Tropical cyclone activity over the western North Pacific associated with El Nino and La Nina events

This paper presents the results of an investigation on the variations of tropical cyclone (TC) activity over the western North Pacific (WNP) associated with both El Nino (EN) and La Nina (LN) events. The study is based on the monthly number of TCs that occurred during the period 1959-97. Anomalies within each 5°lat X 5°long box from the year before (EN - 1 and LN - 1) to the year after (EN + 1 and LN + 1) are examined. During an EN - 1 year, more (less) TCs are found in September and October over the South China Sea (southeast of Japan). In an EN year, TC activity is below normal during these two months over the South China Sea (SCS) but above normal especially in the late season in the eastern part of the WNP. After the mature phase of the warm event (i.e., during an EN + 1 year), TC activity over the entire ocean basin tends to be below normal. No significant anomalies are found during an LN - 1 year. However, in an LN year, the SCS tends to have more TCs in September and October, but for the rest of the WNP, TC activity tends to be below normal from August to November. During the year after an LN event, the entire basin generally has more TCs. Such a situation is especially true over the SCS from May to July. All these anomalous activities are apparently linked to anomalies in the large-scale flow patterns at 850 and 500 hPa. Because the 850-hPa flow is related to TC genesis and development, areas with anomalous cyclonic (anticyclonic) flow are generally found to be associated with above- (below-) normal TC activity. Anomalous 500-hPa flow is identified as responsible for steering TCs toward or away from a region, thus rendering the TC activity in that region above or below normal.