Climate
variability and El Niño
Fluctuations
causes
"El Niño"
El Niño's Global Effects
The
Walker Circulation
The
Southern Oscillation
Climate
Clues to El Niño
El
Niño's Opposite Phase
Forecasting
El Niño
Ecologically
Sustainable Development in Australia
The
Future
Generations
of Australian schoolchildren learned a poet's phrase which described
their country as “a land of droughts and flooding rains”.
Today's scientists talk in terms of the continent's large climate variability
from season to season, and from year to year.
The
impact of climate variability on Australia has been highlighted by the
fluctuating events during the 1990s. While Queenslanders suffered drought
for much of the first half of the decade, people in southeast Australia
contended with severe spring floods in 1992 and 1993. Drought spread
nationwide in 1994, but the pendulum swung the other way in 1995 and
1996 as heavy rain and flooding returned to many parts. Drought once
again visited the southeast of the country during 1997 and 1998.
What
causes these fluctuations?
They are connected
with the climate phenomenon called the Southern Oscillation, a major
air pressure shift between the Asian and east Pacific regions whose
best-known extremes are El Niño events. The Southern Oscillation
(strength and direction) is measured by a simple index, the SOI.
 Rural
productivity, especially in Queensland and New South Wales, is linked
to the behaviour of the Southern Oscillation. The graph opposite shows
how Australia's wheat yield, (with the trend over time removed),
has fluctuated with variations in the Southern Oscillation. Negative
phases in the oscillation (drier periods) tend to have been linked with
reduced wheat crops, and vice versa.
Tourism is another
industry vulnerable to large swings in seasonal climate. Because climate
variability can affect the Australian economy, Australians need the
best possible understanding of the physical mechanisms controlling this
dramatic feature of their climate.
contents
Why
"El Niño" ?
 El
Niño translates from Spanish as “the boy-child”. Peruvian
anchovy fishermen traditionally used the term - a reference to the Christ
child - to describe the appearance, around Christmas, of a warm ocean
current off the South American coast, adjacent to Ecuador and extending
into Peruvian waters.
El Niño affects
traditional fisheries in Peru and Ecuador. In most years, colder nutrient-rich
water from the deeper ocean is drawn to the surface near the coast (upwelling),
producing abundant plankton, food source of the anchovy. However, when
upwelling weakens in El Niño years, and warmer low-nutrient water
spreads along the coast, the anchovy harvest plummets. It was ruined
in the four or five most severe El Niño events this century.
contents
El
Niño's Global Effects
 The
South American El Niño current is caused by large-scale interactions
between the ocean and atmosphere. Nowadays, the term El Niño refers
to a sequence of changes in circulations across the Pacific Ocean and
Indonesian archipelago when warming is particularly strong (on average
every three to eight years). Characteristic changes in the atmosphere
accompany those in the ocean, resulting in altered weather patterns
across the globe.
The Pacific
Ocean's Circulation Features
The Pacific Ocean
is a huge mass of water which controls many climate features in its
region. Its equatorial expanse, far larger than the Indian or Atlantic
Oceans, is critical to the development of the Southern Oscillation and
El Niño. In most years the Humboldt current brings relatively cold
water northward along the west coast of South America, an effect increased
by upwelling of cold water along the Peruvian coast. The cold water
then flows westward along the equator and is heated by the tropical
sun. These normal conditions make the western Pacific about 3°C
to 8°C warmer than the eastern Pacific. However,
in El Niño years the central or eastern Pacific may become as warm
as the western Pacific.
contents
The
Walker Circulation
The Walker circulation
is named after Sir Gilbert Walker, a Director-General of British observatories
in India who, early this century, identified a number of relationships
between seasonal climate variations in Asia and the Pacific region.
“I cannot
help believing that we shall gradually find out the physical mechanism
by which these [relationships] are maintained …” –
Sir Gilbert T. Walker, 1918
 The
easterly trade winds are part of the low-level component of the Walker
circulation. Typically, the trades bring warm moist air towards the
Indonesian region. Here, moving over normally very warm seas, moist
air rises to high levels of the atmosphere. The air then travels eastward
before sinking over the eastern Pacific Ocean. The rising air is associated
with a region of low air pressure, towering cumulonimbus clouds and
rain. High pressure and dry conditions accompany the sinking air. The
wide variations in patterns and strength of the Walker circulation from
year to year are shown in the diagram above.
See
the animated
model of El Niño for more information.
contents
The
Southern Oscillation
“By
the Southern Oscillation is implied the tendency of pressure at stations
in the Pacific ... to increase, while pressure in the region of the
Indian Ocean … decreases.” – Sir Gilbert T. Walker,
1924
This definition
remains valid. We now say that the Southern Oscillation occurs because
of the large changes in the Walker circulation closely linked to the
pattern of tropical Pacific sea temperatures.
