Atypical forcing embedded in typical forcing leading to the extreme summer 2020 precipitation in Nepal.

Nepal experienced record-breaking summer monsoon precipitation in 2020, becoming the wettest year of the last four decades (1980–2020). This paper explores the role of large-scale atmospheric circulation and sea surface temperature patterns in tipping an otherwise typical wet summer into an extreme one in 2020. The unusually high precipitation was fuelled by the Western North Pacific Anticyclone (WNPAC), concomitant with the strong presence of moist static energy and enhanced vertical velocity in and near Nepal. Sea surface temperature anomalies showed an intense Indian Ocean warming and features of a La Niña, succeeding a weak El Niño event that had sustained the WNPAC. The southwesterly winds on the northern flank of then WNPAC contributed excess moisture transport and convergence to Nepal, consequently resulting in elevated precipitation. This feature is consistent with previous wet summer years, but the presence of WNPAC in those years followed the El Niño event. Therefore, it appears that the Indo-western Pacific Ocean Capacitor (IPOC) effect exerts the above-normal precipitation for the summer of 2020 without involving the El Niño event, while anthropogenic warming may have further tipped the balance. [ABSTRACT FROM AUTHOR]