Possible Relationship between NAO and Western North Pacific Typhoon Genesis Frequency

This study examined a strong positive correlation between the North Atlantic Oscillation (NAO) index during June and the total tropical cyclone (TC) genesis frequency in the western North Pacific during July and August. To investigate a possible cause for this relationship, the mean difference between the highest positive NAO years and the lowest negative NAO years was analyzed by dividing into when the El Nino and La Nina years were included and when the El Nino and La Nina years were not included. When the El Nino and La Nina years were included, for the positive NAO years, the TCs mostly occurred in the northwestern region of tropical and subtropical western Pacific, and showed a pattern that migrate from the sea northeast of the Philippines, pass the East China Sea, and move toward the mid-latitudes of East Asia. In contrast, for the negative NAO years, the TCs mostly occurred in the southeastern region of tropical and subtropical western Pacific, and showed a pattern that migrate westward from the sea southeast of the Philippines, pass the South China Sea, and move toward the southern coast of China and Indochinese peninsula. These two different TC migration patterns affect the recurving location of TC, and for the positive NAO years, the recurving of TC was averagely found to take place in the further northeast. In addition, the migration patterns also affect the TC intensity, and the TCs of positive NAO years had stronger intensity than the TCs of negative NAO years as sufficient energy can be absorbed from the ocean while moving north in the mid-latitudes of East Asia. The TCs of negative NAO years showed weak intensity as they get weaken or disappear shortly while landing on the southern coast of China and the Indochinese peninsula. On the other hand, the above result of analysis is also similarly observed when the El Nino and La Nina years were not included.