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Verification of Forecasts of Tropical Cyclone Activity over the Western North Pacific and
Number of Tropical Cyclones Making Landfall in South China
in 2009


1. Introduction

 

Issued on 29 Jan 2010

Since 2000, 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. We also began in 2009 to predict the number of TCs making landfall in South China.

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 Goh and Chan (2009a).


2. Verification of the 2009 forecasts

a. Summary of the forecasts issued

1) TC activity over the WNP

Our April forecasts (issued on 20 April 2009) suggested “near-normal activity for all the categories”.  The June forecasts made on 18 June 2009 gave a similar forecast.  Detailed numbers are summarized in Table 1, together with the observed numbers based on the warnings from JTWC and the Tokyo Regional Specialised Meteorological Center (RSMC) (Table 2). 

Disagreements occurred among the warning centres on the intensity of some of the systems.  Two systems (18W and 25W) were classified by the JTWC as having reached tropical storm intensity but were not named by the RSMC.  Linfa was considered by JTWC as having reached typhoon intensity but not by RSMC Tokyo.  In addition, two TCs (02C and Maka) formed over the Central Pacific and moved into the WNP.

2) TC landfall in South China

The April forecast called for a slightly above normal number of TCs making landfall along the South China coast in the early season (May to August), a below-normal number in the late season (September to December), and a slightly below normal number overall. The forecast issued in June also suggested the number of landfalling tropical cyclones (TCs) to be slightly below normal for the main season (July to December). Table 1 shows a summary of these forecasts, along with the observed number in 2009 and the normal values.

 b. Verification and discussion

1) TC activity over the WNP

Based on the JTWC warnings, TC activity in 2009 over the WNP was slightly below-normal (Fig. 1), with 25 TCs reaching at least tropical storm intensity and 14 TCs reaching typhoon intensity (Table 2).  The number of tropical storms and typhoons is 2 less than the normal number (the normal being 27) while the number of typhoons is 3 less than the normal number (the normal being 17).  Our forecasts from both April and June slightly over-estimated the number of TCs and the number of TCs with at least tropical storm intensity, but with small errors, and are therefore considered to be acceptable.  However, our forecasts over-estimated the number of typhoons, the possible reasons of which are discussed below.

An El Niño event developed in the summer of 2009 and the mean Jun-Nov Niño3.4 index is 0.97.  Normally, in an El Niño year, westerly anomalies are generally found over the eastern part of the tropical WNP, resulting in an eastward extension of the monsoon trough and hence a higher TC activity in this region (Wang and Chan 2002).  TCs generally have the longer life span and therefore more chance to attain typhoon intensity (Table 3).  This is a main reason for our forecast of the slightly above-normal number of typhoons.  However, in the 2009 TC season, although the sea-surface temperatures in the central equatorial Pacific are above normal, easterly anomalies are found at the low-levels (850-hPa) over the eastern part of the tropical WNP, with the maximum amplitude near 170oE (Fig. 2) and therefore no eastward extension of the monsoon trough was observed.  As a result, no significant eastward shift in the mean genesis location was found and only a few storms formed over the tropical WNP east of 160oE, which is quite different from a TC season associated with a typical El Niño event.  These partly explain the slightly below-normal TC activity especially for the number of typhoons. On the other hand, the June-November 500-hPa geopotential height shows positive anomalies over the subtropical WNP, indicating a stronger than normal subtropical high (Fig. 3). Such atmospheric conditions are therefore less favourable for TC genesis and this partly explains the slightly below-normal TC activity.  Indeed, the predictor related to the subtropical high gave the better predictions (predicted number of TCs with at least tropical storm intensity being 27 and number of typhoons being 16) than the ENSO predictors. 

It should also be noted that during the past five decades, the TC activity exhibited a significant interdecadal variation, with the active periods of 1960-76 and 1989-97 and the inactive periods of 1977-1988 and 1998-2008.  The inactive TC period 1998–2008 appears to continue into 2009.  The number of tropical storms and typhoons is below the climatological mean in the 2009 TC season, which is the 10th out of the last 12 years since 1998 with a below-normal TC activity (Fig. 1).

2) TC landfall in South China 

The number of TCs making landfall along the coast of South China was 4 in the early season, 3 in the late season, 6 in the main season, and 7 in the whole season (Table 1). The prediction for the early season was perfect. However, the predictions for the late and main seasons, even if TC Parma, which had taken on a highly unusual track of looping around the northern tip of the Philippines, were not considered, had underestimated the number of landfalling TCs, which also led to an underestimation of the predicted value for the whole season.

Since the error is most prominent in the late season, it is worth studying the atmospheric patterns during that period. In our forecasts issued in April and June, the year 2009 was expected to be an El Niño year, which according to Goh and Chan (2009b), is supposed to see below-normal number of TCs making landfall in South China. However, as mentioned above, the atmospheric patterns are not typical of an El Niño year. In addition to the 850-hPa flow pattern found to be different from that in a typical El Niño year, the formation factors of 850-hPa geopotential height (Fig. 4a) and 200-850 vertical wind shear (Fig. 4b) both show negative anomaly relative to the average during El Niño years in the South China Sea and around the Philippines, while both 1000-500-hPa moist static energy (Fig. 4c) and 850-hPa vorticity (Fig. 4d) show positive anomaly. These factors are included in the current prediction scheme (Goh and Chan 2009a), and are conditions favourable for the formation of TCs, and thus lead to more TCs making landfall along the coast of South China. Therefore the underestimation of this year’s late season forecast appears to be related to the atypical situation of the atmospheric patterns compared to those during typical El Niño years.

