Statistical characteristics of the tropical cyclone activities over the Northwest Pacific and the Bay of Bengal during the Asian summer monsoon onset

Climatologically, the Asian summer monsoon (ASM) onset first occurs at the Bay of Bengal (BOB) and then spreads to the South China Sea (SCS) and Indian subcontinent. The onset usually lasts for about one month. The summer monsoon onsets in different areas show apparent inter-annual variations. The relationship between the tropical cyclone (storm) over the northwestern Pacific and BOB during the ASM onset is analyzed using the datasets from 1951 to 2010. Results show that there are tropical storm activities over the BOB during the BOB summer monsoon onset in 36 a. The earlier onset years are coincident with years of higher probability of up to 80% for tropical storm occurrence. For the three types of the BOB summer monsoon onset (early, normal and late), the high frequency of the BOB tropical storm appears several days before the BOB summer monsoon onset, and the tropical cyclones over the northwestern Pacific become active before the high frequency occurrence of the BOB tropical storm. For the early BOB summer monsoon onset years, tropical cyclones occur in about 40%-50% of the years over the northwestern Pacific, and the tropical cyclones usually occur over the SCS and near the Philippines in the second pentad of April. When the BOB summer monsoon onset is normal, the tropical cyclones usually occur to the east of the Philippines in the fourth pentad of April. When the BOB summer monsoon onset is late, the tropical cyclones most likely occur over the Pacific, which is the easternmost among the areas of tropical cyclone occurrence corresponding to the three types of monsoon onset in early May. During the SCS summer monsoon onsets from 1951 to 2010, there are tropical cyclone activities over the northwestern Pacific in 29 a. In the early and normal SCS summer monsoon onset years, the frequency of tropical cyclone activities is high. The tropical cyclone over the northwestern Pacific usually becomes active on the SCS summer monsoon onset date or several days later. That is, the tropical cyclone activity over the northwestern Pacific intensifies first before the Asian summer monsoon onset, followed by the enhanced BOB tropical storm activity and the Asian summer monsoon onset. In early and normal onset years of the Asian summer monsoon, the tropical cyclone activity over the northwestern Pacific intensifies again after the BOB summer monsoon onset, and the SCS summer monsoon breaks out subsequently.