How Strong ENSO Events Affect Tropical Storm Activity over the Western North Pacific*

Abstract

An analysis of 35-yr (1965-99) data reveals vital impacts of strong (but not moderate) El Nin o and La Nin a events on tropical storm (TS) activity over the western North Pacific (WNP). Although the total number of TSs formed in the entire WNP does not vary significantly from year to year, during El Nin o summer and fall, the frequency of TS formation increases remarkably in the southeast quadrant (0-17N, 140E-180) and decreases in the northwest quadrant (17-30N, 120-140E). The July-September mean location of TS formation is 6 latitude lower, while that in October-December is 18 longitude eastward in the strong warm versus strong cold years. After the El Nin o (La Nin a), the early season (January-July) TS formation in the entire WNP is suppressed (enhanced). In strong warm (cold) years, the mean TS life span is about 7 (4) days, and the mean number of days of TS occurrence is 159 (84) days. During the fall of strong warm years, the number of TSs, which recurve northward across 35N, is 2.5 times more than during strong cold years. This implies that El Nin o substantially enhances poleward transport of heat-moisture and impacts high latitudes through changing TS formation and tracks.