1. RECENT DECLINE IN TYPHOON ACTIVITY IN THE SOUTH CHINA SEA
LEUNG, Y K WU, M C YEUNG, K K

2. Content Observation: Investigation:
Decline in Typhoon (TY) & Category 4-5 TY activities in the South China Sea (SCS) in the last 10 years ( )
Investigation:
The causes of recent decline in TY activities
Interdecadal variations of Typhoons formed east of 120E in the Western North Pacific (WNP)
Shift of recent tropical cyclone (TC) tracks, possibly
due to a change in 500 hPa steering flow or SST in SCS
but not likely due to change in TC genesis locations

3. Data Tropical cyclone (TC) best track data of TC best track data of
Hong Kong Observatory (HKO)
TC best track data of
Regional Specialized Meteorological Centre (RSMC)
Joint Typhoon Warning Centre (JTWC)

4. High Tropical Cyclone Activity Region ( Western North Pacific Ocean & South China Sea )
Hong Kong
South China Sea
Western North Pacific
East of 120E

5. Annual number of Typhoons occurred in the SCS

6. Anomaly in the annual number
TYs
A decline in the recent 10 years ( )
Gaussian filtered time series with periodicities of < 10 years
CAT 4-5 TYs

7. Annual number of Typhoons occurred east of 120 E in the WNP

8. Frequency Spectrum by Multitaper Method ( annual no
Frequency Spectrum by Multitaper Method ( annual no. of TYs formed east of 120E)

9. Genesis positions of typhoon ( 1996 – 2005 )

10. Genesis position for TYs (2)

11. TY track density
more
fewer

12. Difference in the 500 hPa steering flow (1995-2005 mean)-(1961-1995 mean)
Difference in 500 hPa geopotential height

13. Difference in SST (1996-2005 mean)-(1961-1995 mean)
Average SST over (10-20 N, E)
Correlation with 500 hPa height

14. Correlation between 500 hPa heights and the mean SST in the SCS
Correlation coefficient

15. Summary of results
Typhoon (TY) activity in the South China Sea (SCS) has been found to decline since the mid-1990s.
Reasons for the Decline:
inter-decadal variations, i.e. fewer TYs were formed in the Western North Pacific (mid-1990s to 2005 being quieter period).
More TY tracks towards East China Sea and Japan than towards the SCS
Shift in TC tracks may be a result of a change in the mid-tropospheric flow, the latter of which correlates well with the rising mean SST of the South China Sea

16. Thank You

17. 12 out of 21 typhoons occurred over South China Sea in 1971

18. Only 1 out of 14 typhoons occurred over South China Sea in 2002

19. Sea Surface Temperature (SCS) & 500 hpa Geopotential Heights Trends

20. Good correlation between SST(SCS) and H500(140-150E,10-15N)

21. poor correlation between SST(SCS) and H500(170-180E,25-30N)