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 


1. Introduction

Issued on 01 May 2004

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 Niņo/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 Niņo 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 Niņo forcing on TC activity over the WNP might not be dominant this year.


2. The predictions

Predictors other than those directly related to El Niņo 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 Niņo develops.  The quantitative predictions are given in Table 1.

For the SCS, as it is unlikely for a La Niņa event to occur, TC activity will likely be normal to slightly below normal, again the latter occurring if an El Niņo 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

Entire western North Pacific

 

All TC

near normal

Tropical storms and typhoons

near normal to slightly above normal

Typhoons

near normal to slightly above normal

 

 

South China Sea

 

All TC

near normal to slightly below normal

Tropical storms and typhoons

near normal

 

 

Landfall along South China coast near normal


Table 1. Forecasts from various predictors and the weighted average of the forecasts.

 

Entire western North Pacific

All TC
HWNP HIB WP NINO4 Final forecast Normal
Prediction 35 33 29 29 31 31
Weight 0.68 0.73 0.59 0.75

Tropical storms and typhoons

HWNP HIB WP NINO3.4 Final forecast Normal
Prediction 31 28 27 30 29 27
Weight 0.75 0.73 0.60 0.72
Typhoons
HWNP HIB WP NINO3.4 ESOI Final forecast Normal
Prediction 18 18 18 18 19 18 17
Weight 0.60 0.67 0.61 0.73 0.72
 
South China Sea
All TC
HIB NINO3.4 NINO4 SOI ESOI Trend Final forecast Normal
Prediction 10 15 10 13 15 14 13 13
Weight 0.69 0.80 0.57 0.65 0.58 0.90
Tropical storms and typhoons
HIB NINO3.4 NINO4 SOI ESOI Trend Final forecast Normal
Prediction 8 11 10 10 12 12 11 10
Weight 0.71 0.63 0.65 0.60 0.57 0.88
Landfall along South China coast
HZC HIB NINO3.4 ESOI Final forecast Normal
Prediction 5 4 5 6 5 5
Weight 0.69 0.59 0.76 0.66
HWNP Index of the westward extent of the subtropical high over the western North Pacific
HZC Index of the zonal circulation (60o-150oE)
HIB Index of the strength of the India-Burma trough (15o-20oN, 80o-120oE)
WP Primary mode of low-frequency variability over the North Pacific
NINO3.4 Sea surface temperature (SST) anomalies in the NINO3.4 region (5oS-5oN,
170o-120oW)
NINO4 Sea surface temperature (SST) anomalies in the NINO4 region (5oS-5oN, 160oE-150oW)
SOI Southern Oscillation Index (SOI)
(Standardized Tahiti - Standardized Darwin) sea level pressure
ESOI Equatorial Southern Oscillation Index (Equatorial SOI)
Equatorial Eastern Pacific SLP - Indonesia SLP (standardized anomalies)
Trend Trend of the interannual variations in TC activity

 


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