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Assessing trends in observed and modelled climate extremes over Australia in relation to future projections

This study examines observed and modelled climate extremes in Australia, assessing their accuracy and projecting future changes. The study finds that global climate models can generally reproduce observed trends and variability, but some indices are not well reproduced. Future projections indicate significant increases in warm nights and heat waves, but little agreement on projected changes in precipitation extremes.

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Assessing trends in observed and modelled climate extremes over Australia in relation to future projections

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  1. Assessing trends in observed and modelled climate extremes over Australia in relation to future projections • Extremes in a changing climate, KNMI, The Netherlands, 14th-15th May, 2008 • Lisa Alexander, Julie Arblaster and Rob Smalley

  2. Aims Given that changes in climate extremes have greater impact on society and ecosystems than changes in mean climate: • Can global climate models adequately reproduce observed climate extremes over Australia? • If so, how are these extremes projected to change in the future?

  3. Can models reproduce mean change? temperature Models capture most of the overall changes except for NW precip

  4. Extremes indices (Frich et al., 2002) • Warm nights (%) • Frost days (days) • Extreme temperature range (°C) • Heat wave duration (days) • Heavy precipitation days (days) • Consecutive dry days (days) • Daily intensity (mm/day) • Maximum 5-day precipitation (mm) • Very heavy precipitation contribution (%)

  5. Observations • HadEX dataset (Alexander et al., 2006) • 3.75 x 2.5 gridded fields calculated from daily high quality temperature (Trewin, 1999) and precipitation (Haylock & Nicholls, 2000) • One value per grid box, per year, per index • www.hadobs.org

  6. Model data • Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset archived at the Program for Climate Model Diagnosis and Intercomparison (PCMDI) in California • CCSM3 (1), PCM (4), GFDL-CM2.0 (3), GFDL-CM2.1 (3) • MIROC3.2_med (3), MIROC3.2_hi (1), MRI-CGCM2.3.2 (5) • CNRM-CM3 (1) • INM-CM3.0 (1) • Total 22 runs • Models interpolated onto HadEX grid and masked to observational grid points

  7. Temperature extreme trends observations multi-model

  8. Precipitation extreme trends observations multi-model

  9. Improvements in data coverage R10mm RX5day R95pT Source: Rob Smalley

  10. Extremes timeseries comparison Warm nights Frost days Extreme temperature range All model runs capture trend and interannual variability well Some over or underestimate of actual variable amount for some or all model runs Heat waves Heavy precipitation days Max 5-day precip Difference in definition Daily intensity Consecutive dry days Very heavy precip contib

  11. Index Obs Multi-model Warm nights 1.11 ±0.06 1.15 (0.48/1.87) Frost days -0.89 ±0.07 -0.19 (-1.46/0.22) Extreme temperature range -0.19 ±0.02 0.04 (-0.29/0.31) Heat wave duration 7.05 ±0.33 0.26 (-0.31/0.91) Heavy precipitation days 0.28 ±0.06 -0.06 (-0.79/0.89) Maximum 5-day precipitation 0.42 ±0.33 0.32 (-1.37/2.32) Simple daily intensity 0.04 ±0.02 0.02 (-0.06/0.13) Consecutive dry days -0.14 ±0.15 1.04 (-1.68/3.36) Very heavy precipitation contribution 0.60 ±0.12 0.26 (-0.58/1.23) Decadal trends 1957-1999 for Australia Individually most models get the correct sign of trend (except for CDD)

  12. Warm nights Frost days Extreme temperature range Heat waves Heavy precipitation days Max 5-day precip Daily intensity Consecutive dry days Very heavy precip contib Measuring model trend uncertainty

  13. Warm nights Frost days Extreme temperature range Heat waves Heavy precipitation days Max 5-day precip Daily intensity Consecutive dry days Very heavy precip contib Pattern similarity

  14. Verification using improved data coverage RX5day Pattern correlation with data using Taylor diagram. Climate models for 1980-1999 compared with observations R10mm R95pT poor O: cnrm, O: gfdl cm2.0, O: inmcm3.0, O: gfdl cm2.1, O: miroc3.2hi, O: miroc3.2med, O: pcm1, O: mri-cgcm2.3.2, O: ccsm 3.0 Other symbols indicate more than one model run for each model

  15. Anthropogenic versus natural forcing Two models (CCSM/PCM) have output from different forcings Results show that some temperature extremes are inconsistent with natural-only forcings

  16. Interim conclusions • Trends in and interannual variability of warm nights are very well captured by all models • Within uncertainty ranges the multi-model trends overlap with observations (except for heat wave duration because of differences in definition) • However caution is required when interpreting some of the model projections

  17. IPCC future emissions scenarios High population growth, intensive fossil fuel use Low population growth, less fossil fuel use Low population growth, less fossil fuel use We use B1, A1B and A2

  18. Timeseries 1870-2099

  19. Future projections: 2080-2099 minus 1980-1999 • Multi-model agreement across most of Australia for large significant increases in warm nights and heat waves • Little agreement on the significance of projected changes in precipitation extremes

  20. Changes scale with strength of emissions Index Aust/Global (A1B) B1/A1B A2/A1B Warm nights 0.86 0.65 1.11 Frost days 0.45 0.86 1.15 Extreme temperature range -0.53 0.58 2.14 Heat wave duration 0.3 0.50 1.40 Heavy precipitation days 0.17 0.79 -0.54 Maximum 5-day precipitation 0.3 0.61 1.49 Simple daily intensity 1.01 0.76 1.09 Consecutive dry days 4.11 0.58 1.19 Very heavy precipitation contribution 0.80 0.52 1.42

  21. Conclusions (I) obs/model comparison • Generally global climate models are able to simulate the magnitude of observed trends of climate extremes and interannual variability over Australia, particularly for temperature extremes BUT some indices are not well reproduced • Very few models showed significant skill at reproducing the observed spatial pattern of trends • Two models with output from different forcings showed that some changes in temperature indices were consistent with an anthropogenic response

  22. Conclusions (II) future projections • Multi-model agreement for substantial increases in warm nights and heatwaves and decreases in frosts projected by the end of the century irrespective of scenario used • Much longer dry spells interspersed with periods of increased precipitation BUT much less inter-model agreement • In general, the magnitude of changes in both temperature and precipitation indices were found to scale with strength of emissions • But more work is required to improve both the observational coverage and the robustness of projections Alexander and Arblaster (2008), Int. J. Climatol. (in press)

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