2004 Predictions of Seasonal Tropical Cyclone Activity over the Western North Pacific and the South China Sea, and the Number of Landfalling Tropical Cyclones over South China 

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 (TCs) affecting the western North Pacific (WNP) and the South China Sea (SCS).  From 2001 onwards, LAR also issued predictions on the annual number of TCs making landfall along the South China coast.  Verifications of the predictions have shown that the predictions are mostly correct within the error bars.

These are all statistical predictions with predictors drawn from a large group of indices that represent the atmospheric and oceanographic conditions in the previous year up to the spring of the current year.  The most prominent ones include the proxies for El Niño/Southern Oscillation (ENSO), the extent of the subtropical ridge, the intensity of the India-Burma trough.  Details can be found in Chan et al. (1998, 2001) and Liu and Chan (2003).

For 2004, most of the El Niño predictions from different climate centres suggest neutral conditions throughout the summer and fall, although some predictions forecast the possibility of a warm event.  Therefore, it is anticipated the El Niño forcing on TC activity over the WNP might not be dominant this year.

2. The predictions

Predictors other than those directly related to El Niño appear to suggest near-normal overall TC activity (Table 1).  However, for the number of tropical storms and typhoons, most of the predictors forecast normal to above-normal activity.  Thus, it is expected that the overall TC activity over the WNP is not likely to be above normal.  On the other hand, the number of tropical storms and typhoons is likely to be normal, with the possibility of being above normal, the later being the case if an El Niño develops.  The quantitative predictions are given in Table 1.

For the SCS, as it is unlikely for a La Niña event to occur, TC activity will likely be normal to slightly below normal, again the latter occurring if an El Niño develops.  However, the number of tropical storms and typhoons will likely be near normal. The number of tropical cyclones making landfall over South China is expected to be near normal (Table 1). 

In previous years, we have used the predictions from all the past years to estimate the possible errors in the current predictions.  However, since each year, the spread of the predictions from different predictors is likely to be different.  Therefore, such an error estimate is probably not appropriate.  Therefore, we will not provide any error estimate this year.  Instead, based on the predictions from individual predictors, one can give an envelop of the possible error, much like the spread provided in ensemble forecasts.  For a lack of a better estimate, we suggest to use the smallest and largest numbers among the individual predictions as the lower and upper bound of the final predictions.  A larger (smaller) difference between the lower and upper bound might then indicate lower (higher) predictability.

Based on this concept, we could see that for this year, predictions for the number of typhoons have the smallest spread and thus the highest predictability while the confidence of the predictions for the other parameters would be lower, especially for the TC activity for the entire western North Pacific as well as the SCS.

As discussed in Chan et al. (2001), we will provide an updated forecast sometime in June.

Summary of predictions

References

Chan, J. C. L., J. E. Shi and C. M. Lam, 1998: Seasonal forecasting of tropical cyclone activity over the western North Pacific and the South China Sea. Weather Forecasting, 13, 997-1004.
Abstract

Chan, J. C. L., J. E. Shi and K. S. Liu, 2001: Improvements in the seasonal forecasting of tropical cyclone activity over the western North Pacific. Weather Forecasting, 16, 491-498. Abstract

Liu, K. S. and J. C. L. Chan, 2003: Climatological characteristics and seasonal forecasting of tropical cyclones making landfall along the South China coast. Monthly Weather Review, 131, 1650-1662. Abstract