Verification of Forecasts of Tropical Cyclone Activity 
over the Western North Pacific in 2003


Issued on 30 Jan 2004

 


Summary of the forecasts

On 24 April 2003, we predicted that it would be unlikely for tropical cyclone (TC) activity over the entire WNP in 2003 to be above normal and thus called for a normal to below normal number of tropical cyclones, tropical cyclones reaching at least tropical storm intensity, as well as typhoons (Table 1).  While this was based on the average of the statistical forecasts, the predictions were in fact consistent with the results of previous studies that TC activity is in general below normal the year after an El Niņo event (Chan 1985, 2000) and 2002 was one such event.  On the other hand, ENSO predictors predicted above-normal number of tropical cyclones over the South China Sea (SCS).  Past history suggests that during the year after a warm event, TC activity over the SCS tends to be above normal if the warm event transitions to a cold event, but varies from below to above normal if the warm event simply weakens with the transition.  Based on our assessment of the ENSO condition, we therefore predicted a normal to above-normal number of tropical cyclones and of tropical cyclones reaching at least tropical cyclone intensity for the SCS in 2003 (Table 1).  As for the number of landfalling tropical cyclones over South China, the predictors all called for a normal number (Table 1), which appeared to be reasonable given the predictions for the TC activity over the SCS.

On 24 June 2003, we updated our forecasts slightly, with the same qualitative predictions.  These are based on both the statistical forecasts as well as our assessment and prediction of the ENSO condition at the time. 

Summary and discussion of the predictions

Verifications of both sets of predictions showed that indeed over the entire WNP, the number of TCs and the number of TCs with at least tropical storm intensity are below normal, as correctly predicted (Table 1).  The number of typhoons is normal, which is still within the error margin of our predictions.

However, the predictions for the SCS are substantially wrong.  Activity over this region was actually below normal while the predictions called for normal to above-normal activity.

The number of landfalling TCs over South China was correctly predicted within the error margin.

At first, it appears that the reason for the erroneous forecasts for the TC activity over the SCS was the following.  Most of the statistical predictors were ENSO-related, and indeed a La Niņa event was anticipated (see our ENSO discussion on 16 June 2003), but the cold event never developed. The predictions were therefore incorrect.  However, a more detailed examination of the atmospheric flow conditions indicated that the atmospheric flow was very much like that of a La Niņa year (see Chan and Wu 2003).  In addition, the pattern of TC formation locations in 2003 is quite similar to those of previous La Niņa years, with many more TCs forming west of 140oE (Fig. 1).  Indeed, for the late season (Oct to Dec), the mean position of formation was even further than that of the average of all La Niņa years (Fig. 2).  The problem is that many of these TCs did not move into the SCS.  Rather, they migrated northward and/or eventually recurved.

To see how frequent this occurs, we examined the numbers of TCs over the SCS formed within the SCS and outside the SCS but moved into the SCS (Fig. 3).  It is obvious that a significant decrease in the number of TCs in the latter group occurs from 1997.  While part of this decrease was due to a general decrease in the number of TCs formed in the region east of the Philippines (Fig. 4), the percentage of these TCs entering the SCS is also decreasing.  The latter apparently results from an anticyclonic anomaly during June to October over the western North Pacific (Fig. 5) that tends to steer TCs northward.  The anomalous circulation over the SCS during these few months is also anticyclonic.  Thus, the erroneous predictions of the 2003 TC activity over the SCS are apparently due to an interdecadal variations in the strength of the subtropical high, a factor that so far has not been incorporated into the prediction scheme.  We will attempt to include this in the predictions for 2004.  Interdecadal variations shown in Figs. 3-5 will also be investigated.

 


References

Chan, J. C. L., 1985: Tropical cyclone activity in the northwest Pacific in relation to the El Niņo/Southern Oscillation. Mon. Wea. Rev., 133, 599-606. 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. and M. C. Wu, 2003:  Outlook for the 2003/04 East Asia winter monsoon.  Joint Meeting on Seasonal Prediction of the 2003/04 East Asia Winter Monsoon, Japan Meteorological Agency, Nov 11-13, Tokyo. Download

Wang, B. and J. C. L. Chan, 2002: How strong ENSO events affect tropical storm activity over the western North Pacific. J. Climate, 15, 1643-1658. Abstract


Table 1.  Forecasts of TC activity in 2003 using the CSL-4 and CSL-6 schemes.

