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 Niño and La Niña 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 Niño summer and fall, the frequency of TS formation increases remarkably in the southeast quadrant (0°-17°N, 140°E-180°) and decreases in the northwest quadrant (17°-30°N, 120°-140°E). 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 Niño (La Niña), 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 35°N, is 2.5 times more than during strong cold years. This implies that El Niño substantially enhances poleward transport of heat-moisture and impacts high latitudes through changing TS formation and tracks. The enhanced TS formation in the SE quadrant is attributed to the increase of the low-level shear vorticity generated by El Niño-induced equatorial westerlies, while the suppressed TS generation over the NW quadrant is ascribed to upper-level convergence induced by the deepening of the east Asian trough and strengthening of the WNP subtropical high, both resulting from El Niño forcing. The WNP TS activities in July-December are noticeably predictable using preceding winter-spring Niño-3.4 SST anomalies, while the TS formation in March-July is exceedingly predictable using preceding October-December Niño-3.4 SST anomalies. The physical basis for the former is the phase lock of ENSO evolution to the annual cycle, while for the latter it is the persistence of Philippine Sea wind anomalies that are excited by ENSO forcing but maintained by local atmosphere-ocean interaction.