What caused the increase of tropical cyclones in the western North Pacific during the period of 2011–2020?

Based on satellite data after 1979, we find that the tropical cyclone (TC) variations in the Western North Pacific (WNP) can be divided into three-periods: a high-frequency period from 1979 to 1997 (P1), a low-frequency period from 1998 to 2010 (P2), and a high-frequency period from 2011 to 2020 (P3). Previous studies have focused on WNP TC activity during P1 and P2. Here we use observational data to study the WNP TC variation and its possible mechanisms during P3. Compared with P2, more TCs during P3 are due to the large-scale atmospheric favorable conditions of vertical velocity, relative vorticity and relative humidity. Warm sea surface temperature (SST) anomalies are found during P3 and migrate from east to west, which is also favorable for TC genesis. The correlation between the WNP TC frequency and SST shows a significant positive correlation around the equator and a significant negative correlation around 36°N, which is similar to the warm phase of the Pacific Decadal Oscillation (PDO). The correlation coefficient between the PDO and TC frequency is 0.71, above the 95% confidence level. The results indicate that the increase of the WNP TC frequency during 2011–2020 is associated with the PDO and warm SST anomalies.