PEAC'S TROPICAL CYCLONE OUTLOOK SUMMARY

The three forecasts (below) for western North Pacific tropical cyclone activity presented differ slightly: one calls for slightly below normal activity, while the other two indicate that activity will be close to normal. The greatest effect of the ENSO cycle on the tropical cyclones of the western North Pacific is on the locations where they form. During El Nino years, the tropical cyclones of the basin tend to form east of normal, and during La Nina years, the tropical cyclones tend to form west of normal. During the year immediately following an El Niño, the number of tropical storms and typhoons is typically reduced. A weak El Niño occurred in 2004, making 2005 the follow-on year to El Niño. This factor would favor reduced tropical cyclone activity for 2005. However, the recent El Niño was weak, and the climate state of 2005 is expected to gradually revert to ENSO Neutral; therefore, we expect the numbers and locations of the tropical cyclones in the western North Pacific to be close to the long-term average. We agree with the Hong Kong LAR outlook that no above-normal activity is expected in any category (e.g., number of typhoons and number of intense typhoons). Caution: the annual number of tropical cyclones in the western North Pacific basin is approximately 30, and is the highest of any basin in the world. Even reduced activity in this basin still represents a large number of tropical cyclones, and though the risk of a typhoon affecting any of the islands in Micronesia might be reduced, the risk is not zero.

TROPICAL CYCLONE OUTLOOKS

Since 2000, the Laboratory for Atmospheric Research (LAR) at City University of Hong Kong has been issuing real-time predictions of the annual number of tropical cyclones affecting the western North Pacific (WNP) and the South China Sea. For 2005, all the predict

ors suggest slightly below-normal overall tropical cyclone activity; the predictor related to the subtropical high forecasts activity much below normal. For the number of tropical storms and typhoons, most of the predictors forecast below-normal activity except for the ENSO predictor that suggests near-normal activity.  Thus, it is expected that the overall TC activity over the WNP is likely to be slightly below normal.  A similar prediction can be made for the number of tropical storms and typhoons as well as the number of typhoons. No above-normal activity is expected in any of the categories.  The quantitative predictions are given in Table 1.

Table 1. LAR predictions of 2005 tropical cyclone activity
http://aposf02.cityu.edu.hk/~mcg/tc_forecast/2005_forecast_Apr.htm

The International Research Institute for Climate Prediction (IRICP) issued an outlook in May. 2005 for the tropical cyclone activity expected to occur in the western North Pacific for the 4-month period Apry through October. According to the IRICP, the forecast for the number of named tropical cyclones in the western North Pacific during the 2005 peak season is equally divided among the three categories: below normal, normal, above normal (Table 2).

Tropical Storm Risk (TSR) at the Benfield Hazard Research Centre, University College London also issues a northwest Pacific typhoon forecast. According to their May. update, TSR anticipates a near average season for northwest Pacific typhoon activity in 2005 (Table 3). The forecast spans the full western North Pacific season from January 1 to December 31.

Table 3. TSR predictions of 2005 tropical cyclone activity
http://tropicalstormrisk.com