Updated Prediction of Seasonal Tropical Cyclone Activity over the Western North Pacific for 2008


1. Introduction

 

Issued on 24 June 2008

This is an update of the predictions of the annual number of tropical cyclones (TCs) in the western North Pacific (WNP) for 2008 that we issued on 18 April 2008.  These updates are made based on new information for the months of April and May 2008. 


2.  ENSO conditions in 2008

The 2007 La Niña event continues to weaken and the consensus is that the ENSO condition is currently in a neutral status.  In May, SSTs are slightly colder than normal in the central and east-central equatorial Pacific but become warmer than normal in the far eastern equatorial Pacific.  The Niño3.4 index in May is -0.59.  Most model forecasts of ENSO from different climate centres call for a neutral condition in 2 to 7 months time (see summary in Table. 1).  Out of the 6 forecasts, 4 suggest that the ENSO will return to its neutral condition during the summer and fall and 2 suggest a transition to a weak El Niño event.  These predictions suggest that the La Niña event developed in 2007 is not likely to persist for the rest of the 2008 season.  In other words, 2008 will most likely be an ENSO-neutral year although a transition to an El Niño event cannot be ruled out.

An analysis of the TC activity in a year after a La Niña event and becoming neutral or changing to warm ENSO conditions (see Table 3) suggests that the TC season in 2008 tends to be normal or above-normal, which is consistent with our forecast (Table 2).

3.  Predictions for 2008

No significant change is found on the predicted total number of TCs.  All the predictors give numbers similar to those in the April forecast and therefore the final predicted number is 33.

For the number of tropical storms and typhoons, the prediction from the west Pacific index gives an increase in TC number (from 28 to 29) but the Niño3.4 predictor gives a predicted number of 30 compared with 31 in the April forecast (cf. Tables 2a and 2b).  No change occurs using the index of the strength of the subtropical high and the India-Burma trough.  The final predicted number therefore is still 30. 

A similar situation is found for the predicted number of typhoons.  The west Pacific index gives an increase in TC number but the Niño3.4 predictor gives a decrease and no change occurs using other index.  The final predicted number therefore is still 19. 

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

The predictor related to the subtropical high (HWNP) continues to suggest an above-normal TC activity, which is probably related to the weaker-than-normal subtropical high over the western part of the WNP (Fig. 1).

Summary of predictions

Entire western North Pacific

Forecast

Normal

All TC

33 (slightly above-normal)

31

Tropical storms and typhoons

30 (above-normal)

27

Typhoons

19 (slightly above-normal)

17


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

 

MODEL /
GROUP

2-4 MONTHS
(May to Jul)

5-7 MONTHS
(Aug to Oct)

POAMA
(run at Bureau of Met)

Neutral

Neutral

System 3
ECMWF

Neutral

Neutral

GloSea
UK Met Office

Warm

Not Available

CFS
NCEP

Warm

Neutral

CGCMv1
GMAO/NASA

Neutral

Neutral

JMA-CGCM02
Japan Met. Agency

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 34 33 31   33 31
Weight 0.65 0.66 0.73  

Tropical storms and typhoons

HWNP HIB WP NINO3.4 Final forecast Normal
Prediction 29 31 28 31 30 27
Weight 0.68 0.67 0.52 0.67
Typhoons
HWNP HIB NINO3.4 WP Final forecast Normal
Prediction 20 19 18 18 19 17
Weight 0.57 0.59 0.73 0.58

 

(b) June Forecast

Entire western North Pacific

All TC
HWNP HIB NINO4   Final forecast Normal
Prediction 34 33 31   33 31
Weight 0.65 0.66 0.73  

Tropical storms and typhoons

HWNP HIB WP NINO3.4 Final forecast Normal
Prediction 29 31 29 30 30 27
Weight 0.68 0.73 0.57 0.67
Typhoons
HWNP HIB NINO3.4 WP Final forecast Normal
Prediction 20 19 17 19 19 17
Weight 0.61 0.71 0.75 0.69
 
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)

 


Table 3. Number of tropical storms and typhoons and number of typhoons during the years after La Niña events.  Orange and blue shadings indicate the above-normal and below-normal TC activity respectively.

 

 

No. of tropical storms and typhoons

No. of typhoons

ENSO status

1965

34

21

El Niño

1972

30

22

1976

25

14

1985

25

17

Neutral

1989

31

21

1996

33

21

2001

29

20

 


Fig. 1.
 

500-hPa geopotential height anomalies between April and May in 2008. Thick contour indicates the geopotential height (contour interval = 10 m) ³ 5860 m.