MEDIA RELEASE - ISSUED 19th April 2001
Three-month Seasonal Climate Outlook Summary: Rainfall probabilities for May to July 2001
Below average rainfall favoured in the southeast
The National Climate Centre's outlook for total May to July rainfall shows low probabilities for above average falls in the southeast of the country.
Across Tasmania, most of Victoria and the southeast of S.A., the chances of above average (median) three-month totals are below 40%, nearing 25% in some parts (see map below). In most other areas, the chances are close to 50%.
This means that in years with climate patterns like the current, only about 3 out of every 10 are wetter than the long-term seasonal median, whilst about 7 out of 10 are drier in these parts of the southeast. However, these probabilities should be viewed cautiously, because historical tests of the outlook model show it has low skill over much of this area for the May to July period. The exception is in southern Victoria and Tasmania where the outlooks are moderately reliable.
The low probabilities stem from an abrupt rise in eastern Pacific Ocean surface temperatures, one of the inputs to the forecast. The other input is from the Indian Ocean where temperatures have also being warming strongly, although this has had little effect on the outlook.
More information on this outlook is available during normal office hours
from 8:45am 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 MAY 2001.
March 2001 rainfall - Decile Distribution.
January to March 2001 rainfall - Decile Distribution.
|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.