Three-month Seasonal Climate Outlook Statement
Summer rainfall odds near 50:50
Summer rainfall odds are near 50:50 across the country with no strong swings towards wetter or drier conditions. However, this masks the competing effects from the Pacific and Indian Oceans.
According to the Bureau’s National Climate Centre, the current pattern of above average Indian Ocean temperatures favours ABOVE median rainfall over the December to February period across eastern Queensland, eastern NSW and the Gascoyne and Pilbara regions in WA.
Counteracting this signal, the current El Niño pattern of above average Pacific Ocean temperatures favours BELOW median summer rainfall through essentially these same areas. The final probabilities therefore come out close to 50%.
So with climate patterns like the current, about 5 seasons out of 10 are expected to be wetter than average over the country, whilst about 5 out of 10 are drier. This scenario is consistent with how El Niños have behaved in the past with eastern Australian rainfall commonly returning to closer to average values sometime between December and March. As a result, the average summer impact from El Niño is much less than it is in winter or spring. Regular updates of the progress of the El Niño are available at El Niño Wrap-Up page.
The statistical outlook model has moderate reliability in eastern NSW and southeast Queensland, and moderate to high reliability in western WA.
More information on this outlook is available from 9:00am to 5:30pm
(EDT) Monday to Friday by contacting the following climate
meteorologists in the National Climate Centre:
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 17th DECEMBER 2002.
|Information on tropical cyclones|
|The tropical cyclone season around northern Australia extends from November to May. The average number of cyclones per season is 9.4 (mean from 1949/50 season to 1993/94 season), with a standard deviation of 3.4. Cyclone activity in this region is related to the El Niño-Southern Oscillation phenomenon, with fewer than normal cyclones during El Niño episodes, and slightly more during La Niña episodes. Below is a map showing the extreme bounds of the Australian Tropical Cyclone Region (Australia's area of responsibilty).|
|Frequently Asked Questions|
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.
|Definitions and Explanations....|
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.