Understanding and predicting the contrast of Australian springtime rainfall of 1997 and 2002

1. Understanding and predicting the contrast of Australian springtime rainfall of 1997 and 2002 Eunpa Lim & Harry Hendon Seasonal Prediction Science Team R&D Branch Bureau of Meteorology CAWCR 9th Annual Workshop 19-22 Oct 2015

2. All-Australia rainfall (mm/yr) vs the SOI for 1900-2004 of the obs • Occurrence of El Nino is associated with decrease of rainfall over eastern Australia (in spring) • Magnitude of El Nino does not account for amount of the AUS rainfall decrease El Nino La Nina Power et al. (2006) J. Clim

3. Wang and Hendon (2007) J. Clim 1997 2002 • Strongest El Nino of the 20th century • AUS wasn’t as dry as expected • Weak El Nino with its max. SST warming over the dateline • Severe drought in the east

4. 1997 2002 Regression of SSTs onto AUS mean spring rainfall positive positive

5. 1997 2002 Strong negative SAM Strong positive IOD Was it really the tropical central Pacific SSTs that made the Australian spring rainfall response so different in 1997 and 2002? What were the roles of the land and atmosphere conditions in the rainfall anomalies of 1997 and 2002?

6. POAMA • Hindcasts for 1997 and 2002 Sep-Nov • Forecast sensitivity experiments • an atmosphere-ocean coupled dynamical seasonal forecast system • atmosphere and ocean component models are initialised with realistic conditions generated from the Bureau's state-of-the art data assimilation systems - Forecasts initialised on the 1st of Sep 1997 and 2002, and verified in the following Sep-Oct-Nov season (i.e. lead time 0) - Ensemble mean of 10 member forecasts - Forecast anomalies computed against the hindcast monthly climatology over the period of 1982-2005  model’s systematic bias is not present in the forecasts

7. POAMA forecasts for 1997 and 2002 spring climate Obs SSTEOF1 SSTEOF2 POAMA

8. Sensitivity experiments to atm & land initial conditions POAMA (realistic atmosphere, land & ocean initial conditions of 1 Sep 1997 and 2002) POAMA but with climatological land initial conditions POAMA but with unrealistic atmosphere initial conditions (i.e. no realistic noise) It was the difference in the tropical Pacific SSTs that caused the basic difference between the 1997 and 2002 rainfall anomalies Realistic atmosphere conditions of 1 Sep 1997 & 2002 amplified the respective rainfall anomalies forced by El Ninos The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

9. Sensitivity experiments to air-sea interaction POAMA AMIP with POAMA SSTs Sensitivity to air-sea interaction Air-sea coupling amplified the dry response in 2002 while it didn't make any significant difference in 1997

10. Rainfall MSLP POAMA Atmosphere forced by POAMA SSTs SAM is not well predicted  local circulation/AUS drought not well predicted Air-sea coupling is important for the skilful prediction of spring season SAM and Australian rainfall !

11. 1997 springwasn’t as dry as expected because the maximum SST warming of the 1997 El Nino was far shifted to the east Atmospheric noise amplified the wet conditions 2002 spring was dry because the maximum SST warming of this El Nino was shifted to the west Atmospheric noise amplified the dry conditions -ve SAM intensified the dry conditions Air-sea coupling was important to the skilful forecast of –ve SAM and associated dry conditions Good representation of air-sea interaction is important for skilful forecasts for extratropical circulations and associated Australian climate Summary Lim and Hendon 2015 J. Climate, 28, 2804-2822