How Strong ENSO Events Affect Tropical Storm Activity over the Western North Pacific
An analysis of 35-year (1965-1999) data reveals vital impacts of strong (but not moderate) El Nino and La Nina events on tropical storm (TS) activity over the western North Pacific (WNP). Although the total number of TS formed in the entire WNP does not vary significantly from year to year, during El Nino summer and fall, the frequency of TS formation increases remarkably in the southeast quadrant (0-17oN, 140-180oE) and decreases in the northwest quadrant (17-30oN, 120-140oE). The July-September mean location of TS formation is 6o latitude lower, while that in October-December is 18o longitude eastward in the strong warm versus strong cold years. After the El Nino (La Nina), 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 TS, which recurves northward across 35oN, is 2.5 times more than during strong cold years. This implies that El Nino substantially enhances poleward transport of heat/moisture and impacts high latitude 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 Nino-induced equatorial westerly, while the suppressed TS generation over the NW quadrant is ascribed to upper-level convergence induced by the deepening of east Asian trough and strengthening of the WNP subtropical high, both resulting from El Nino forcing. The WNP TS activities in July-December are noticeably predictable using preceding winter/spring NINO3.4 SST anomalies, while the TS formation in March-July is exceedingly predictable using preceding October-December NINO3.4 SST anomaly. 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.