MEDIA RELEASE - ISSUED 15th MAY 2002
Reduced winter rains in eastern Australia
Winter rainfall odds favour below average falls in an area straddling the NSW - Queensland border centred around Moree, according to the Bureau's National Climate Centre. For the June to August period, the chances of above average falls are less than 40% in this area, meaning that below average falls have a greater than 60% chance of occurring. These probabilities have resulted from higher than average sea temperatures across much of the tropical Pacific.
So with climate patterns like the current, about 6 winters out of 10 are expected to be drier than average in this area, whilst about 4 out of 10 are wetter. In addition, based on historical data the outlook scheme has moderate to good reliability through this region for winter.
Winter rainfall chances are also reduced across parts of the southeast and southwest of the country. However, in both these areas the outlook reliability is low so users are urged to interpret the probabilities with caution. In other parts of the nation, the chances of above average winter rainfall are between 40 and 60% with no big swings to wet or dry conditions.
There continues to be speculation about a possible El Niño event this year. Despite a majority of computer models predicting an event, the lack of supporting evidence from the Pacific means the chance of an El Niño has decreased. The expert opinion from Australian climate scientists is that the odds for an El Niño event in 2002 have come back to around 50%. This is still a significant chance, being roughly double the normal level of risk.
If the present climate patterns persist for another month, the chance of an El Niño will be significantly lower because most El Niños are clearly established by the end of June. The National Climate Centre is keeping a very close watch on developments. The El Niño Wrap-Up page provides weekly updates of important data.
More information on this outlook is available during normal office hours
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 JUNE 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 Niņo or La Niņa 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 Niņo, 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 Niņa, 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 Niņo translates from Spanish as "the boy-child", and refers to the extensive warming of the central and eastern Pacific Ocean.
La Niņa 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 Niņo.
See http://www.bom.gov.au/climate/glossary/elnino.shtml for more on SOI and El Niņo.