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

1. Introduction

Issued on 18 Jun 2004

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 2004 that we issued on 1 May 2004.  These updates are made based on new information for the months of April and May 2004.  Details of the predictions methods can be found in Chan et al. Chan et al. (1998, 2001) and Liu and Chan (2003).

2. ENSO conditions in 2004

Most dynamical models continue to indicate ENSO-neutral conditions throughout much of 2004 although a slight bias towards warm conditions can be seen in some of the models and very few predicted cold conditions.  No obvious precursors that were present in previous warm or cold events can be identified from the current atmospheric and oceanographic conditions.  Thus, it is likely that 2004 should be ENSO-neutral with a slight possibility towards warm conditions.  Because of the lack of forcing from ENSO, it is likely that TC activity will be mainly controlled by factors other than those from ENSO.

3. Predictions for 2004

As discussed in section 1 and Chan et al. (2001), the Apr and May predictors are included in our update forecasts.  For the entire WNP, two parameters are found to give a significant change in the predictions: the Nino indices and the western extent of the subtropical high (HWNP).  For the total number of TCs, the prediction from the Nino4 index is 33 compared with 29 in the Apr forecast (cf. Tables 1 and 2).  A similar increase in the predicted number of typhoons also occurs using the Nino3.4 predictor, increasing from 18 to 19, and from 18 to 20 using the HWNP predictor.  With these changes, the average predictions from all the potential predictors therefore call for a normal to above-normal year for all TC categories for the entire WNP.

For the SCS, the number of TCs predicted from the Nino4 predictor also increases from 10 to 14 (see Tables 1 and 2) with the other predictors giving essentially the same predictions as those from Apr.  Averaging all the predictions therefore calls for a normal for all TC categories for the SCS as well as for the number of landfalling TCs over the South China coast.

To summarize, the updated predictions are:

Western North Pacific near- to above-normal numbers of tropical cyclones and those reaching at least tropical storm intensity, as well as the number of typhoons
South China Sea normal numbers of tropical cyclones and those reaching at least tropical storm intensity
Landfall over South China normal

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

 

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

 

Table 2. Updated forecasts from various predictors and the weighted average of the forecasts. Bolded numbers indicate the updates based on new information for the months of April and May 2004.

 

Entire western North Pacific
All TC
HWNP HIB WP NINO4 Final forecast Normal
Prediction 35 33 29 33 33 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 0.66 0.72
Typhoons
HWNP HIB WP NINO3.4 ESOI Final forecast Normal
Prediction 20 18 18 19 19 19 17
Weight 0.57 0.67 0.61 0.78 0.72
 
South China Sea
All TC
HIB NINO3.4 NINO4 SOI ESOI Trend Final forecast Normal
Prediction 10 15 14 13 15 14 13 13
Weight 0.69 0.80 0.66 0.79 0.60 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 0.61 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 0.71 0.76 0.66

 

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