2003

Forecast Observed Normal
  CSL-4 CSL-6    

Western North Pacific

No. of TCs 29 3 30 3 28 31
No. of TCs with at least tropical storm intensity 26 3 26 3 23 27
No. of typhoons 16 2 15 2 17 17
    

South China Sea

No. of TCs 14 2 15 2 9 13
No. of TCs with at least tropical storm intensity 11 2 12 2 7 10
No. of TCs making landfall along the South China coast 5 1 4 1 4 5

 


Table 2. 2003 summary of tropical cyclones over the western North Pacific. 

Western North Pacific (including South China Sea)

2003

Tropical cyclones Tropical cyclones with at least tropical storm intensity Tropical cyclones with typhoon intensity
   01. Yanyan
02. Kujira
03. 03W*
04. Chan-Hom
05. Linfa
06. Nangka
07. Soudelor
08. Koni
09. Imbudo
10. Morakot
11. Etau
12. Krovanh
13. Vamco
14. Dujuan
15. Jimena#
16. Maemi
17. Choi-Wan
18. Koppu
19. 18W
20. 19W
21. Ketsana
22. Parma
23. 22W
24. 23W
25. Melor
26. Nepartak
27. Lupit
28. 27W*
01. Yanyan
02. Kujira
03. 03W*
04. Chan-Hom
05. Linfa
06. Nangka
07. Soudelor
08. Koni
09. Imbudo
10. Morakot
11. Etau
12. Krovanh
13. Vamco
14. Dujuan
15. Maemi
16. Choi-Wan
17. Koppu
18. Ketsana
19. Parma
20. Melor
21. Nepartak
22. Lupit
23. 27W*


 

01. Kujira
02. Chan-Hom
03. Soudelor
04. Koni
05. Imbudo
06. Morakot
07. Etau
08. Krovanh
09. Dujuan
10. Maemi
11. Choi-Wan
12. Koppu
13. Ketsana
14. Parma
15. Melor
16. Nepartak
17. Lupit

 

 


 

 

  * 03W and 27W were considered as having TS intensity by JTWC but no name was given by RSMC Tokyo
# Tropical storm Jimena formed over the eastern North Pacific and entered the western North Pacific as a tropical depression
Total number 28 23 17
Predicted number
(issued in April)
29 3 26 16
Predicted number
(issued in June)
30 3 26 15

 


Table 3. 2003 summary of tropical cyclones over the South China Sea and making landfall along the South China coast.

South China Sea

  Tropical cyclones Tropical cyclones with at least tropical storm intensity Tropical cyclones making landfall along the South China coast
 

01. Linfa
02. Nangka
03. Koni

04. Imbudo
05. Krovanh
06. Dujuan 
07. 18W
08. 22W
09. Nepartak

01. Linfa
02. Nangka
03. Koni

04. Imbudo
05. Krovanh
06. Dujuan 
07. Nepartak

 

01. Koni
02. Imbudo
03. Krovanh
04. Dujuan 

 


otal number 9 7 4
Predicted number
(issued in April)
14 2 11 2 5 1
Predicted number
(issued in June)
15 2 12 2 5 1


Fig. 1. Tracks of tropical cyclones in 2003. Typhoon symbols indicates the formation locations.

 

Fig. 2. Averaged locations of tropical storm formation during (a) JAS, (b) OND, and (c) Jul-Dec of each year from 1965 to 1999.  Heavy squares and solids denote strong warm and strong cold years, respectively. The mean locations of TS formation for each of the five categories of SST anomalies are marked by large-size symbols as shown in (c). The averaged location of TS formation for 2003 is indicated in green (adapted from Wang and Chan 2002).

 

Fig. 3. (a) Number of tropical cyclones formed over the South China Sea (SCS) and (b) number of tropical cyclones formed over the western North Pacific (WNP) and entered the SCS from 1960 to 2003. Red and blue dots indicate El Niņo and La Niņa years respectively.

 

Fig. 4. (a) Number of TCs formed in the region east of the Philippines and (b) the percentage of these TCs entering the SCS

 

Fig. 5. Mean 500-hPa wind (vector) and geopotential height (contour) anomalies between June and October in the years 1998-2003.