On the mechanisms of the historical lowest/highest tropical cyclone genesis over the western North Pacific in July/October 2020

Abstract

The tropical cyclone (TC) genesis over the western North Pacific (WNP) displayed two distinct features in 2020. The WNP experienced unprecedented TC silence in July 2020, whereas the TC genesis frequency reached the historical highest in October 2020 with all the TCs generated over the western part of the WNP. The present study reveals that the strong Indian Ocean basin warming (IOBW) and extremely prolonged Madden-Julian Oscillation (MJO) Phases 1 and 2 (MJO12) contributed to the TC genesis condition in different regions for the July TC silence. The IOBW explained the unfavorable TC genesis condition over the latitudes around 20°N, whereas the MJO12 was responsible for the unfavorable condition over the latitudes around 10°N. Although both IOBW and MJO12 resulted in a diminution in relative humidity and absolute vorticity, they exerted opposite influences on the vertical wind shear, which made IOBW more important than MJO12. The impacts of other factors, including the cooling in the Niño3.4 region and warming in the tropical Atlantic Ocean, were relatively smaller compared to IOBW and MJO12. The abnormal TC genesis in October 2020 is mainly attributed to the moderate La Niña and extremely prolonged MJO phase 5 (MJO5). The La Niña condition induced the shift of supportive genesis condition to the western part of the WNP, and MJO5 superimposed its impacts around the South China Sea and the Philippines. The analysis of the genesis potential index (GPI) highlights the crucial importance of the relative humidity to the October's aberrance. The La Niña played a major role in increasing the relative humidity over a vast area of the major genesis region (MGR), and the MJO5 played a subsidiary role. By inducing an increase in absolute vorticity over MGR and a decrease in vertical wind shear over the eastern MGR, the Pacific meridional mode also played a supportive role in October 2020.