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Juan Feng and Jianping Li LASG, IAP, Chinese Academy of Sciences Yun Li

A monsoon-like Southwest Australian circulation and its relation with rainfall in Southwest Western Australia. Juan Feng and Jianping Li LASG, IAP, Chinese Academy of Sciences Yun Li CSIRO Mathematics, Informatics and Statistics CSIRO Climate Adaptation Flagship. IMSC 2010, Edinburgh

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Juan Feng and Jianping Li LASG, IAP, Chinese Academy of Sciences Yun Li

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  1. A monsoon-like Southwest Australian circulation and its relation with rainfall in Southwest Western Australia Juan Feng and Jianping Li LASG, IAP, Chinese Academy of Sciences Yun Li CSIRO Mathematics, Informatics and Statistics CSIRO Climate Adaptation Flagship IMSC 2010, Edinburgh 12-17 July 2010

  2. Acknowledgement • Australian-China Climate Change Partnership Program-Department of Climate Change • Indian Ocean Climate Initiative • CSIRO Climate Adaptation Flagship

  3. Outline • Motivations • Existence of a monsoon-like southwest Western Australia circulation (SWAC) • Relationship between the winter SWAC and SWWA winter rainfall • The impacts of large-scale circulations on the SWAC-SWWA winter rainfall relationship • Possible drivers of the SWAC • Conclusion

  4. Motivation: A riddle that has puzzled researchers for decades • ‘Why has south-west Western Australia experienced dramatic declines in rainfall since the 1970s?’ • Possible factors include 1. Natural variability (Cai et al. 2005) 2. Land-use changes (Pitman et al. 2004) 3. Anthropogenic contribution through ozone depletion and carbon dioxide increase (Cai and Cowan 2006, 2007; Karoly 2003) 4. Trend in the Southern Annular Modes (Cai and Cowan 2006, 2007; Hendon et al. 2007; Li et al. 2005) 5. Indian Ocean SST (England et al. 2006; Ummenhofer et al. 2008) • SWWA rainfall and Antarctic snowfall are reversely linked (van Ommen and Morgan 2010) Aim: investigate regional circulations and rainfall decline over SWWA

  5. Data • Observed rainfall and 0.25 x0.25  gird rainfall (National Centre of the Australian Bureau of Meteorology, Lo et al. 2007) • NCEP-NCAR reanalysis data (Kalnay et al 1997) • The 40-year ECMWF Reanalysis (ERA-40) data (Uppala et al. 2005): as a comparison to verify results from NCEP-NCAR reanalysis data • CMAP pentad mean rainfall data (Xie and Arkin 1997) • IERSST (Smith and Reynolds 2004) • Nino 3, SAM, IOD and ENSO-Modoki indices

  6. Existence of a Monsoon-Like Southwest Australia Circulation (SWAC) Evidence 1: The alternating of wet and dry seasons over SWWA Winter Summer

  7. Existence of a Monsoon-Like Southwest Australia Circulation (SWAC) Evidence 2: there is a circulation system with a strong seasonal variation controlling the climate in SWWA Summer Winter

  8. Existence of a Monsoon-Like Southwest Australia Circulation (SWAC) V925 U925 U925 Vertical Velocity at 925 hPa Zonal vapour trasport at 925 hPa

  9. Existence of a Monsoon-Like Southwest Australia Circulation (SWAC) Evidence 3: Strong difference and the reversal thermal gradient between land and sea SST/SAT (C) SAT-SST (C) Difference SAT-SST (C) in the region (70-130E, 35-25S)

  10. A Monsoon-Like Southwest Australia Circulation (SWAC) • There exists a monsoon-like Southwest Circulation (SWAC) system over the wider Southwest Australia region. • The conception and the measurement of monsoon strength might be reasonable to delineate the magnitude and variability of the SWAC.

  11. Seasonal March of Winter SWAC and Rainfall Winter SWAC phases: 1. “preonset” (1 May-20 May) 2. “active” (21 May-29 June) 3. “break” (30 June-14 July) 4. “revival” (15 July-24 July) 5. “decay” (25 July-13 Aug) 6. “die out” (14 Aug-17 Sep) SWWA winter rainfall is strongly linked to the winter SWAC evolution phases.

  12. Seasonal March of Winter SWAC and Rainfall Phase difference in winds at 850 hPa Phase difference in Rainfall Break - Active Active - Preonset Revival - Break Decay - Rivival • Active – Preonset • Break – Active • Revival – Break • Decay – Revival • Die-out – Decay Die-out - Decay

  13. A New Climate Index Explains Winter Rainfall Decline in SWWA The SWAC Index (SWACI): derived from a unified Dynamic Normalized Seasonality (DNS) monsoon index (GRL, Li and Zeng 2002) The SWAC domain: (100°-145E, 35 °- 25S)

  14. A New Climate Index Explains Winter Rainfall Decline in SWWA R=0.73 (0.73) MJJASO No ENSO R=0.75(0.74) No ENSO No SAM No SAM R=0.68(0.68)

  15. Estimated coefficients of regression models and associated AIC values for the relationship between SWWA rainfall and various normalizedindices

  16. A New Climate Index Explains Winter Rainfall Decline in SWWA Late winter (ASO) in 1979-2006 Cor SWACI & Rain No IOD No EM No ENSO

  17. Composite SLP and 850 hPa wind differences between high and low SWACI polarities MJJ ASO No SAM years The SWAC is largely Independent of well-known large-scale circulations. No IOD years No EM years No ENSO years

  18. Possible drivers of the SWAC Early winter (MJJ) Late winter (ASO) Driver 1: Polarward shift of the subtropical high -the strength of the SWAC is related to the location of subtropical high ridge The stronger SWAC in early and late winter imply the ridge of subtropical high locates to the north of the SWWA region and brings more synoptic rainfall events impinging on this region.

  19. Possible drivers of the SWAC Driver 2:Warming SSTs in equatorial northern Australia and westward of SWWA over the Indian-Ocean The warming/cooling SSTs westward of SWWA over the Indian-Ocean are associated with the weak/strong SWAC which as a result leads to negative/positive rainfall anomalies over SWWA

  20. Conclusion • There is a monsoon-like Southwest Western Australia Circulation (SWAC) based on evidences characterizing a monsoonal system. • SWWA winter rainfall is strongly linked to the winter SWAC evolution phases. • The well-coupled SWAC-SWWA rainfall relationship seems to be largely independent of the well-known effects of large-scale atmospheric circulationssuch as the SAM, ENSO, IOD and ENSO and ENSO-Modoki. • The weakening of the SWAC may contribute to SWWA rainfall decline in early winter. • A new climate index (SWACI) to explain rainfall decline of SWWA

  21. On going work • Further investigation using global climate models to confirm whether warming oceans or natural fluctuations are responsible for the weakening SWAC circulation; • Investigating the linkage between reduction of extreme rainfall and the monsoon-like SWAC; • Investigating the relationship between the summer SWAC and hot spells and heat waves in SWWA; • The SWACI will be used to see if it can improve skill in forecasting seasonal rainfall and crop growth potential.

  22. Contact: Yun Li Phone: (08) 93336388 Email: Yun.Li@csiro.au; Web:http://www.cmis.csiro.au/Yun.Li/ Thank you and welcome your comments! Feng, J., J. Li, and Y. Li, 2010:A monsoon-like Southwest Australian circulation and its relation with rainfall in Southwest Western Australia.Journal of Climate23, 1334-1353 Web: http://www.cmis.csiro.au/yun.li/paper.htm

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