Climatological relationships among the tropical cyclone(TC)frequency,duration,intensity and activity regions over the Western Pacific are explored based on long-term best track data.Frequent TC occurrence does not necessarily imply a long duration of TCs in the same periods.Three types of relationship between TC number and duration in the period 1945-2007 were identified in this study:low frequency and short duration during 1945-1955(Period I);high frequency and short duration in the 1960s(Period II);and high frequency and long duration in the 1990s(Period III).TC activity regions differed among the three periods.During Period I,the main activity regions were over the ocean east of the Philippines(120°-140°E).During period II,two prevailing storm tracks extended west-northwest between 110° and 147°E.During period III,TCs had an extensive activity region from 110° to 160°E.TC intensity is related closely to activity regions.Most strong TCs developed over the ocean far from the Philippines,and had a northwestward track.Our results also show that the relationships between TC frequency,duration and their active regions are modulated strongly by broad-scale vertical motion,geopotential height and horizontal wind anomalies.
