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Understanding the Weather

What is the Nino3.4 Index?

The Nino3.4 Index, similar to the Southern Oscillation Index, is closely tied in with the El Nino phenomenon. Nino3.4 is the average sea surface temperature anomaly in the area bounded by 5N to 5S and 170W to 120W. Changes in sea-surface temperatures in this region are important in shifting rainfall areas across the far western Pacific Ocean. An El Nino event is indicated if the 5-month running average of the Nino3.4 index exceeds 0.4C for at least six consecutive months. A La Nina event is indicated if this average is below -0.4C for at least six consecutive months.

A graph of the Nino3.4 index is updated weekly on Weatherzone, making it easier to track changes in the Pacific Ocean sea-surface temperatures.

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Jack becomes Australia's tenth cyclone

12:38 EST Tropical Cyclone Jack has rapidly developed over the warm waters of the Indian Ocean in the past 24 hours, making it the 10th system to be named this season.

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