Odds favour warmer spring in tropics & parts of southern Australia

The national outlook for average spring (September to November) maximum temperatures shows a moderate to strong shift in the odds favouring warmer than normal conditions over most of the tropics and parts of southern Australia.

The pattern of seasonal temperature odds across Australia is mostly a result of continued warmth in the central to southeastern Indian Ocean, especially off the west coast of WA. Averaged over September to November, the chances are mainly between 60 and 75% for above average maximum temperatures in northern Queensland, the northern two-thirds of the NT and the far north of WA (see map). In some areas near the Gulf of Carpentaria, the probabilities approach 80%. There are also 60-65% chances for a warmer than normal spring in southwest WA, southeast SA, and western and central Victoria. So for every ten years with ocean patterns like the current, about six to eight springs are expected to be warmer than average across the north of the country as well as in parts of southwest and southeast Australia, while about two to four are expected to be cooler. Across the rest of the country, the chances of a warmer than average spring are between 50 and 60%, indicating roughly equal chances for warmer or cooler than normal conditions. Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During spring, history shows this effect on maximum temperatures to be moderately consistent across large parts of the country, with the exception of the southeast and far west where it is only weakly consistent (see background information). Users therefore need to be cautious in applying this outlook in southwest and southeastern Australia. Average spring minimum temperatures are also favoured to be warmer than normal across most of the country, with the exception of Queensland's southern half and the northeast half of NSW (see map). The chances of increased overnight warmth (averaged over the coming three months) are between 60 and 80% over most of Australia. Or to put it another way, for every ten years with ocean patterns like the current, about six to eight would be expected to warmer than average in terms of overnight temperatures averaged over spring. History shows the oceans' effect on minimum temperatures during spring to be moderately to highly consistent over most of the country, except in parts of southeast and eastern Australia where the effect is generally only weakly consistent.

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Regional versions of this media release are available: | Northern Aust | Southeastern Aust | WA |

Regional commentary is available from the Climate Services Sections in the Bureau's Regional Offices:

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 25^th September 2008

Corresponding rainfall outlook

Maximum temperature departures from average for May to July 2008 - base period 1961-1990

Minimum temperature departures from average for May to July 2008 - base period 1961-1990

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/).