2008 Predictions of Seasonal Tropical Cyclone Activity over the Western North Pacific


1. Introduction

Issued on 18 April 2008

    Since 2000, the Laboratory for Atmospheric Research (LAR) at City University of Hong Kong has been issuing real-time predictions of the annual number of tropical cyclones (TCs) affecting the western North Pacific (WNP) and the South China Sea (SCS).  Verifications of the predictions have shown that the predictions are mostly correct within the error bars.   

    These are all statistical predictions with predictors drawn from a large group of indices that represent the atmospheric and oceanographic conditions in the previous year up to the spring of the current year.  The most prominent ones include the proxies for El Niño/Southern Oscillation (ENSO), the extent of the subtropical ridge, and the intensity of the India-Burma trough.  Details can be found in Chan et al. (1998, 2001) and Liu and Chan (2003).


2. ENSO conditions in 2008

    As an important determinant is the status of the ENSO condition, it is useful to have a discussion on the possible ENSO situation in 2008.  A strong La Niña event has developed in 2007 and shows a sign of weakening in the recent month.  In March, SSTs remain colder than normal in the central equatorial Pacific but become warm than normal in the far eastern equatorial Pacific.  A summary of the various ENSO model forecasts from different climate centres suggests that most of them predict a neutral condition in 5 to 7 months time (Table. 1).  Out of the 6 forecasts, 5 suggest that the ENSO will return to its neutral condition during the summer.  Based on these results, it appears that 2008 will likely be an ENSO-neutral year.  


3. 2008 predictions     

    The ENSO predictor (NINO4 index) suggests a near-normal overall TC activity while the other predictors give an above-normal activity (Table 2).  The final forecast is therefore for a slightly above-normal overall TC activity.

    For the number of tropical storms and typhoons, all the predictors forecast a slightly above-normal or above-normal activity (ranging from 28 to 31).  Therefore, an above-normal activity (30 tropical storms and typhoons) is expected for this category. 

    Similar forecast is obtained for the number of typhoons.  The predicted numbers range from 18 to 20 and the final forecast is 19 typhoons, which is slightly higher than the normal number.

    Thus, it is expected that the overall TC activity, the number of tropical storms and typhoons as well as the number of typhoons are likely to be slightly above-normal or above-normal.  The quantitative predictions are given in Table 2.  It should be noted that the Equatorial Southern Oscillation Index is not included in the forecast this year because this predictor give below-normal TC activity, which is not consistent with the other predictors.

    As pointed out in section 2, 2008 is a year after a strong La Niña event. Therefore, it is useful to discuss the TC activity in a year after a La Niña event.  Out of the 12 such years, 6 are associated with above-normal TC activity and the TC activity is normal in 4 years (Table 3, see also Fig. 1).  The two years (1975 and 99) with below-normal TC activity are found in the years in which the La Niña event developed in the previous year persisted into the TC season.  Therefore, the 2008 TC season will likely to be normal or above normal, which is consistent with our forecast.

    The predictor related to the subtropical high (HWNP) suggests an above-normal TC activity.  Such forecasts are consistent with the weaker-than-normal subtropical high over the western part of the WNP observed between February and March in 2008 (Fig. 2).  This is the first time since 2001 that the subtropical high during these two months is weaker than normal (Fig. 3).  Even for 2001, which is actually a year after the persistent La Niña event of 1998-2000, only the region over the South China Sea has below-normal geopotential heights.  In other words, if the subtropical high in this early season is an indication of TC activity for the summer, it is highly likely that TC activity is above normal in 2008.

    The possible error in the current predictions is given by an envelope of the possible errors, which are based on the predictions from individual predictors.  The smallest and largest numbers among the individual predictions are considered as the lower and upper bound of the final predictions.  A larger (smaller) difference between the lower and upper bound might then indicate lower (higher) predictability.  Based on this concept, we could see that for this year, prediction for the number of typhoons has the smallest spread and thus the highest predictability.

    It might also be noted that this predicted above-normal activity is consistent with the results of Chan (2000) for the TC activity in a year after a La Niña year (Fig. 4).  Most of the western North Pacific has positive anomalies in his composite study.  The pattern also suggests perhaps more TCs are likely to make landfall in the southern part of East Asia (Philippines, Vietnam and South China).  At the time of writing, a rather intense typhoon (Neoguri) is about to hit South China.

    As discussed in Chan et al. (2001), we will provide an updated forecast sometime in June.


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

Cool

Not Available

CFS
NCEP

Neutral

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.

 

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
 
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

1971

35

24

La Niña

 

1974

32

15

1975

20

14

1999

24

12

2000

25

15

1985

25

17

Neutral

1989

31

21

1996

33

21

2001

29

20

 


 

Fig. 1.
 
Time series of the annual number of tropical storms and typhoon.  Red circle and blue squares indicate the El Niño and La Niña years respectively.   The green triangle indicated the predicted number in 2008.

 

Fig. 2.
 
500-hPa geopotential height anomalies between February and March in 2008. Thick contours indicate the geopotential height (contour interval = 10 m) ³5860 m.

 

 

 2001
 2002
 2003
 2004
 2005
 2006
 2007

 

Fig. 3.
 
500-hPa geopotential height anomalies between February and March in the years from 2001 to 2007. Thick contours indicate the geopotential height (contour interval = 10 m) ³5860 m.

 

Fig. 4.
 
Composite annual TC activity anomaly during the year after a La Niña event (from Chan 2000).  The pink and blue shadings indicate that the anomalies are significant at the 95% and 90% confidence levels respectively.

 


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., 2000: Tropical cyclone activity over the western North Pacific associated with El Nio and La Nia Events. J. Climate, 13, 2960-2972. 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