Prediction of growing season rainfall and crop yields in southern Australia

Most research into long and medium range rainfall and crop forecasting in Australia has focused on the El Niño/Southern Oscillation. The Southern Oscillation Index (SOI) has been found to be of limited use in southern Australia for rainfall forecasting in the critical April to October growing season period. In southern Australia during this period one of the more common rain mechanisms is the general uplift in ‘northwest cloudbands’ which often have their origin in ocean areas to the northwest of the Australian continent. A set of seven model indices were devised using climatic variables which are related to the formation of these ‘northwest cloudbands’, namely surrogate measurements of upper-level Rossby waves and northeast Indian Ocean sea-surface temperatures and gradients. The indices were formulated from a dependent dataset comprising 27 years of surface pressure and 500 hPa geopotential height from sites mostly about Australia. These indices were then tested on a further ten years of independent data for correlation with rainfall over part and full growing seasons, and with (de-trended) wheat yields. The results showed significant correlations typically 0.6 to 0.9 over much of southern Australia.