Neutral outlook for summer rainfall in SA

The latest seasonal rainfall outlook issued by the Bureau of Meteorology shows a moderate shift in the odds towards below-normal summer (December to February) rainfall for parts of north Queensland. However, across South Australia the chances of accumulating at least average (median) rain during the coming three months are close to 50%.

The pattern of seasonal rainfall odds across Australia is a result of competing effects from higher than average temperatures in both the Pacific and Indian Oceans. The warm Pacific (because of El Niño) biases the climate towards drier than average in eastern Queensland and NSW as well as in northwest WA, while the warm Indian Ocean promotes wetter than average conditions in these same areas. Hence, the two signals largely cancel each other.

The chances of exceeding the median rainfall for the December to February period, are mainly between 50 and 55% over SA. So in years with ocean patterns like the current, about five summers out of ten are expected to be drier than average over the State, with about five out of ten being wetter.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During summer, history shows this effect to be only weakly or very weakly consistent across SA (see background information).

The Southern Oscillation Index (SOI), negative for the sixth straight month as a result of the El Niño, fell from −5 in September to −15 in October. The approximate SOI for the 30 days ending 20th November was −7.

The current Pacific El Niño pattern, of which the low SOI is but one indicator, is expected to persist for most of the summer. For routine updates and comprehensive discussion on the latest data relating to El Niño, together with details on what the phenomenon is and how it has affected Australia in the past, please see the ENSO Wrap-Up.

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THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 20^th DECEMBER 2006

Corresponding temperature outlook

October 2006 rainfall in historical perspective

August to October 2006 rainfall in historical perspective

Background Information

• The Bureau's seasonal outlooks are general statements about the probability or risk of wetter or drier than average weather over a three-month period. The outlooks are based on the statistics of chance (the odds) taken from Australian rainfall/temperatures and sea surface temperature records for the tropical Pacific and Indian Oceans. They are not, however, categorical predictions about future rainfall, and they are not about rainfall within individual months of the three-month outlook period. The temperature outlooks are for the average maximum and minimum temperatures for the entire three-month outlook period. Information about whether individual days or weeks may be unusually hot or cold, is unavailable.

• This outlook is a summary. More detail is available from the contact people or from SILO (www.bom.gov.au/silo/products/SClimate.shtml).

• Probability outlooks should not be used as if they were categorical forecasts. More on probabilities is contained in the booklet The Seasonal Climate Outlook - What it is and how to use it, available from the National Climate Centre. These outlooks should be used as a tool in risk management and decision making. The benefits accrue from long-term use, say over 10 years. At any given time, the probabilities may seem inaccurate, but taken over several years, the advantages of taking account of the risks should outweigh the disadvantages. For more information on the use of probabilities, farmers could contact their local departments of agriculture or primary industry.

• Model Consistency and Outlook Confidence: Strong consistency means that tests of the model on historical data show a high correlation between the most likely outlook category (above/below median) and the verifying observation (above/below median). In this situation relatively high confidence can be placed in the outlook probabilities. Low consistency means the historical relationship, and therefore outlook confidence, is weak. In the places and seasons where the outlooks are most skilful, the category of the eventual outcome (above or below median) is consistent with the category favoured in the outlook about 75% of the time. In the least skilful areas, the outlooks perform no better than random chance or guessing. The rainfall outlooks perform best in eastern and northern Australia between July and January, but are less useful in autumn and in the west of the continent. The skill at predicting seasonal maximum temperature peaks in early winter and drops off marginally during the second half of the year. The lowest point in skill occurs in early autumn. The skill at predicting seasonal minimum temperature peaks in late autumn and again in mid-spring. There are also two distinct periods when the skill is lowest - namely late summer and mid-winter. However, it must always be remembered that the outlooks are statements of chance or risk. For example, if you were told there was a 50:50 chance of a horse winning a race but it ran second, the original assessment of a 50:50 chance could still have been correct.

• The Southern Oscillation Index (SOI) is calculated using the barometric pressure difference between Tahiti and Darwin. The SOI is one indicator of the stage of El Niño or La Niña events in the tropical Pacific Ocean. It is best considered in conjunction with sea-surface temperatures, which form the basis of the outlooks. A moderate to strongly negative SOI (persistently below –10) is usually characteristic of El Niño, which is often associated with below average rainfall over eastern Australia, and a weaker than normal monsoon in the north. A moderate to strongly positive SOI (persistently above +10) is usually characteristic of La Niña, which is often associated with above average rainfall over parts of tropical and eastern Australia, and an earlier than normal start to the northern monsoon season. The Australian impacts of 23 El Niño events since 1900 are summarized on the Bureau's web site (www.bom.gov.au/climate/enso/).