C20C Workshop, ICTP Trieste 2004 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

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; ° 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 (every 5 th track) Data source: Australian Bureau of Meteorology

C20C Workshop, ICTP Trieste 2004 Average TC track density & intensity 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 26 o C.  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 each SST (HadISST 1.1) only SST and solar (monthly, Lean) SST, solar and CO 2 Plus ensemble of 5 simulations for SST, solar, CO 2 & O 3 Ensemble of 5 simulations for with SST Ensemble of 5 simulations for with SST & O 3 Data saved 4x day from all experiments and used in track detection processing

C20C Workshop, ICTP Trieste 2004 Detection criteria used   vorticity  > 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

Regional model simulation, model “El Nino” and “La Nina” conditions Walsh and Syktus, 2003, Atmos. Sci. Letters El Nino La 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 + CO 2 case JFM

C20C Workshop, ICTP Trieste 2004 in Australian region

C20C Workshop, ICTP Trieste 2004 Vertical Wind Shear JFM SST case ensemble average (m/s)

C20C Workshop, ICTP Trieste 2004 VWS difference SST_O 3 _CO 2 - SST JFM

C20C Workshop, ICTP Trieste 2004 VWS Trend SST_O JFM

C20C Workshop, ICTP Trieste 2004 VWS Trend Difference O 3 – SST JFM (m/s per 100 yrs)

C20C Workshop, ICTP Trieste 2004 Tropical Cyclone Occurrence (per year/20) Region E, 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 O 3 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