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

Goh, A. Z. C., and J. C. L. Chan, 2009a: An improved statistical scheme for the prediction of tropical cyclones making landfall in South China. Submitted to Weather and Forecasting.

Goh, A. Z. C., and J. C. L. Chan, 2009b: Interannual and interdecadal variations of tropical cyclone activity in the South China Sea. International Journal of Climatology, DOI: 10.1002/joc.1943.

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 1. Forecasts of TC activity in 2009 issued in April and June.  The observed activity based on both the JTWC and RSMC-Tokyo warnings and the normal values are also shown.

 

2009

Forecast

Observed

Normal

 

April

June

 JTWC

RSMC

 

Entire western North Pacific

No. of TCs

31

30

29

---

31

No. of TCs with at least tropical storm intensity

27

27

25

22

27

No. of typhoons

18

19

14

13

17

Landfall in South China
Early Season (May to Aug) 4 --- 4 ---

3

Late Season (Sep to Dec) 0 --- 3 --- 2
Main Season (Jul to Dec) --- 3 6 --- 4
Whole Season (May to Dec) 4 --- 7 --- 5

 


Table 2. 2009 summary of tropical cyclones over the western North Pacific and tropical cyclones making landfall in South China.

 

 

Tropical cyclones

Tropical cyclones with at least tropical storm intensity

Tropical cyclones with typhoon intensity

Tropical cyclones making landfall in South China

 

01. Kujira
02. Chan-hom
03. Linfa%
04. Nangka
05. Soudelor
06. 06W
07. Molave
08. Goni
09. Morakot
10. Etau
11. Maka*
12. Vamco
13. Krovanh
14. 02C*
15. Dujuan
16. Mujigae
17. Choi-wan
18. Koppu
19. Ketsana
20. 18W
#
21. Parma
22. Melor
23. Nepartak
24. Lupit
25. Mirinae

26. 24W
27. 25W
#
28. Nida
29. 27W

01. Kujira
02. Chan-hom
03. Linfa%
04. Nangka
05. Soudelor
06. Molave
07. Goni
08. Morakot
09. Etau
10. Maka*
11. Vamco
12. Krovanh
13. Dujuan
14. Mujigae
15. Choi-wan
16. Koppu
17. Ketsana
18. 18W
#
19. Parma
20. Melor
21. Nepartak
22. Lupit
23. Mirinae

24. 25W#
25. Nida
 

01. Kujira
02. Chan-hom
03. Linfa%
04. Molave
05. Morakot
06. Vamco
07. Choi-wan
08. Koppu
09. Ketsana
10. Parma
11. Melor
12. Lupit
13. Mirinae

14. Nida

 

 

 

 

01. Nangka
02. Soudelor
03. Molave
04. Goni
05. Mujigae
06. Koppu
07. Parma

 

 

Total number

29 (JTWC)

25 (JTWC) /
22 (RSMC)

14 (JTWC) /
13 (RSMC)

7

Predicted number
(issued in April)


31


27


18


 

Predicted number
(issued in June)


30


27


1
9


 

# 18W and 25W were considered as having TS intensity by JTWC but no name was given by RSMC Tokyo
*
  02C and Maka formed over the Central Pacific and moved into the western North Pacific.

% Linfa
was considered by JTWC as having reached typhoon intensity but not by RSMC Tokyo

 


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.

 

 

El Niño Year

Number of tropical storms and typhoons

Number of typhoons

Active period

1963

25

19

1965

34

21

1969

19

13

1972

30

22

1976

25

14

1991

30

20

1994

36

21

1997

31

23

 

 

 

 

Inactive period

1982

26

19

1986

27

19

1987

24

18

2002

26

18

2004

30

21

2006

22

14

2009

25

14

 


Fig. 1.
 
Annual number of tropical storms and typhoons between 1960 and 2009. The horizontal line indicates the climatological mean.  Red circle and blue squares indicate the El Niño and La Niña years respectively. 

 

 


Fig. 2. 850-hPa wind anomalies (vector) between June and November in 2009.  Shadings indicate the zonal wind (interval = 0.5 m s-1).

 



Fig. 3.
 
500-hPa geopotential height anomalies between June and November in 2009. Thick contour indicates the geopotential height (contour interval = 10 m) ³ 5860 m.

 

 


Fig. 4. Differences in the late season (September-December) atmospheric fields between 2009 and other El Niño years. (a) 850-hPa geopotential height (contour interval = 2 m), (b) 200-850 hPa vertical wind shear (contour interval = 1 m s-1), (c) MSE (contour interval = 3 x 106 W m-2) and (d) 850-hPa relative vorticity (contour interval = 2 x 10-6 s-1).