basin Mode (IOBM), which shows a strong significant negative relationship with the Indian Ocean from preceding spring to concurrent summer. Furthermore, PC1 is associated with strong easterly monsoonal wind flow from the Pacific Ocean and strengthening monsoonal trough over Nepal. However, in PC2, an anomalous low-level vertical circulation was observed between the Indian Ocean (divergence and anticyclonic circulation) and northern India (convergence and cyclonic circulation). The anomalous descending motion and cyclonic circulation strengthen the moisture transport to northern India, which plays a crucial role in increasing precipitation in western Nepal than eastern Nepal. This study provides useful information and underlying mechanism related to the summer monsoon variability of Nepal.