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 Niño-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.

对尼泊尔夏季降水年际变化的研究至关重要，因为尼泊尔表现出极端事件，因此影响了该国的社会经济。这项研究提出了尼泊尔1987-2015年夏季降水的主要模式及其控制现象。在这里，我们使用0.25°×0.25°APHRODITE和ERA5重新分析数据集来计算经验正交函数（EOF），小波，回归和相关分析。结果表明，尼泊尔夏季降水EOF的两种主要模式。EOF1显示具有36.3％的方差的单一变异模式，而EOF2显示具有19.2％的方差的偶极变异模式，其特征是尼泊尔西部（东部）的正（负）异常。两种主导模式的潜在机制是不同的。PC1与夏季厄尔尼诺-南方涛动（ENSO）高度负相关，而PC2受印度洋海盆模式（IOBM）的影响，从先前的春季到夏季的夏季，印度洋海盆模式显示出与印度洋的强烈负相关。此外，PC1与来自太平洋的强东风季风风和尼泊尔上空的季风槽有关。但是，在PC2中，在印度洋（发散和反气旋环流）和印度北部（会聚和气旋环流）之间观察到了异常的低水平垂直循环。异常的下降运动和气旋环流加强了向印度北部的水分输送，这在尼泊尔西部比尼泊尔东部增加降水方面起着至关重要的作用。