C20C Workshop, ICTP Trieste 2004

The impact of stratospheric ozone depletion and CO2 on tropical

cyclone behaviour in the Australian region

Syktus J. and K. Walsh

Department of Natural Resources, Mines and Energy, Brisbane, Australia

School of Earth Sciences, University of Melbourne, Melbourne, Australia

C20C Workshop, ICTP Trieste 2004

• Introduction• Detecting and tracking model tropical

cyclones in AGCMs• Vertical Wind Shear changes in Australian

region in CSIRO C20C simulations• Detection - preliminary results• Conclusions

C20C Workshop, ICTP Trieste 2004

Issues• Observations show a marked decrease in numbers of TC

approaching Queensland coast during the past decade, this decrease has been linked to decrease in summer rainfall in Central Queensland

• The numbers of TC approaching the coast of Western and Northern Australia has increased in recent times

• Changes to the vertical structure of cyclones under global warming has been postulated, however model uses fixed SST so it may be a fishing expedition

• Extra-tropical cyclone and anticyclone tracks and trends for the Southern Hemisphere show overall decrease (cyclones and anticyc.) but a clear increase of cyclones stronger than 980 hPa (Pezza & Ambrizzi, 2003)

• Main purpose to study the inter-annual characteristics of model simulated TCLVs

C20C Workshop, ICTP Trieste 2004

Observed Cyclone Numbers in SW Pacific 1970 to 1998

Source: Nicholls et al., 1998

• The total number of cyclones have decreased in recent decades (south of equator; 105-160° E)

• However, the number of stronger cyclones (minimum central pressure less than 970 hPa) appears to have increased

C20C Workshop, ICTP Trieste 2004

Observed cyclone tracks, Australian region 1967-2000 (every 5th track)

Data source: Australian Bureau of Meteorology

C20C Workshop, ICTP Trieste 2004

Average TC track density & intensity

1970-97 in Australian region(Sinclair, 2002)

C20C Workshop, ICTP Trieste 2004

Influence of El Nino and La Nina

C20C Workshop, ICTP Trieste 2004

Influence of El Nino and La Nina

C20C Workshop, ICTP Trieste 2004

Tropical Cyclones in AGCMs

• Numerous studies showed that AGCMs can create model tropical cyclones with strong similarities to observed tropical cyclones:

Cyclonic vorticity, convergence and high moisture content at lower levels.

Heavy precipitation and local maximum of surface winds.

Strong upward motion, positive local temperature anomaly throughout the troposphere.

Anti-cyclonic vorticity and divergency at upper levels.

C20C Workshop, ICTP Trieste 2004

Development in areas of SSTs above 26oC. Vertical structure similar to observed tropical

cyclones composites.

Model tropical cyclones in low-resolution AGCMs have deficiencies:

Lack the presence of an eye, eye-wall and rainbands.

Horizontal extension larger than observed tropical cyclones.

Non-interactive SST is a limitation factor!

C20C Workshop, ICTP Trieste 2004

• Deficiencies mostly impact the intensity of the model tropical cyclones.

• Unlikely to have a strong impact on the seasonal variability of the tropical cyclone activity.

C20C Workshop, ICTP Trieste 2004

Experiments & Analysis

Ensemble of 5 simulations for 1871-2003 each• SST (HadISST 1.1) only• SST and solar (monthly, Lean)• SST, solar and CO2

Plus ensemble of 5 simulations for 1961-2003• SST, solar, CO2 & O3

• Ensemble of 5 simulations for 1949-2003 with SST• Ensemble of 5 simulations for 1961-2003 with SST & O3

Data saved 4x day from all experiments and used in track detection processing

C20C Workshop, ICTP Trieste 2004

Detection criteria used vorticity> -10-5 s-1 (i.e. cyclonic, Southern

Hemisphere)• a closed pressure minimum within a radius of 250

km of above point• Sum of temperature anomalies at 700, 500 and 300

hPa around the centre of the storm > 0.• Wind speed higher at 300 hPa than at 850 hPa• Temp. anomaly at 300 hPa > 850 hPa• Wind speed at lowest sigma level (about 45m) > 12

ms-1

Some of the detection criteria need to be optimized

C20C Workshop, ICTP Trieste 2004

C20C Workshop, ICTP Trieste 2004

C20C Workshop, ICTP Trieste 2004

C20C Workshop, ICTP Trieste 2004

Regional model simulation, model “El Nino” and “La Nina” conditions

Walsh and Syktus, 2003, Atmos. Sci. Letters

El NinoLa Nina

RCM 75 km has correctly captured the ENSO relationship for TC distribution, but the contrast was underestimated

C20C Workshop, ICTP Trieste 2004

El Nino

La Nina

AGCM simulation of TC tracks for El Nino & La Nina conditions SST + CO2 case JFM 1949-2003

C20C Workshop, ICTP Trieste 2004

in Australian region

C20C Workshop, ICTP Trieste 2004

Vertical Wind Shear JFM 1997-2003 SST case ensemble average (m/s)

C20C Workshop, ICTP Trieste 2004

VWS difference SST_O3_CO2- SST 1961-2003 JFM

C20C Workshop, ICTP Trieste 2004

VWS Trend SST_O3 1961-2003 JFM

C20C Workshop, ICTP Trieste 2004

VWS Trend Difference O3 – SST 1961-2003 JFM (m/s per 100 yrs)

C20C Workshop, ICTP Trieste 2004

Tropical Cyclone Occurrence (per year/20)Region 145-180E, North of 30S

C20C Workshop, ICTP Trieste 2004

East coast of Australia, with region of coastal-approaching storms defined as west of the indicated line

Walsh & Syktus 2003

C20C Workshop, ICTP Trieste 2004

Coastal-approaching storms

C20C Workshop, ICTP Trieste 2004

Preliminary conclusions

• Model able to capture a basic contrast between El Nino and La Nina years

• Increased Vertical Wind Shear for the O3 case

indicate possible changes in vertical structure of atmosphere in SW Pacific region

• Tropical cyclone occurrence show strong inter-annual variability. Not clear if there is a difference….

• The numbers of calculated coastal approaching cyclones are much less than observed

• More work is required before any useful conclusions could be derived