Prediction of Tropical Cyclone Activity over the Western North Pacific in 2000 - an Update

Issued in June 2000

1. Background

In our previous discussion, we showed that during the 1997 and 1998 seasons, our prediction accuracies appear to be affected by the abrupt occurrence of the warm and cold episodes of the El Nio/Southern Oscillation (ENSO). The main problem is that the predictors contain information only up to the current March. However, the signal of ENSO may not be present until later in the year. Therefore, it might be necessary to derive an updated forecast based on the information of the current April and May. This discussion presents our latest forecast for 2000 using such information.

2. Predictors and methodology

In addition to the predictors described in Chan et al. (1998, hereafter CSL), the following new predictors are included:

1. Meridional wind anomalies at 850-hPa northeast of Australia (5-20^oS, 140-170^oE)
   
   

2. Sea-level pressure anomalies in two regions in the Southern Hemisphere: (5-35^oS, 180-150^oW), (5-35^oS, 120-150^oW)
   
   

3. Time changes of the Southern Oscillation Index (SOI) and the parameters in a and b.

The parameters in a and b are chosen based on the studies by Chan and Xu (2000) and Xu and Chan (2001) that examined the precursors of ENSO.

The first step in making the updated predictions is to perform the same procedure as described in CSL using all the possible predictors (from previous June to current May). The best [in terms of the root-mean-square (RMS) errors from the jackknife test] six predictors are then chosen.

The second step is to choose six predictors from this set and those from the original forecast (which has been re-derived with six predictors). The final forecast is then made from a weighted average of that from each of the six final predictors. Details of this updated methodology are being written up and will be posted here in due course.

3. Forecasts for 2000

Table 1. Original and updated predictions of the number of tropical cyclones over the western North Pacific and the South China Sea in 2000.

Compared with the forecasts made in April, the updated ones generally reduce the number of more intense TCs (to be closer to normal) and increase the number of tropical depressions.

References

Chan J. C. L. and J. J. Xu, 2000: Physical mechanisms responsible for the transition from a warm to a cold state of the El Nio Southern Oscillation. J. Climate, 13, 2056-2071. Abstract

Xu J. J. and J. C. L. Chan, 2001: The role of the Asian-Australian monsoon system in the onset time of El Nio events.. J. Climate, 14, 418 - 433. Abstract