MEDIA RELEASE - ISSUED 15th AUGUST 2002
Drier than average spring for parts of eastern Australia
Spring rainfall odds show moderate to large swings towards drier than average conditions in the northeast and southeast of Australia, but towards wetter than average conditions in the southwest, according to the Bureau’s National Climate Centre.
The chances that total September to November rainfall will be BELOW the long-term median are between 60 and 70% over north Queensland, together with southern SA, most of Victoria, southwestern NSW and northern Tasmania. These probabilities have mainly resulted from higher than average sea temperatures in the tropical Pacific Ocean, one of the features of the currently developing El Niño. A weekly update of the progress of El Niño is available on the Bureau's El Niño Wrap-Up page.
So with climate patterns like the current, about 6 or 7 springs out of 10 are expected to be drier than average over these parts of the country, whilst about 3 or 4 out of 10 are wetter. Furthermore, large parts of eastern Australia have been very much drier than average since early autumn.
In contrast, the chances that the total spring rainfall for southwestern WA will be ABOVE the long-term median are 60 to 70%. Elsewhere across Australia, the chances of below median falls are between 40 and 60% with no strong bias towards wetter or drier conditions.
For spring, the outlook model has moderate to high reliability in north and central Queensland, moderate skill in southern NSW, but low reliability elsewhere.
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:
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 17th SEPTEMBER 2002.
|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.