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Syktus J. and K. Walsh Department of Natural Resources, Mines and Energy, Brisbane, Australia

The impact of stratospheric ozone depletion and CO 2 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. Introduction

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Syktus J. and K. Walsh Department of Natural Resources, Mines and Energy, Brisbane, Australia

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  1. 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

  2. Introduction • Detecting and tracking model tropical cyclones in AGCMs • Vertical Wind Shear changes in Australian region in CSIRO C20C simulations • Detection - preliminary results • Conclusions

  3. 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

  4. 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

  5. Observed cyclone tracks, Australian region 1967-2000 (every 5th track) Data source: Australian Bureau of Meteorology

  6. Average TC track density & intensity 1970-97 in Australian region (Sinclair, 2002)

  7. Influence of El Nino and La Nina

  8. Influence of El Nino and La Nina

  9. 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.

  10. 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!

  11. Deficiencies mostly impact the intensityof the model tropical cyclones. • Unlikely to have a strong impact on the seasonal variability of the tropical cyclone activity.

  12. 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 simulationsfor 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

  13. Detection criteria used • vorticity z > -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

  14. Regional model simulation, model “El Nino” and “La Nina” conditions El Nino La Nina RCM 75 km has correctly captured the ENSO relationship for TC distribution, but the contrast was underestimated Walsh and Syktus, 2003, Atmos. Sci. Letters

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

  16. in Australian region

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

  18. VWS difference SST_O3_CO2- SST 1961-2003 JFM

  19. VWS Trend SST_O3 1961-2003 JFM

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

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

  22. East coast of Australia, with region of coastal-approaching storms defined as west of the indicated line Walsh & Syktus 2003

  23. Coastal-approaching storms

  24. Preliminary conclusions • Model able to capture a basic contrast between El Nino and La Nina years • Increased Vertical Wind Shear for the O3 caseindicate 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

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