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


1. Introduction

Issued on 23 June 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:

Entire western North Pacific

 

All TC

near normal

Tropical storms and typhoons

near normal

Typhoons

near normal


Table 1. Summary of model forecasts extracted from the Australian Bureau of Meteorology homepage.

 

GROUP

5 MONTHS
(Oct 2006)

8 MONTHS
(Jan 2007)

POAMA
(run at Bureau of Met)

Neutral

Neutral

CPC

Neutral

Neutral

ECMWF

Neutral

Not Available

UKMO

Neutral

Not Available

LDEO

Neutral

Warm

NCEP

Neutral

Neutral

NOAA LINEAR INVERSE

Neutral

Neutral

SCRIPPS/MPI

Neutral

Neutral

NSIPP/NASA

Neutral

Neutral

JMA

Neutral

Not Available

SSES (Ohio)

Neutral

Not Available

CLIPER

Neutral

Neutral

 

Table 2. 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 NINO4   Final forecast Normal
Prediction 31 31 32   32 31
Weight 0.66 0.68 0.70  

Tropical storms and typhoons

HWNP HIB WP NINO3.4 Final forecast Normal
Prediction 25 26 26 32 27 27
Weight 0.71 0.65 0.62 0.68
Typhoons
HWNP HIB NINO3.4 ESOI Final forecast Normal
Prediction 15 15 19 17 17 17
Weight 0.64 0.58 0.73 0.67

 

(b) June Forecast

Entire western North Pacific

All TC
HWNP HIB NINO4   Final forecast Normal
Prediction 30 30 32   31 31
Weight 0.74 0.72 0.70  

Tropical storms and typhoons

HWNP HIB WP NINO3.4 Final forecast Normal
Prediction 25 27 29 32 28 27
Weight 0.71 0.67 0.60 0.69
Typhoons
HWNP HIB WP NINO3.4 ESOI Final forecast Normal
Prediction 16 17 18 19 17 18 17
Weight 0.61 0.58 0.67 0.73 0.65
 
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)

 


Fig. 1.
 
500-hPa geopotential height anomalies for the periods (a) Feb-Mar and (b) Apr-May in 2006. Thick contours indicate the geopotential height (contour interval = 10 m) 5860 m.

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