Updated Prediction of Seasonal Tropical Cyclone Activity over the Western North Pacific for 2007


1. Introduction

Issued on 25 June 2007

This is an update of the predictions of the annual number of tropical cyclones (TCs) affecting the western North Pacific (WNP) for 2007 that we issued on 23 April 2007.  These updates are made based on new information for the months of April and May 2007. 


2.  ENSO conditions in 2007

The ENSO condition is currently in a neutral status.  In May, SST anomalies remain near-zero in the central equatorial Pacific but below normal in the eastern equatorial Pacific and becoming increasingly negative.  The Niño3.4 index in May becomes negative (-0.19), the first time this year.  A summary of the various ENSO model forecasts from different climate centres suggests that all of them predict a cool condition in 2 to 7 months time (Table. 1).  All the 6 forecasts suggest a possible cold event throughout the summer and fall.  Based on these results, it appears that 2007 will likely be a La Niña year.  An analysis of the TC activity in the past years with La Niña events suggests that the TC season in 2007 tends to be normal or below-normal, which is consistent with our forecast.

 

3.  Predictions for 2007

For the total number of TCs, the index of the western extent of the subtropical high (HWNP) gives a predicted number of 28 compared with 30 in the April forecast (cf. Tables 2a and 2b).  No change occurs using the Niño4 predictor.  The final predicted number therefore decreases from 28 to 27.

For the number of tropical storms and typhoons, the predictions from the index of the strength of the west Pacific index and the Niño3.4 index give decreases in TC number especially for the west Pacific index.  The final predicted number therefore decreases from 25 to 24.

No significant change is found on the predicted number of typhoons.  All the predictors give similar numbers as compared with the April forecast and therefore the final predicted number is still 14.

With these changes, the average predictions from all the potential predictors therefore call for a below-normal year for all TC categories for the entire WNP (See Table 2b).

The predictor related to the subtropical high (HWNP) continues to suggest a below-normal TC activity.  The subtropical high between April and May is even stronger (Fig. 1), resulting in the decreases of the predicted numbers in all categories.

To summarize, the updated predictions are:

Entire western North Pacific

 

All TC

Below-normal

Tropical storms and typhoons

Below-normal

Typhoons

Below-normal


 

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

 

MODEL /
GROUP

Forecast Start Date

2-4 MONTHS
(Jul to Sep)

5-7 MONTHS
(Oct to Dec)

POAMA
(run at Bureau of Met)

29/04-28/05

Cool

Cool

System 3
ECMWF

01 May

Cool

Cool #

GloSea
UK Met Office

01 May

Cool

Not Available

CSF
NCEP

02/05-21/05

Cool

Cool

CGCMv1
GMAO/NASA

May

Cool

Neutral

JMA-CGCM02
Japan Met. Agency

April

Cool

Cool #

# Model forecast is run only until the end of November

 

 

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 NINO4     Final forecast Normal
Prediction 30 27     28 31
Weight 0.70 0.73    

Tropical storms and typhoons

HWNP WP NINO3.4   Final forecast Normal
Prediction 24 26 24   25 27
Weight 0.73 0.62 0.67  
Typhoons
HWNP NINO3.4 ESOI   Final forecast Normal
Prediction 14 13 16   14 17
Weight 0.60 0.77 0.68  

 

(b) June Forecast

Entire western North Pacific

All TC
HWNP NINO4     Final forecast Normal
Prediction 28 27     27 31
Weight 0.68 0.73    

Tropical storms and typhoons

HWNP WP NINO3.4   Final forecast Normal
Prediction 24 25 23   24 27
Weight 0.70 0.56 0.62  
Typhoons
HWNP WP NINO3.4 ESOI Final forecast Normal
Prediction 14 14 13 16 14 17
Weight 0.61 0.67 0.75 0.68
 
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 between April and May in 2007. Thick contour indicates the geopotential height (contour interval = 10 m) ³ 5860 m.