Three-month Seasonal Climate Outlook Statement
Rainfall odds mostly neutral for the March quarter
According to the Bureau’s National Climate Centre, rainfall odds for the first three months of the new year are near 50:50 across most of the country with no strong swings towards wetter or drier conditions. There are however two exceptions: an enhanced chance of below average falls in northeast Australia, and an enhanced chance of above average falls in parts of SA. These probabilities have mainly resulted from the current El Niño pattern of above average Pacific Ocean temperatures.
The chances of above median rainfall for the January to March period are only 35 to 40% in parts of northern and central Queensland, meaning that BELOW median falls have a 60 to 65% chance of occurring. The reverse is true for some of southern and central South Australia, that is, ABOVE median falls have a 60 to 65% of occurring. On a cautionary note though, the statistical outlook scheme has generally low reliability in these areas for this period.
So with climate patterns like the current, about 6 seasons out of 10 are expected to be drier than average in north Queensland, whilst about 4 out of 10 are wetter. The reverse logic applies in southern SA.
January to March is the period when the impact from El Niño on Australian rainfall most commonly breaks down with a return to average to above average totals. There are four points to note regarding this:
Supplementary Information concerning the breakdown of El Niño-related droughts
As mentioned above, the January to March period is the most likely three-month period for a significant change in Australia’s rainfall patterns at the end of an El Niño event. These changed rainfall patterns, which usually mark the beginning of the end of drought conditions, can take various forms ranging from a succession of moderately wetter than average months, to a widespread deluge that heralds several months of significantly above average rainfall.
What precisely will happen at the end of the current drought is impossible to say, but because it is so widespread the breakdown is unlikely to be uniform in either time or space. Furthermore, what constitutes drought-breaking rain depends on one’s perspective. For example, an engineer concerned with water storages is likely to have different ideas on what breaks the drought, from those of a farmer or grazier. Most importantly, predictions of when and how the drought might break are beyond the scope of this or any other seasonal outlook scheme.
A quick survey of the details on Australian rainfall impacts in El Niño years reveals the following information about recent El Niño-related droughts:
1997 - Rainfall deficiencies were confined to relatively small areas in the southeast of the country and to parts of eastern Queensland. Rainfall was generally average to above average across Victoria in January and February 1998. From April 1998 onwards rainfall was above to very much above average across wide areas of eastern and northern Australia as the climate moved into a La Niña state.
1994 - Similar to this year with widespread rainfall deficiencies across the country. Average to above average falls fell in parts of eastern Australia (especially NSW) in November and December, and then very heavy rain and flooding occurred over inland NSW, SW Qld and northern Victoria in January effectively ending the event in these areas. In eastern Queensland it wasn’t until August to October 1995 that rainfall was consistently average to above average.
1991 - This marked the beginning of a long dry period in eastern Queensland that culminated in the 1994 event. About three-quarters of the State recorded rainfall totals in the driest 10% of records (decile 1 or rainfall deficient) for the nine months from March to November 1991, and it was the driest such period on record in parts of the Darling Downs. The northern half of NSW was also seriously affected with about half this region in decile 1 for the nine months. Above to very much above average rain fell in December over SE Qld and the eastern half of NSW marking the beginning of the end of this event in these regions. Although falls were patchy in January 1992, heavy rain fell again in February, particularly in the southeast of Queensland and NSW. Drought-breaking failed to eventuate in tropical Queensland.
1982 - Drought was widespread across eastern and southern Australia culminating with heat-wave conditions and bushfires in southern Australia in February 1983, including the Ash Wednesday disaster. For the 11 months from April 1982 to February 1983 the vast bulk of Victoria, the southern halves of both NSW and South Australia, together with large tracts of central and western Qld recorded record low falls for this particular 11-month period. The pattern changed abruptly in March 1983 when flood rains in central and southern Australia heralded several months of above average rainfall across much of the country.
1972 - For the ten months from March to December, rainfall deficiencies occurred over the majority of Victoria, SA, southern WA, north and east Tasmania, western and southern NSW, and far southwest and central Queensland. Above average falls occurred in January, followed by widespread very much above average to record falls in February, thereby ending the drought and heralding the wettest two-year period in Australia’s history as strong La Niña conditions were established.
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:
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 15th JANUARY 2003.
|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 Niño-Southern Oscillation phenomenon, with fewer than normal cyclones during El Niño episodes, and slightly more during La Niña 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 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.