Updated Prediction of Seasonal Tropical Cyclone Activity over the Western North Pacific for 2005


1. Introduction

Issued on 24 June 2005

This is an update of the predictions of the annual number of tropical cyclones (TCs) affecting the western North Pacific (WNP) for 2005 that we issued on 27 April 2005.  These updates are made based on new information for the months of April and May 2005.  Details of the predictions methods can be found in Chan et al. (1998, 2001) and Liu and Chan (2003).

 

2. ENSO conditions in 2005

Most dynamical models, 10 out of 12, continue to predict ENSO-neutral conditions throughout much of 2005.  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 2005 should be ENSO-neutral.  Because of the lack of an obvious forcing from ENSO, it is likely that TC activity will be mainly controlled by factors other than those from ENSO.

 

3. Predictions for 2005

As discussed in section 1 and Chan et al. (2001), the Apr and May predictors are included in our updated forecasts.  It is noted that because NCEP changes the way the WP index is calculated (see http://www.cpc.noaa.gov/data/teledoc/telecontents.shtml), we decided to drop this index as one of the predictors until a more thorough study on the revised index has been completed.

For the total number of TCs, changes in the predictions from two parameters are found.  The prediction from the index of the western extent of the subtropical high (HWNP) is 26 compared with 25 in the Apr forecast, which still gives below-normal TC activity (cf. Tables 1a and 1b).  A greater increase, from 29 to 31 occurs based on the Nino4 predictor.  The final predicted number is therefore increased from 28 to 29. 

For the number of tropical storms and typhoons, only the prediction from the index of the strength of the India-Burma trough changes, increasing from 23 to 24.  The final predicted number is therefore increased from 24 to 25. 

No obvious change for the predicted number of typhoons.  All predictors give essentially the same predictions as those from Apr. However, with the drop of  the WP index as one of the predictors, the final predicted number becomes 16. 

With these changes, the average predictions from all the potential predictors therefore call for a near-normal to slightly below-normal year for all TC categories for the entire WNP.

 

Summary of predictions

Entire western North Pacific

 

All TC

near normal to slightly below normal

Tropical storms and typhoons

near normal to slightly below normal

Typhoons

near normal to slightly below normal


Table 1. Forecasts from various predictors and the weighted average of the forecasts issued in (a) April and (b) June.


(a) April Forecast

Entire western North Pacific

All TC
HWNP HIB WP NINO4 Final forecast Normal
Prediction 25 30 29 29 28 31
Weight 0.70 0.69 0.59 0.72

Tropical storms and typhoons

HWNP HIB WP NINO3.4 Final forecast Normal
Prediction 21 23 25 28 24 27
Weight 0.66 0.63 0.72 0.72
Typhoons
HWNP HIB WP NINO3.4 ESOI Final forecast Normal
Prediction 13 13 13 19 18 15 17
Weight 0.60 0.66 0.61 0.74 0.60

(b) June Forecast

Entire western North Pacific

All TC
HWNP HIB NINO4 Final forecast Normal
Prediction 26 30 31 29 31
Weight 0.72 0.69 0.76

Tropical storms and typhoons

HWNP HIB NINO3.4 Final forecast Normal
Prediction 21 24 28 25 27
Weight 0.66 0.64 0.72
Typhoons
HWNP HIB NINO3.4 ESOI Final forecast Normal
Prediction 13 13 19 18 16 17
Weight 0.60 0.66 0.76 0.60

 

HWNP Index of the westward extent of the subtropical high over the western North Pacific
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)
ESOI Equatorial Southern Oscillation Index (Equatorial SOI)
Equatorial Eastern Pacific SLP - Indonesia SLP (standardized anomalies)

 


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