Increased warmth indicated in southwest and far northern WA
The temperature outlook for autumn (March to May) shows a moderate
shift in the odds towards above normal temperatures in both the southwest
and far north of WA. However, over most of the State the chances of a
warmer than average season are close to 50:50.
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The pattern of seasonal temperature odds across WA is due to roughly equal
contributions from higher than average temperatures in both the Pacific and Indian
Oceans.
Averaged over March to May, the chances are between 60 and 70% for higher than
normal maximum temperatures in southwest WA as well as in the far north of
the State (see map).
So in years with ocean patterns like the current, about six or seven autumns
out of every ten are expected to be hotter than average in these
parts of WA, with about three or four out of ten being cooler.
Outlook confidence is related to how consistently the Pacific and Indian
Oceans affect Australian temperatures. During autumn, history shows this effect on maximum temperatures
to be moderately consistent in the Kimberley and in the far southwest corner of WA,
but over most of the State the effect is only weakly or very weakly consistent
(see background information).
Average autumn minimum temperatures are favoured to be warmer than normal across
the whole State, with the chances generally being between 60 and 75%.
History shows the oceans' effect on minimum temperatures during autumn
to be moderately consistent over most of WA.
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Background Information
The Bureau's seasonal outlooks are 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 Australian rainfall/temperatures
and sea surface temperature records for the tropical Pacific and Indian Oceans.
They are not, however, categorical predictions about future rainfall,
and they are not about rainfall within individual months
of the three-month outlook period.
The temperature outlooks are for the average maximum and minimum temperatures
for the entire three-month outlook period.
Information about whether individual days or weeks may be
unusually hot or cold, is unavailable.
This outlook is a summary.
More detail is available from the contact people or from SILO
(www.bom.gov.au/silo/products/SClimate.shtml).
Probability outlooks should not be used as if they were categorical forecasts.
More on probabilities is contained in the booklet
The Seasonal Climate Outlook - What it is and how to use it,
available from the National Climate Centre.
These outlooks should be used as a 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 should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.
Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
outlook probabilities.
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% 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.
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 moderate to strongly negative SOI (persistently below –10) is
usually 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 moderate to strongly positive SOI (persistently above +10)
is usually 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.
The Australian impacts of 23 El Niño events since 1900 are summarized
on the Bureau's web site
(www.bom.gov.au/climate/enso/).
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