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.
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.
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
Southern Oscillation Index (SOI)
The "SOI", or Southern Oscillation
Index, is a measure that scientists use to establish whether El Niņo
is present and how strong it is. A negative SOI value indicates an El
Niņo. The lower the value of the SOI, the stronger the El Niņo event.
If the SOI value is positive, it means that there was no El Niņo event
in that year.
The Southern Oscillation
Index (SOI) is determined by the difference in barometric pressure between
Tahiti and Darwin. Pressure fluctuates between the two locations, but
in El Niņo years (SOI is negative), the pressure is higher in Darwin
than in Tahiti. See the SOI
graph for the current
values.
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.
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).
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