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Australian Summer Monsoon Harry Hendon and Matthew Wheeler BMRC Melbourne

Australian Summer Monsoon Harry Hendon and Matthew Wheeler BMRC Melbourne • Broadscale Evolution and Onset • Intraseasonal Variability • Interannual Variability • Seasonal Prediction Recent Trend (wetter in west). DJF. X. Define monsoon by deep baroclinic zonal flow. Climatological

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Australian Summer Monsoon Harry Hendon and Matthew Wheeler BMRC Melbourne

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  1. Australian Summer Monsoon Harry Hendon and Matthew Wheeler BMRC Melbourne •Broadscale Evolution and Onset • Intraseasonal Variability • Interannual Variability • Seasonal Prediction • Recent Trend (wetter in west) DJF X

  2. Define monsoon by deep baroclinic zonal flow Climatological onset late Dec retreat late Mar Drosdowsky 1996 Onset associated with poleward contraction of subtropical jet (Troup 1961)

  3. As in Indian Monsoon: composed of intraseasonal bursts and breaks intraseasonal variability large interannual variability relatively weak (~15% of the mean) Wheeler and McBride 2005

  4. 7. Eastward MJO accounts for ~1/3 of ISV, but some years dominant Westward Rossby waves (provide focus for TC)

  5. n=1 ER case from 2001 16th April, 2001

  6. DJFM MJO Composite evolution (OLR/850 winds) little indication of poleward propagation

  7. u T MJO evolution at Darwin +20 time (d) -20 rain r.h. v

  8. MJO provides broadscale conditions for onset Onset tends not to occur in suppressed phase. Active phase Onset date at Darwin relative to phase of MJO Wheeler and Hendon (2004) Suppressed phase

  9. Interannual Variability and Seasonal Prediction

  10. pre-monsoon: low rainfall/highly variable SON NDJFM monsoon: high rainfall/low variability

  11. ENSO most prominent influence in transition season Correlation seasonal rainfall with Nino3 DJF SON McBride and Nicholls

  12. Lagged Correlation SST and Winds with Nino34 in SON SON (0) DJF (+1) Pre-monsoon: easterly basic state, positive air-sea feedback Monsoon: westerly basic state, negative air-sea feedback Hendon Wu and Kirtman Wang et al

  13. Darwin Pressure June-Aug Nicholls et al. 1982 onset date

  14. Coupled model dynamical forecasts of seasonal mean rainfall POAMA 3-member ensemble Lead 0 Hindcast Skill 1982-2005 SON DJF MAM

  15. DJF Rainfall Potential Predictability (%) ANOVA of 8-member ensemble forced with observed SST 1982-2002 >60% <10% Observed DJF rainfall correlation withNino4

  16. Decadal Variability and Recent Wet Trend

  17. wet decadal La Nina dry decadal El Niño 10 yr running mean Nino4

  18. Natural variability (decadal ENSO) can’t be dismissed • Ozone hole-induced trend in the SAM high SAM rainfall signal weak in Tropics • Greenhouse gas induced climate change IPCC AR4 models don’t reproduce obs trend • Asian aerosols CSIRO Coupled model bias?

  19. Rotstayn et al. 2007 aerosols excite an unrealistic negative IOD response in summer (driven by weak “La Niña” conditions in Pacific?)

  20. Australian Summer Monsoon • Large intraseasonal variability, low interannual variability, low predictability (post-onset) • Despite limited predictability of mean monsoon, opportunities for useful predictions statistical and dynamical onset-related, intraseasonal (MJO/cyclones) • Poor representation of monsoon processes in climate/forecast models hampers attribution of recent climate variationslimits utility of dynamical forecasts

  21. Response to Asian Aerosols (Rotstayn et al. 2007)

  22. SAM trended positive in DJF result of springtime ozone hole trend H 500mb DJFM 1979-1999 Thompson and Solomon 2002

  23. Rotstayn et al. 2007

  24. n=1 Eq. Rossby waves * ~7 to 35-day period * Westward propagating (~5m/s) * Appear to often influence TC development.

  25. MJO modulation of probability of extreme weekly rainfall 1 degree gridded rainfall. DJF 1974-1999 OLR during 8 phases

  26. Correlation N. Aust rainfall with SST 1900-2006 SOND DJFM

  27. Statistical forecast model for probability of late onset (based on rainfall accumulation): Logistic regression P = predicted probability x = predictor: (July-August SOI) b0, b1 = best fit model coefficients Brier skill score over climatology (forecasts 1950-2005) Lo and Wheeler 2007

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