Decadal Variations of Intense Decadal Variations of Intense Typhoon ActivityTyphoon Activity

Johnny ChanJohnny Chan

CityU-IAP Laboratory for Atmospheric ScienceCityU-IAP Laboratory for Atmospheric Science

Laboratory for Atmospheric ResearchLaboratory for Atmospheric Research

Dept. of Physics & Mat. Sci.Dept. of Physics & Mat. Sci.

City University of Hong KongCity University of Hong Kong

in Relation to the Global Warming Issuein Relation to the Global Warming Issue

OutlineOutline

• What has been said

• What is actually observed

• Controls on typhoon intensity on climate scales– thermodynamic– dynamic

• Conclusion

Webster et al.’s (2005) Webster et al.’s (2005) ScienceScience paper paper

1975-89 1990-2004

Number 75 115

Percentage 32 42

No. of Category 4 and 5 TyphoonsNo. of Category 4 and 5 Typhoons

ACE ACE vs.vs. May-Nov SSTA (5-30May-Nov SSTA (5-30OON, 120-180N, 120-180OOE)E)

[standardized = (raw-mean)/S.D.][standardized = (raw-mean)/S.D.]

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

ACE

SSTA

correlation = -0.30

1975-89 1990-2004

Number 75 115

Percentage 32 42

No. of Category 4 and 5 TyphoonsNo. of Category 4 and 5 Typhoons

1960-74 1975-89 1990-2004

Number 105 75 115

Percentage 37 32 42

No. of Category 4 and 5 TyphoonsNo. of Category 4 and 5 Typhoons

Controls on Typhoon IntensityControls on Typhoon Intensityon Climate Scaleson Climate Scales

Standardized Number of Cat 4 & 5 TyphoonsStandardized Number of Cat 4 & 5 Typhoons

-2

-1

0

1

2

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Year

Wavelet Analysis of Intense Typhoon Occurrence FrequencyWavelet Analysis of Intense Typhoon Occurrence Frequency

2-7 yr

16-32 yr Period A1 Period B Period A2

Thermodynamic FactorsThermodynamic Factors

Sea-surface Sea-surface Temperature Temperature AnomaliesAnomalies

Period A1

Period B

Period A2

Reconstructed Reconstructed Moist Static Moist Static

Energy (EOF2)Energy (EOF2)

Period A1 Period A2

Period B

Vertical Gradient of Vertical Gradient of Saturated Moist Saturated Moist

Static EnergyStatic Energy

(1000 minus 600 (1000 minus 600 hPa)hPa)

Period A1 Period A2

Period B

Precipitation Rate Precipitation Rate AnomaliesAnomalies

Period A1 Period A2

Period B

Summary on thermodynamic controlSummary on thermodynamic control

•Southeastern part of western North Pacific appears to Southeastern part of western North Pacific appears to be a critical region – the be a critical region – the keykey region region

During periods of above-normal number of intense During periods of above-normal number of intense typhoonstyphoons, the following thermodynamic characteristics , the following thermodynamic characteristics are found in the key region:are found in the key region:

•SST is higherSST is higher

•more energy for convectionmore energy for convection

•lower troposphere more convectively unstable – indeed lower troposphere more convectively unstable – indeed more rainfall observedmore rainfall observed

higher potential for tropical cyclones to form in the higher potential for tropical cyclones to form in the key regionkey region

Dynamic FactorsDynamic Factors

Lower Tropospheric Lower Tropospheric Streamfunction Streamfunction

AnomaliesAnomalies

Period A1 Period A2

Period B

200-hPa minus 850-hPa Zonal Wind Shear200-hPa minus 850-hPa Zonal Wind ShearPeriod A1 minus Period B Period A2 minus Period B

Tracks of Intense Tracks of Intense TyphoonsTyphoons

Period A1 Period A2

Period B

Frequency of Frequency of Occurrence of Occurrence of

Intense TyphoonsIntense Typhoons

Period A1 Period A2

Period B

Blue shading: 95% Green shading: 90%

Period A1 minus Period B Period A2 minus Period B

Difference in the Frequency of Occurrence Difference in the Frequency of Occurrence of Intense Typhoonsof Intense Typhoons

Summary on dynamic controlSummary on dynamic control

During periods of above-normal number of intense During periods of above-normal number of intense typhoonstyphoons, the following dynamic characteristics are , the following dynamic characteristics are found in the key region:found in the key region:

•higher cyclonic rotationhigher cyclonic rotation

•less vertical wind shearless vertical wind shear

SummarySummaryDuring periods of above-normal number of intense During periods of above-normal number of intense typhoonstyphoons::

•both thermodynamic and dynamic conditions in the key both thermodynamic and dynamic conditions in the key region favour the formation of tropical cyclonesregion favour the formation of tropical cyclones

•once formed, these cyclones move northwestward once formed, these cyclones move northwestward along paths that continue to have these favourable along paths that continue to have these favourable conditionsconditions

•the atmospheric flow (steering) patterns also allow the atmospheric flow (steering) patterns also allow them to have low-latitude recurvaturethem to have low-latitude recurvature

these cyclones can stay over the ocean under these cyclones can stay over the ocean under favourable conditions for intensification for a longer favourable conditions for intensification for a longer period of time, and hence can become more intenseperiod of time, and hence can become more intense

ConclusionConclusion

•Frequency of intense typhoon occurrence Frequency of intense typhoon occurrence undergoes decadal or multi-decadal undergoes decadal or multi-decadal changes in response to changes in the changes in response to changes in the planetary-scale thermodynamic and planetary-scale thermodynamic and dynamic conditions in the atmospheredynamic conditions in the atmosphere

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

ConclusionConclusion•However, although thermodynamic However, although thermodynamic conditions may have become more conditions may have become more favourable as a result of global warming, favourable as a result of global warming, evidence has yet to be identified for the evidence has yet to be identified for the dynamic conditions to have become dynamic conditions to have become favourable favourable

•Thus, it is not clear whether global Thus, it is not clear whether global warming will, or can, indeed, lead to more warming will, or can, indeed, lead to more intense typhoons in the western North intense typhoons in the western North PacificPacific