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SST phases and Australian rainfall

Each month the National Climate Centre (NCC) of the Australian Bureau of Meteorology issues a Seasonal Climate Outlook (SCO) for the following three-month season. This outlook, in the form of the probability of exceeding various thresholds for rainfall and maximum and minimum temperatures, is derived using discriminant analysis with large-scale patterns of sea-surface temperature as predictors (Drosdowsky and Chambers 1998; Jones 1998). An alternative seasonal forecast, based on phases of the Southern Oscillation (Stone and Auliciems 1992), is also widely available from the Queensland Department of Primary Industry (QDPI) via the internet and the commercial RAINMAN software package. This phase system is popular with users who can relate to conditions in the analogue years with similar phase to the current year. Since these forecasts are based on different predictors and different statistical methodologies, they occasionally give conflicting results. This paper describes an attempt to reconcile these differences by developing a phase system based on the SST predictors. An additional SOI analogue system, developed by Drosdowsky (1994) and based on non-linear dynamical systems theory, is also examined during the formulation of the SCO. To compare the hindcast skill of these systems a single rainfall dataset which covers the 100-year period from 1900 to 1999 with monthly temporal resolution is used. This dataset was created by the National Climate Centre (Jones and Weymouth 1997) by analysing all the available quality controlled station data onto a one-quarter degree grid using a successive correction scheme. A reduced resolution subset (at one degree spatial resolution) is used in this study. Two versions of the Global Ice and Sea Surface Temperature data set (GISST) developed by the UK Meteorological Office are used in this study; Version 1.1; (Parker et al. 1995), available from 1949 to 1991, and Version 3.1 (Rayner et al. 2000) from 1871 to 1997. More recent data is obtained from the Bureau of Meteorology operational SST analyses (Smith 1995). A detailed description of the SST phase system, and a comparison with the SOI phases, is presented in the next section. The forecast methodology and the evaluation of hindcast skill are then outlined in the following section, and finally, the conclusions and discussion of the results are presented. Aust. Met. Mag. 51 (2001) 1-12