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Based on observations, the number of tropical cyclones (TCs) approaching the southern coast of Japan, including Tokyo, has increased over the last 40 years, and these TCs are increasing in strength when they approach land. The environmental conditions for TC development have become more favorable, with warmer sea surface temperature, less vertical wind shear, and more moisture in the atmosphere. In addition, the translation speed of TCs has decreased, which indicates a longer influence time. Comparison of the synoptic environment during July– October between the first (1980 – 1999, P1) and second (2000 – 2019, P2) 20 years shows that the sub-tropical high is strengthened in P2, where the western and northern edge of the high extends further into the west and north, respectively. Also, the westerly jet is weakened in P2 over and south of Japan in the middle-to-upper troposphere. These changes in the synoptic environment are considered to play a role in the increase in the number of TCs approaching Tokyo and also in the creation of more favorable conditions for TC development. The relationship between the changes in TC characteristics over the last 40 years and global warming is unclear. As the Pacific Decadal Oscillation is in a positive phase in P1 and in a negative phase in many years of P2, decadal oscillations may have played some role in the increase in the number of approaching TCs and in the changes in the synoptic environment.

Increase in the Number of Tropical Cyclones Approaching Tokyo since 1980