S –O 2014 19How La Niña
happensHow El Niño
happens(^1) Pacifi c trade winds weaken
or reverse, allowing warm surface
water to fl ow east along the
equator towards the Americas,
where it collects.
(^1) The prevailing Pacifi c trade
winds blow to the west. When
they intensify, they push warm
surface water closer to the Asia
than normal.
(^2) The SOI records negative
values, the result of higher-
than-normal atmospheric
pressure near Darwin and
lower than normal atmospheric
pressure near Tahiti.
(^2) The SOI records positive
values, the result of lower-than-
normal atmospheric pressure
near Darwin and higher than
normal atmospheric pressure
near Tahiti.
(^3) The Walker Circulation
moves east, bringing increased
cloud and rainfall to the eastern
Pacifi c, while northern Australia,
Indonesia, South-East Asia,
India and even the east coast
of Africa experience drier than
usual conditions.
(^3) The east–west air circulation
over the Pacifi c Ocean, the
Walker Circulation, moves west,
bringing increased cloud and
rainfall to the western Pacifi c.
50%
Likelihood of an El Niño event
occurring in the spring of 2014,
as forecast in August by the
Bureau of Meteorology.
Major bushfire
(More than 2000sq.km burnt)
or reverse, allowing warm surface
water to fl ow east along the
equator towards the Americas,
where it collects.
values, the result of higher-
than-normal atmospheric
pressure near Darwin and
lower than normal atmospheric
pressure near Tahiti.
normal atmospheric pressure
near Darwin and higher than
normal atmospheric pressure
near Tahiti.
moves east, bringing increased
cloud and rainfall to the eastern
Pacifi c, while northern Australia,
Indonesia, South-East Asia,
India and even the east coast
of Africa experience drier than
usual conditions.
over the Pacifi c Ocean, the
Walker Circulation, moves west,
bringing increased cloud and
rainfall to the western Pacifi c.
1
3
2
Reduced
upwelling of
cold water
Tahiti
Warmer water
than normal Equator
Major flooding
(Sustained above-average rainfall)
Major drought
(Sustained below-average rainfall)
1
3
2
1995 2000
2005
2010
2013
MILLENIUM, QLD/NSW/VIC
2002–
EAST COAST, NSW
DEC 1994
BRISBANE, QLD
JAN 2011
EAST ALPINE, VIC
BLACK CHRISTMAS, NSW JAN 2003
DEC 2001
BLACK SATURDAY, VIC
GREAT DIVIDE, VIC FEB 2009
JAN 2007
Warmer water
and more rainfall
Walker Circulation
Cold^ water
Darwin Tahiti
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