The
Southern Oscillation Index (SOI)
 The
Southern Oscillation Index (SOI) gives us a simple measure of the strength
and phase of the Southern Oscillation, and indicates the state of the
Walker circulation. The SOI is calculated from the monthly or seasonal
fluctuations in the air pressure difference between Tahiti and Darwin.
The “typical” Walker circulation pattern shown in the diagram
has an SOI close to zero (Southern Oscillation close to the long-term
average state). When this pattern is strong the SOI is strongly positive
(Southern Oscillation at one extreme of its range). When the Walker
circulation enters its El Niño phase, the SOI is strongly negative
(Southern Oscillation at the other extreme of its range).
Positive values
of the SOI are associated with stronger Pacific trade winds and warmer
sea temperatures to the north of Australia. Together these give a high
probability that eastern and northern Australia will be wetter than
normal. During El Niño episodes, the Walker circulation weakens,
seas around Australia cool, and slackened trade winds feed less moisture
into the Australian/Asian region. There is then a high probability that
eastern and northern Australia will be drier than normal.
contents
Climate
Clues to El Niño
 Meteorologists
watch for changes to the atmosphere and ocean circulation which help
them detect an El Niño, or forecast its lifetime.
Indicators are:
- The Walker circulation
and trade winds weaken. During more intense El Niño episodes,
westerly winds are observed over parts of the equatorial western and
central Pacific.
- The area of
warm water usually over the western tropical Pacific cools and the
warmest water is displaced eastward to the central Pacific.
- The normally
cold waters on the South American coast warm by 2°C to 8°C.
- The Southern
Oscillation Index remains negative.
- Enhanced cloudiness
develops over the central equatorial Pacific.
contents
El
Niño's Opposite Phase
When the Southern
Oscillation Index sustains high positive values, the Walker circulation
intensifies, and the eastern Pacific cools. These changes often bring
widespread rain and flooding to Australia - this phase is sometimes
called anti-El Niño (or La
Niña). Australia's strongest recent examples were in 1973-74
(Brisbane's worst flooding this century in January 1974) and in 1988-89
(vast areas of inland Australia had record rainfall in March 1989).
Forecasting
El Niño
Scientists have
made important advances in understanding El Niño/Southern Oscillation
phenomena in recent decades. These led to the National Climate Centre's
launch of the Seasonal Climate Outlook Service in 1989. The service
offers medium-term (three-months ahead) outlooks of rainfall. Useful
predictions of seasonal rainfall have the potential to contribute to
the goals of sustainable development in the rural sector.
contents
Ecologically
Sustainable Development in Australia
A large proportion
of Australia's natural environment is farmed, harvested or managed by
farmers. Many renewable resources, from topsoil to wildlife, are broadly
under rural sector management. Rural communities need the best climate
advice to help them protect and sustain national ecological resources
in the face of climate extremes. Improved understanding of climate variability,
and application of appropriate management techniques, will be crucial
to achieving sustainable development goals.
The
Future
Sustainable development
requires improved management in all climate ranges, especially during
climate extremes, which bring the greatest risk of environmental degradation.
The diagram below suggests how improved climate understanding and forecast
skill may increase the range of low-risk conditions, and enhance our
capacity to better manage high-risk periods.
contents
More
Information
The National Climate
Centre monitors the climate of the Australian region. It provides routine
information on current climate conditions in Australia. If you would
like more information about Australia's climate, El Niño or the
Seasonal Climate Outlook service, contact us at the:
- NSW Regional
Office Bureau of Meteorology PO Box 413 Darlinghurst NSW 2010 Tel:
(02) 9296 1555 e-mail:reqnsw@bom.gov.au
- Victorian Regional
Office Bureau of Meteorology PO Box 1636M Melbourne VIC 3001 Tel:
(03) 9669 4915 e-mail:climate.vic@bom.gov.au
- Queensland Regional
Office Bureau of Meteorology GPO Box 413 Brisbane QLD 4001 Tel: (07)
3239 8700 e-mail:climate.qld@bom.gov.au
- South Australian
Regional Office Bureau of Meteorology PO Box 421 Kent Town SA 5071
Tel: (08) 8366 2600 e-mail:climate.sa@bom.gov.au
- Western Australian
Regional Office Bureau of Meteorology PO Box 1370 West Perth WA 6872
Tel: (08) 9263 2222 e-mail:climate.wa@bom.gov.au
- Tasmania & Antarctica
Regional Office Bureau of Meteorology GPO Box 727G Hobart TAS 7001
Tel: (03) 6221 2043 e-mail:climate.tas@bom.gov.au
- Northern Territory
Regional Office Bureau of Meteorology PO Box 40050 Casuarina NT 0811
Tel: (08) 8920 3819 e-mail:climate.nt@bom.gov.au
- National Climate
Centre Bureau of Meteorology GPO Box 1289K Melbourne VIC 3001 Tel:
(03) 9669 4603 e-mail:webclim@bom.gov.au
|
