Seasonal Climate Outlook Rainfall Archive
Frequently Asked Questions|
Three-month Seasonal Climate Outlook Statement El Niño over: prospects for increased rainWith the demise of the 2002/03 El Niño, there are improved prospects for winter rains over much of Australia. However it is still possible for the effects of the drought to linger for some time yet, especially in those areas where heavy winter rain is uncommon. Whilst a return of El Niño cannot be ruled out entirely, this is much less likely than either neutral or La Niña conditions. For more detail see the El Niño Wrap-Up The Bureau’s winter rainfall outlook shows the odds of above average seasonal falls are better than 50:50 over most of Queensland, NSW and Tasmania. In remaining parts of eastern Australia the odds are near 50:50, but in southwest WA below average winter rainfall is the more likely outcome. The Bureau’s winter rainfall outlooks have moderate to high reliability over much of Queensland and northern NSW, but mainly low reliability elsewhere, including southwest WA (see background). The chances of ABOVE median rainfall for the June to August period are between 60 and 65% over most of east and central Queensland south of about Townsville, as well as over the northeast quarter of NSW. So with climate patterns like the current, about 6 winters out of 10 are expected to be wetter than average in these areas, whilst about 4 out of 10 are drier. In contrast, the chances of above median rainfall are between 35 and 40% over southwest WA, meaning BELOW median falls have a 60 to 65% chance of occurring. This equates to about 6 winters out of 10 being drier than average under the present climate scenario. The overall pattern of probabilities is almost entirely a result of warmer than average temperatures in the Indian Ocean. Regional versions of this media release are available in
Portable Document Format (PDF):
Background Information:
More information on this outlook is available from 9:00am to 5:30pm
(EST) Monday to Friday by contacting the following climate
meteorologists in the National Climate Centre:
Regional commentary is available from the Climate and Consultancy
sections in the Bureau's Regional Offices:
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 17th JUNE 2003. Archive of previous Seasonal Climate Rainfall Outlooks Archive of previous Seasonal Climate Temperature Outlooks |
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Q: WHAT ARE THE BUREAU OF METEOROLOGY'S SEASONAL CLIMATE OUTLOOKS? A: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 rainfall and sea surface temperature records. They are not, however, categorical predictions about future rainfall, and they are not about rainfall within individual months of the three-month outlook period. Q: WHAT DO WE MEAN BY "WETTER OR DRIER THAN AVERAGE, OR "WARMER OR COOLER THAN AVERAGE""? A:Being above or below the median rainfall, average maximum temperature, or average minimum temperature for the three-month period. The median is a useful measure of "normal" rainfall. In the long term, rainfall is above median in one half of years, and below median in the other half. For example, from July to September at Mackay in Queensland, one-half of 3-month rainfall totals have been below 80mm, and one-half have been above. If rainfall was above 80mm in that period it would be "wetter than average" or above median. Over the long haul there is a 50% chance of this occurring. In terms of odds this is even money. Note that the average maximum temperature is the average of all the daily highest temperatures for the period. Similarly, the average minimum temperature is the average of all the daily lowest temperatures for the period Q: HOW ACCURATE ARE THE OUTLOOKS? A: In the places and seasons where the outlooks are most skilful, the eventual outcome (above or below median) is correctly given the higher chance about 70 to 80% 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. Q: WILL CATEGORICAL OUTLOOKS EVER BE ISSUED? (Eg. It WILL be drier than average.) A: Very unlikely. There is a certain level of natural variability in the climate which is chaotic and unpredictable. This is particularly the case with rainfall. For example, rainfall in a season can be significantly above average in one region, and significantly below average less than 50km away. Q: HOW SHOULD THE OUTLOOKS BE USED? A: As another 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 will outweigh the disadvantages. For more information on the use of probabilities, farmers could contact their local departments of agriculture or primary industry. |
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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 strongly negative SOI (below -10) is 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 strongly positive SOI (above +10) is 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. El Niño & La Niña El Niño translates from Spanish as "the boy-child", and refers to the extensive warming of the central and eastern Pacific Ocean. La Niña translates from Spanish as "the girl-child", and refers to the extensive cooling of the central and eastern Pacific Ocean. The term has recently become the conventional label for the opposite of El Niño. See http://www.bom.gov.au/climate/glossary/elnino.shtml for more on SOI and El Niño. |