Forecast of Tropical Cyclone Activity over the Western North Pacific

[Normals (calculated from JTWC data for 1965-94) are in parentheses]

Annual for the WNP:

No. of TCs : 33 +/- 3 (31)
No. of TCs reaching at least tropical storm intensity: 30 +/- 3 (27)
No. of typhoons: 19 +/- 2 (17)

May to December for the WNP:

No. of TCs from May to Dec: 30 +/- 3 (28)
No. of TCs reaching at least tropical storm intensity: 27 +/- 2 (25)
No. of typhoons: 18 +/- 2 (16)

Annual for the South China Sea (SCS):

No. of TCs: 12 +/- 2 (13)
No. of TCs reaching at least tropical storm intensity: 9 +/- 2 (10)

Boundaries:

WNP: 100-180°E, 0-40°N
SCS: 100-120°E, 0-25°N

The predictions are based on three sets of monthly parameters for the months from April of the previous year to March of the current year. Not all parameters are chosen. The three sets are as follows:

1. SOI and NINO-4 anomalies

2. Indices describing the winter conditions over East Asia and the Western North Pacific. These include the following:

NOAA/CAC West Pacific Pattern Index
Strength of the subtropical high over the South China Sea
Area of the 500-hPa polar vortex over the Pacific sector
500-hPa height anomalies over the Tibetan Plateau
500-hPa trough over the India-Burma region
Western extent of the 500-hPa ridge
Cold-air intrusions into China

3. Climatology-Persistence of the number of TCs (CLIPER-type)

The test data set consists of TCs for the years 1965-94 and the predictions are based on the projection-pursuit regression method for individual parameters and then a weighted combination of individual forecasts (up to a maximum of five) gives the final forecast. Tests have been made using the jackknife method and found to be satisfactory.

This study was sponsored by the Risk Prediction Initiative of the Bermuda Biological Station for Research.

A paper is being written up for publication. Comments are welcome.