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The Southern Oscillation Index (SOI) is calculated from the monthly or
seasonal fluctuations in the air pressure difference between Tahiti and Darwin.
Sustained negative values of the SOI often indicate El Niņo episodes. These
negative values are usually accompanied by sustained warming of the central
and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific
Trade Winds, and a reduction in rainfall over eastern and northern Australia.
The most recent strong El Niņo was in 1997/98.
Positive values of the SOI are associated with stronger Pacific trade winds
and warmer sea temperatures to the north of Australia, popularly known as a
La Niņa episode. Waters in the central and eastern tropical Pacific Ocean
become cooler during this time. Together these give an increased probability
that eastern and northern Australia will be wetter than normal.
The most recent strong La Niņa was in 1988/89; a moderate La Niņa event
occurred in 1998/99, which weakened back to neutral conditions before reforming
for a shorter period in 1999/2000. This last event finished in Autumn 2000. |
