The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology 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 9 th Annual Workshop Oct 2015

El Nino La Nina All-Australia rainfall (mm/yr) vs the SOI for 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 Power et al. (2006) J. Clim

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

Regression of SSTs onto AUS mean spring rainfall positive

Strong positive IOD Strong negative SAM 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?

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 1 st 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  model’s systematic bias is not present in the forecasts

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

The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology 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

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

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 ! Rainfall MSLP

Summary 1997 spring wasn’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 Lim and Hendon 2015 J. Climate, 28,

The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Statistical analysis – multiple linear regression OBS SSTPC1, SSTPC2, IOD & SAM No SSTPC1 No SSTPC2 No IOD No SAM Suppressed warming of tropical central Pacific SSTs captured by –ve SSTPC2 was critical to the near normal rainfall in 1997 SAM and SSTPC2 were important to the intense dry condition in the east in 2002