Verification of Forecasts of Tropical Cyclone Activity 
over the Western North Pacific in 2007


Issued on 11 Jan 2008

 

 

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 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. Verification of the 2007 forecasts

a. Summary of the forecasts issued

    On 23 April 2007, our forecasts suggested “below-normal activity for all the categories”.  A similar forecast was made on 25 June 2007 that we expected below-normal conditions for all categories.  Detailed numbers are summarized in Table 1, together with the observed numbers based on the warnings from JTWC and RSMC Tokyo (Table 2).

    Disagreements exist between the two warning centres on the intensity of some of the systems.  One system (06W) was classified by JTWC as having reached tropical storm intensity but not named by the RSMC.  On the other hand, RSMC Tokyo issued tropical storm warning for two systems (Haiyan and Podul) but no warning was given by JTWC.  Lekima was considered by JTWC as having reached typhoon intensity but not by RSMC Tokyo.

 b. Verification and discussion

    The TC activity in 2007 over the WNP was below-normal, with 23 (24) TCs reaching at least tropical storm intensity and 15 (14) TCs reaching typhoon intensity according to JTWC (RSMC-Tokyo) warnings (Table 2). Our forecasts for 2007 are therefore correct in all categories for both the April and June forecasts.  It is worth to note that it is the eighth year that the WNP has a below-normal TC activity since 1998 except for 2001 and 2004 (Fig. 1) and the inactive TC activity period seems to persist. 

    A moderate La Niña event has developed in 2007 and the historical records suggest that the TC activity over the WNP tends to be slightly below normal or below normal in a La Niña year (Table 3).  The TC activity in 2007 appears to be consistent with the atmospheric conditions associated with a La Niña event. 

    The summertime (June-September) 500-hPa geopotential height show positive anomalies over the subtropical WNP (Fig. 2), indicating the stronger than normal subtropical high.  The predictor related to the subtropical high suggested a stronger-than-normal subtropical high in the TC season and hence the below-normal TC activity, which were consistent with the observational results.

 

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


Table 1Forecasts of TC activity in 2007 issued in April and June.  The observed activity based on both the JTWC and RSMC-Tokyo warnings and the normal values are also shown.

2007

Forecast

Observed

Normal

 

April

June

 JTWC

RSMC

 

No. of TCs

28

27

26

---

31

No. of TCs with at least tropical storm intensity

25

24

23

24

27

No. of typhoons

14

14

15

14

17

 


Table 2. 2007 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. Kong_Rey
02. Yutu

03.
Toraji
04. Man-yi
05. Usagi
06. 06W#
07. Pabuk
08. Wutip
09. Sepat
10. Fitow
11. Danas
12. Nari
13. Wipha
14. 14W
15. Francisco
16.
Lekima%
17. Krosa
18. Haiyan*
19. Podul*
20. Lingling

21. Kajiki
22. Faxai
23. Peipah
24. Tapah
25. Hagibis
26. Mitag
27. 25W
28. 26W

01. Kong_Rey
02. Yutu

03.
Toraji
04. Man-yi
05. Usagi
06.
06W#
07. Pabuk
08. Wutip
09. Sepat
10. Fitow
11. Danas
12. Nari
13. Wipha
14. Francisco
15. Lekima
%
16. Krosa
17. Haiyan*
18. Podul*

19. Lingling
20. Kajiki
21. Faxai
22. Peipah
23. Tapah
24. Hagibis
25. Mitag

01. Kong_Rey
02. Yutu

03.
Man-yi
04. Usagi
05. Pabuk
06. Sepat
07. Fitow
08. Nari
09. Wipha
10. Lekima
%
11. Krosa
12. Kajiki
13. Peipah
14. Hagibis
15. Mitag

 

 

Total number @

26 (JTWC)

23 (JTWC) / 24 (RSMC)

15 (JTWC) / 14 (RSMC)

Predicted number
(issued in April)


28


25


1
4

Predicted number
(issued in June)


27


2
4


1
4

# 06W wase considered as having TS intensity by JTWC but no name was given by RSMC Tokyo
*  Haiyan and Podul were considered by RSMC Tokyo as having reached TS intensity but no warning was given by JTWC.
% Lekima was considered by JTWC as having reached typhoon intensity but not by RSMC Tokyo

@
See above notes for the reason for these differences in numbers.


Table 3.  Number of TCs with at least tropical storms intensity and number of typhoons in La Niña years.

La Niña year

No. of TCs with at least tropical storm intensity

No. of  typhoons

1964

39

26

1970

24

12

1971

35

24

1973

21

12

1974

32

15

1975

20

14

1984

27

16

1988

26

14

1995

26

15

1998

17

9

1999

24

12

2000

25

15

2007

23

15

 

   


Fig. 1.  Annual number of tropical storms and typhoons in the western North Pacific between 1960 and 2007. The horizontal line indicates the climatological mean.


Fig. 2.  500-hPa geopotential height anomalies between June and September in 2007. Thick contours indicate geopotential heights (contour interval = 10 m) ³ 5860 m.