LARGE-SCALE CIRCULATION CONDITIONS AFFECTINGTHE TYPHOON FORMATION OVER THENORTHWEST PACIFIC

A contrasting study of the large-scale circulation features responsible for months with many typhoons and months with tew typhoons has revealed that the frequency of typhoon formation over the Northwest Pacific is related to the following conditions:
Over the Northwest Pacific, a well-defined ITCZ (Intertropical Convergence Zone) extending eastward to 160°E was displaced to 20°N.At 200 mb, an extensive anomalous anticyclonic circulation prevails over the western and central Pacific.The condition characteristic of a break in the monsoon prevailed in India.The monsoon trough at 500 mb and at the surface over the Indian Peninsula was relatively weak and was accompanied by higher-than-normal rainfall in the northern part of India and lower-than-normal rainfall over the peninsula.In addition, the polar vortex tends to be weak and move to the side of the Northern Hemisphere, opposite to the North Pacific.Finally, abnormally warm water was observed over the Central and Eastern Pacific and abnormally cold water over the West Pacific in summer as well as in the preceding spring.