Linking Tropical Cyclone Number Over the Western North Pacific with Sea Surface Temperatures

The analyses based on the observational data show that the tropical cyclone (TC) numbers tend to become decreasing over some oceanic basins during the recent multiple decades despite a rise in their sea surface temperatures (SSTs). A methodology of understanding the mechanism responsible for such seemingly counter intuitive phenomena is suggested in this chapter and thus the causality between the unusually heterogeneous pattern of SSTs in warming environment and TC frequency over, as an example, the western North Pacific (WNP) is explained. This methodology is important in that it provides an insight into the problem why under global warming high SSTs as one of the necessary conditions for TC genesis should unexpectedly contribute to TC number reduction, though locally, and furthermore, in view of that such phenomena have also been observed over the other oceanic basins than the WNP and that the principle of the technique is universal, this procedure of analysis can be expected to apply globally to examine the extreme weather/climate extent frequency trend in the worldwide warning conditions.