Verification of Forecasts of Tropical Cyclone Activity 
over the Western North Pacific in 2006


Issued on 17 Jan 2007

 

 

1. Introduction

    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).  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 Niņo/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. Verification of the 2006 forecasts

a. Summary of the forecasts issued

    On 24 April 2006, our forecasts suggested “near-normal activity for all the categories”.  A similar forecast was made on 23 June 2006 that we expected near-normal conditions for all categories.  Detailed numbers are summarized in Table 1, together with the observed numbers based on the warnings from JTWC (individual TCs listed in Table 2).

 b. Verification and discussion

    The TC activity in 2006 over the WNP was slightly below-normal (Table 1), with 24 TCs reaching at least tropical storm intensity (the normal being 27) and 15 TCs reaching typhoon intensity (the normal being 17). Our forecasts from both April and June slightly over-estimated the TC activity, but with small errors, and are therefore considered to be acceptable.

     This below-normal activity follows that of 2005 (Fig. 1).  Although an El Niņo developed in the latter part of 2006, it was not strong and apparently began too late in the season to have much effect.  While the 850-hPa zonal wind anomalies are westerly in the central and eastern equatorial Pacific, they are only weakly positive in the equatorial western Pacific and only west of 150oE (Fig. 2).  As a result, other than a few high-latitude TCs, none of the TCs formed east of this longitude (Fig. 3), quite unlike the situation in a strong El Niņo year (e.g. 1997, 2002 and 2004).  This happened despite an above-normal SST in the eastern part of the WNP (Fig. 4).  Thus, without the support of favourable dynamic conditions, a slight increase in SST cannot cause the formation of TCs in the WNP, consistent with results of previous studies (e.g. Chan and Liu 2004).  The below-normal activity is therefore likely a result of the favourable dynamic conditions not extending into the eastern part of the WNP.  As a result, TCs could only form west of 150oE and hence the number of formations is less than normal.

 

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., 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

Chan, J. C. L. and K. S. Liu, 2004:  Global warming and western North Pacific typhoon activity from an observational perspective.  J. Climate, 17, 4590-4602. Abstract


 Table 1.  Forecasts of TC activity in 2006 issued in April and June.

2006

Forecast

Observed

Normal

 

April

June

 

 

No. of TCs

32

31

27

31

No. of TCs with at least tropical storm intensity

27

28

24

27

No. of typhoons

17

18

15

17

 


Table 2. 2006 summary of tropical cyclones over the western North Pacific. 

 

Tropical cyclones

Tropical cyclones with at least tropical storm intensity

Tropical cyclones with typhoon intensity

 

01. 01W *
02. Chanchu

03. Jelawat
04. Ewiniar
05. Bilis
06. Kaemi
07. Prapiroon
08.Saomai
09. Maria
10. Bopha
11. Wukong
12. Sonamu
13. 13W
14. Ioke ^
15. Shanshan
16. 15W
17. Yagi
18. 17W *
19. Xangsane
20. Bebinca
21. Rumbia
22. Soulik
23. Cimaron
24. Chebi
25. Durian
26. Utor
27. Trami
#

01. 01W*
02. Chanchu

03. Jelawat
04. Ewiniar
05. Bilis
06. Kaemi
07. Prapiroon
08.Saomai
09. Maria
10. Bopha
11. Wukong
12. Sonamu
13. Ioke
14. Shanshan
15. Yagi
16. 17W*
17. Xangsane
18. Bebinca
19. Rumbia
20. Soulik
21. Cimaron
22. Chebi
23. Durian

24. Utor

 

01. Chanchu
02. Ewiniar
03. Kaemi
04. Prapiroon

05. Saomai
06. Maria
07. Ioke
08. Shanshan
09. Yagi
10. Xangsane
11. Soulik
12. Cimaron
13. Chebi
14. Durian

15. Utor

 

Total number

27

24

15

Predicted number
(issued in April)


32


27


17

Predicted number
(issued in June)


31


28


18

* 01W and 17W were considered as having TS intensity by JTWC but no name was given by RSMC Tokyo
#  Trami was considered by RSMC Tokyo as having reached TS intensity but JTWC classified it as TD only.

^ Typhoon Ioke formed over the central North Pacific and entered the western North Pacific as a typhoon


Fig. 1.  Annual number of tropical storms and typhoons in the western North Pacific between 1960 and 2006. The horizontal line indicates the climatological mean.


Fig. 2.  850-hPa zonal wind anomalies between June and Nov 2006 over the WNP.


Fig. 3. Tracks of tropical cyclones in 2006 based on JTWC warnings. Typhoon symbols indicate formation locations.


Fig. 4.  As in Fig. 2 except for SST.