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RECENT DECLINE IN TYPHOON ACTIVITY IN THE SOUTH CHINA SEA

RECENT DECLINE IN TYPHOON ACTIVITY IN THE SOUTH CHINA SEA. LEUNG, Y K WU, M C YEUNG, K K. Content. Observation: Decline in Typhoon (TY) & Category 4-5 TY activities in the South China Sea (SCS) in the last 10 years (1996-2005) Investigation: The causes of recent decline in TY activities

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RECENT DECLINE IN TYPHOON ACTIVITY IN THE SOUTH CHINA SEA

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

  2. Content • Observation: • Decline in Typhoon (TY) & Category 4-5 TY activities in the South China Sea (SCS) in the last 10 years (1996-2005) • 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 • Hong Kong Observatory (HKO) • 1961-2005 • TC best track data of • Regional Specialized Meteorological Centre (RSMC) • 1977-2005 • Joint Typhoon Warning Centre (JTWC) • 1961-2005

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

  5. Annual number of Typhoons occurred in the SCS

  6. TYs Gaussian filtered time serieswith periodicities of < 10 years CAT 4-5 TYs A decline in the recent 10 years (1996-2005) Anomaly in the annual number

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

  8. 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. more fewer TY track density

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

  13. Average SST over (10-20 N, 110-120 E) Correlation with 500 hPa height Difference in SST (1996-2005 mean)-(1961-1995 mean)

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

  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 betweenSST(SCS) and H500(140-150E,10-15N)

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

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