2002 Prediction 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).  In 2001, 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 Nio/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 (2002).

2. ENSO conditions in 2002

The ENSO conditions up to mid April are not really clear as to whether a warm event is likely to take place.  Based on previous studies, if a warm event is to take place, this previous winter over East Asia should be either cold or normal.  However, although December 2001 was cold over Asia, temperatures in Jan to March were way above normal (Fig. 1).  Equatorial westerlies also do not extend beyond about 160^oE, with easterlies prevailing throughout the central equatorial Pacific (Fig. 2).  In addition, the subtropical high is stronger than normal in the eastern North Pacific and positive anomalies extend all the way to the South China Sea (Fig. 3).  For a warm event to occur, negative anomalies should be present in the western North Pacific, especially to the Philippine Sea.

Fig. 1. Surface air temperature anomaly between Jan and Mar in 2002.

Fig. 2. (a) 925-hPa and (b) 850-hPa wind anomalies between Jan and Mar in 2002.

Fig. 3. Anomalous sea-level pressure between Jan and Mar in 2002.

Predictions from climate models also differ, with some predicting neutral conditions while others predicting a mild warm event.  Even if a warm event is to occur, most predictions favour a warming in August or beyond. 

Given these uncertainties, it appears that the ENSO, even if it is to occur, may not be an important factor in the TC predictions this year.

3. Predictions for 2002

An examination of the individual predictors suggests two groups of forecasts.  The ENSO predictors tend to predict above-normal number of TCs over the WNP but below normal for SCS probably because past history suggests the likelihood of a warm event.  On the other hand, predictors that are based on indices of atmospheric circulations of the previous year and winter predict below-normal TC activity over the WNP and the SCS.  Since the final predictions are weighted averages of the individual forecasts, the predictions for 2002 are for a near-normal TC activity over the WNP with below-normal activity over the SCS and below-normal number of landfalling TCs over South China.  Details of the forecasts are given in Table 1.

As discussed in Chan et al. (2001), we will provide an updated forecast sometime in June when the signal for the occurrence or non-occurrence of a warm event may be stronger.

Table 1. Predictions of the number of tropical cyclones over the western North Pacific and the South China Sea in 2002.

Data for the predictors of the various prediction schemes have been either provided by the National Climate Center of the China Meteorological Administration, or extracted from the website of the US Climate Prediction Center or the NCEP/NCAR reanalysis dataset. The plots in Figs. 1 to 3 are also generated from this latter dataset.

The development of the various prediction schemes has been supported over the last few years by grants from the City University of Hong Kong and the Risk Prediction Initiative of the Bermuda Biological Station for Research.

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. Wea. 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. Wea. Forecasting, 16, 491-498. Abstract

Liu, K. S. and J. C. L. Chan, 2002: Climatological characteristics and seasonal forecasting of tropical cyclones making landfall along the South China coast. (Submitted to Monthly Weather Review)