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

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

Webster et al.’s (2005) Science paper

Number75115 Percentage3242 No. of Category 4 and 5 Typhoons

ACE vs. May-Nov SSTA (5-30 O N, O E) [standardized = (raw-mean)/S.D.] correlation = -0.30

Number75115 Percentage3242 No. of Category 4 and 5 Typhoons

Number Percentage No. of Category 4 and 5 Typhoons

Controls on Typhoon Intensity on Climate Scales

Standardized Number of Cat 4 & 5 Typhoons

Wavelet Analysis of Intense Typhoon Occurrence Frequency 2-7 yr yr Period A1Period BPeriod A2

Thermodynamic Factors

Sea-surface Temperature Anomalies Period A1 Period B Period A2

Reconstructed Moist Static Energy (EOF2) Period A1Period A2 Period B

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

Precipitation Rate Anomalies Period A1Period A2 Period B

Summary on thermodynamic control Southeastern part of western North Pacific appears to be a critical region – the key regionSoutheastern 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 higherSST is higher more energy for convectionmore energy for convection lower troposphere more convectively unstable – indeed more rainfall observedlower troposphere more convectively unstable – indeed more rainfall observed  higher potential for tropical cyclones to form in the key region

Dynamic Factors

Lower Tropospheric Streamfunction Anomalies Period A1Period A2 Period B

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

Tracks of Intense Typhoons Period A1Period A2 Period B

Frequency of Occurrence of Intense Typhoons Period A1Period A2 Period B

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

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 rotationhigher cyclonic rotation less vertical wind shearless vertical wind shear

Summary During periods of above-normal number of intense typhoons: both thermodynamic and dynamic conditions in the key region favour the formation of tropical cyclonesboth 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 conditionsonce 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 recurvaturethe 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

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 atmosphereFrequency 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 occurrenceAny changes in these conditions, e.g. caused by global warming, could therefore change such frequency of occurrence

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 favourableHowever, 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 PacificThus, it is not clear whether global warming will, or can, indeed, lead to more intense typhoons in the western North Pacific