Updated 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

This is an update of the predictions of the annual number of tropical cyclones (TCs) affecting the western North Pacific (WNP) and the South China Sea (SCS), as well as the annual number of TCs making landfall along the South China coast for 2002 that we issued on 7 May 2002.  These updates are made based on new information for the months of April and May 2002.  Details of the predictions methods can be found in Chan et al. (1998, 2001) and Liu and Chan (2002).

2. ENSO conditions in 2002

In our 7 May 2002 predictions, we discussed the unclear situation of whether a warm episode of ENSO would occur.  However, the situation apparently became much clearer at the end of May.  A dramatic change occurred in the mean-sea-level pressure anomaly over the Pacific (Fig. 1).  In both hemispheres, the pressure anomalies are negative, which are indicative of a weakening of the trades.  The circulation anomalies at 850 hPa (Fig. 2) also show features similar to the conditions prior to past warm events that occurred in the summer months (Fig. 3), the most important being cyclonic anomalies in the western Pacific in both hemispheres.  Thus, a warm event is very likely to occur (and may have already begun) this year.  Such a prediction is similar to those from most climate models. 

As to the impact of a summer warm event on the TC activity over the WNP, past history suggests that the number of tropical storms as well as typhoons would likely be slightly above normal while TC activity over the SCS should be reduced (Table 1).

Fig. 1. Anomalous sea-level pressure in May 2002.

Fig. 2. 850-hPa wind anomalies in May 2002.

(pink - positive SSTA, blue - negative SSTA)

Fig. 3. 850-hPa wind and SST anomalies in Jun-Jul for summer El Nino.

Table 1. TC activity over the WNP and the SCS in years associated with summer warm events.

3. Predictions for 2002

As in the May predictions, the individual predictors again show 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 of 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 continued to 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 normal to slightly above-normal TC activity over the WNP but below-normal activity over the SCS and below-normal number of landfalling TCs over South China.  Details of the forecasts are given in Table 2.

Table 2. Updated predictions of the number of tropical cyclones over the western North Pacific and the South China Sea in 2002.

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)