Decadal Variations of Intense Typhoon Activity

1. Decadal Variations of Intense Typhoon Activity in Relation to the Global Warming Issue Johnny Chan CityU-IAP Laboratory for Atmospheric Science Laboratory for Atmospheric Research Dept. of Physics & Mat. Sci. City University of Hong Kong

2. Outline • What has been said • What is actually observed • Controls on typhoon intensity on climate scales • thermodynamic • dynamic • Conclusion

3. Webster et al.’s (2005) Science paper

4. No. of Category 4 and 5 Typhoons

5. ACE vs.May-Nov SSTA (5-30ON, 120-180OE) [standardized = (raw-mean)/S.D.] correlation = -0.30

6. No. of Category 4 and 5 Typhoons

7. No. of Category 4 and 5 Typhoons

8. Controls on Typhoon Intensityon Climate Scales

9. Standardized Number of Cat 4 & 5 Typhoons

10. Wavelet Analysis of Intense Typhoon Occurrence Frequency 2-7 yr 16-32 yr Period A1 Period B Period A2

11. Thermodynamic Factors

12. Period A1 Period A2 Period B Sea-surface Temperature Anomalies

13. Period A1 Period A2 Period B Reconstructed Moist Static Energy (EOF2)

14. Period A2 Period A1 Vertical Gradient of Saturated Moist Static Energy (1000 minus 600 hPa) Period B

15. Period A1 Period A2 Period B Precipitation Rate Anomalies

16. Summary on thermodynamic control • Southeastern part of western North Pacific appears to be a critical region – the key region • During periods of above-normal number of intense typhoons, the following thermodynamic characteristics are found in the key region: • SST is higher • more energy for convection • lower troposphere more convectively unstable – indeed more rainfall observed • higher potential for tropical cyclones to form in the key region

17. Dynamic Factors

18. Period A1 Period A2 Period B Lower Tropospheric Streamfunction Anomalies

19. 200-hPa minus 850-hPa Zonal Wind Shear Period A1 minus Period B Period A2 minus Period B

20. Period A1 Period A2 Period B Tracks of Intense Typhoons

21. Period A1 Period A2 Period B Frequency of Occurrence of Intense Typhoons

22. Difference in the Frequency of Occurrence of Intense Typhoons Period A1 minus Period B Period A2 minus Period B Blue shading: 95% Green shading: 90%

23. Summary on dynamic control • During periods of above-normal number of intense typhoons, the following dynamic characteristics are found in the key region: • higher cyclonic rotation • less vertical wind shear

24. Summary • During periods of above-normal number of intense typhoons: • both thermodynamic and dynamic conditions in the key region favour the formation of tropical cyclones • once formed, these cyclones move northwestward along paths that continue to have these favourable conditions • the atmospheric flow (steering) patterns also allow them to have low-latitude recurvature • these cyclones can stay over the ocean under favourable conditions for intensification for a longer period of time, and hence can become more intense

25. Conclusion • Frequency of intense typhoon occurrence undergoes decadal or multi-decadal changes in response to changes in the planetary-scale thermodynamic and dynamic conditions in the atmosphere • Any changes in these conditions, e.g. caused by global warming, could therefore change such frequency of occurrence

26. Conclusion • However, although thermodynamic conditions may have become more favourable as a result of global warming, evidence has yet to be identified for the dynamic conditions to have become favourable • Thus, it is not clear whether global warming will, or can, indeed, lead to more intense typhoons in the western North Pacific