Three-month Seasonal Climate Outlook Statement
Wetter for parts of Qld/NSW, drier in parts of SA/Vic.
The latest seasonal rainfall outlook from the Bureau’s National Climate Centre shows a contrasting pattern of odds, particularly in the east and southeast of the country.
The chances of ABOVE median rainfall for the May to July period are between 60 and 75% in the southeast quarter of Queensland and much of the eastern half of NSW. So with climate patterns like the current, about 6 to 7 seasons out of 10 are expected to be wetter than average in these areas, whilst about 3 to 4 out of 10 are drier. The probabilities are above 60% across much of the Northern Territory, but these should be treated with caution because it is the dry season there.
In contrast, the chances of above median rainfall are between 40 and 45% over much of southern SA, western/central Victoria and northwest Tasmania, meaning BELOW median falls have a 55 to 60% chance of occurring. This equates to about 6 seasons out of 10 being below average under the present climate scenario. The objective statistical outlook scheme has moderate to strong reliability over the areas where the chances for above median rainfall are more than 60%, but lower reliability elsewhere (see background).
The overall pattern of probabilities is almost entirely a result of warmer than average temperature in the Indian Ocean, with little contribution from the Pacific Ocean. As far as the El Niño is concerned, most of the main indicators show the event is nearly over. Whilst a regeneration of El Niño cannot be ruled out entirely, this is much less likely than neutral conditions. For more detail see El Niño Wrap-Up.
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:
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 15th MAY 2003.
|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.