Tropical cyclone genesis over the western north Pacific in La Niña decay summers: Comparison between 2018 and 2021

As the primary interannual signal of variability in the tropical ocean-atmosphere interaction, the El Niño-Southern Oscillation has a considerable impact on tropical cyclone (TC) activity over the western North Pacific (WNP). Both 2018 and 2021 were La Niña decay years, but TC activity over the WNP during the two summers (June–August) showed notable differences. In 2018, summer TC activity was unusually high with a total of 18 TCs, and the region of TC genesis was mainly in the central and eastern WNP. In contrast, only 9 TCs were generated in summer 2021, and the region of TC genesis was primarily in the western WNP. By comparing the characteristics of the large-scale environmental conditions over the regions of TC genesis, the thermal factors of the tropical oceans, and the activity of the Madden-Julian Oscillation (MJO), this study revealed the possible causes for the marked differences in TC genesis over the WNP during the two summers, which both had a similar background of La Niña decay. The Indian Ocean Basin Mode (IOBM) transitioned of a cold anomaly in the winter of 2017/2018 and persisted until summer 2018. At the same time, the Pacific Meridional Mode (PMM) maintained a positive phase, leading to eastward and northward displacement of the Western Pacific Subtropical High in summer, and eastward extension of the tropical monsoon trough, which presented conditions conducive to TC genesis over the Northwest Pacific. Moreover, the days when the MJO stagnated in phases 5 and 6 in the summer of 2018 increased by approximately 150% relative to climatological state, providing dynamic conditions favorable for TC formation. In 2021, the IOBM quickly turned to a warm anomaly in March and persisted until summer, whereas the PMM became a negative phase in January and remained so until summer. At the same time, the MJO stagnated in phases 2 and 3 for up to 47 days, with the center of convection located over the western Maritime Continent, producing conditions unconducive to TC genesis over the Northwest Pacific. Thus, despite being under a similar background of La Niña decaying year, the distinct evolutions of the IOBM, PMM, and MJO in spring and summer of 2018 and 2021 were the main causes of the notable differences in TC activity over the WNP during these two summers, and the anomalies in IOBM and MJO contributed more significantly than those of the PMM.