MEDIA RELEASE - ISSUED 15th November 2000
Three-month Seasonal Climate Outlook Summary: Rainfall probabilities for Summer 2000/01 (Dec - Feb)
Potentially wetter in west and far east
The National Climate Centre's outlook for total summer rainfall shows increased potential for above average (median) totals in both the west of Western Australia and through SE Queensland and eastern NSW (see map below). The chances of a wetter than normal summer are generally around 60 to 70% in these areas. This means that if, for the sake of argument, we had the current climate patterns in another 10 summers, then in about 6 or 7 of these summers, rainfall would be above the long-term median in these areas. In the other 3 or 4 rainfall would be below.
In both the west and far east the skill of the outlooks is moderate, but in other parts of the country it is low for the summer season. In addition, the chances in these low skill areas are generally close to 50% - ie above and below average rainfall are about as equally likely.
Research into tropical cyclone numbers and the SOI (see below for SOI), indicates that there is likely to be a similar number to last cyclone-season in the Australian area of responsibility. In the 1999/2000 season there were eleven tropical cyclones.
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:
October 2000 rainfall - Decile Distribution
August to October 2000 rainfall - Decile Distribution
|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 Niņo-Southern Oscillation phenomenon, with fewer than normal cyclones during El Niņo episodes, and slightly more during La Niņa 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 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.