Updated Prediction of Seasonal Tropical Cyclone Activity over the Western North Pacific for 2006

This is an update of the predictions of the annual number of tropical cyclones (TCs) affecting the western North Pacific (WNP) for 2006 that we issued on 24 April 2006.  These updates are made based on new information for the months of April and May 2006. 

2.  ENSO conditions in 2006

A summary of the various ENSO model forecasts from different climate centres continues to predict a neutral condition in 5 to 8 months time (Table. 1).  All the forecasts suggest neutral conditions throughout the summer and fall.  Based on these results, it appears that 2006 will likely be a neutral year.  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 2006

As discussed in section 1 and Chan et al. (2001), the Apr and May predictors are included in our update forecasts.   

For the total number of TCs, two parameters are found to give a significant change in the predictions.  Both the index of the western extent of the subtropical high (HWNP) and the index of the strength of the India-Burma trough give a predicted number of 30 compared with 31 in the Apr forecast (cf. Tables 2a and 2b).  No change occurs using the Nino4 predictor.  The final predicted number therefore decreases from 32 to 31. 

For the number of tropical storms and typhoons, the predictions from the index of the strength of the India-Burma trough, the west Pacific index and the NINO3.4 index give increases in TC number especially for the west Pacific index.  The final predicted number therefore increases from 27 to 28. 

Similar changes are found for the predicted number of typhoons.  A greater increase, from 15 to 17, occurs using the index of the strength of the India-Burma trough.  The final predicted number increases from 17 to 18.

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

As discussed in the April forecast, the predictor related to the subtropical high (HWNP) suggests a slightly below-normal TC activity, which are partly based on the stronger-than-normal subtropical high observed between February and March in 2006 (Fig. 1a).  These anomalies persist into the months April-May (Fig. 1b) and therefore the updated forecast still gives a slightly below-normal TC activity.

To summarize, the updated predictions are:

(a) April Forecast

(b) June Forecast

References

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