What is La Nina and what does it mean?

(Australian Associated Press)

 

Australia’s weather is influenced by many climate drivers. El Nino and La Nina have the strongest influence on year-to-year climate variability for most of the country. They are part of a natural cycle known as the El Nino-Southern Oscillation cycle that loosely operates over timescales from one to eight years.

LA NINA TYPICALLY MEANS:

  • Increased rainfall across much of Australia
  • Cooler daytime temperatures (south of the tropics)
  • Warmer overnight temperatures (in the north)
  • Shift in temperature extremes
  • Decreased frost risk
  • Greater tropical cyclone numbers
  • Earlier monsoon onset

The Bureau of Meteorology has issued an alert saying there’s a 70 per cent chance of La Nina developing in Australia this year, bringing cooler temperatures, more rain and an increased chance of widespread flooding.

The increased rainfall and cloudiness associated with La Nina usually means above-average winter and spring rainfall for Australia, particularly across the east and north, raising the risk of flooding in some parts with wetter soils bringing milder daytime temperatures.

“We can also see an earlier start to the tropical cyclone season,” a BOM spokeswoman said.

The six wettest winter-spring periods on record for eastern Australia occurred during La Nina years.

In the Murray-Darling Basin, winter/spring rainfall averaged over all 18 La Nina events since 1900 was 22 per cent higher than the long-term average, with the severe floods of 1955, 1988, 1998 and 2010 all associated with La Nina.

Unlike El Nino years, the impacts of La Nina often continued into the warm months, the BOM said.

In eastern Australia, the average December-March rainfall during La Nina years is 20 per cent higher than the long-term average, with eight of the ten wettest such periods occurring during La Nina years.

The east coast, which tends to be less affected by La Nina during the winter months, can experience severe flooding during La Nina summers.

Of the 18 La Nina events since 1900, 12 resulted in floods, with the east coast experiencing twice as many severe floods during La Nina years than El Nino years.

Some areas of northern Australia typically experience flooding during La Nina because of an increase in tropical cyclone numbers.

The wettest years on record for Australia occurred during the strong 2010-2012 and 1974 La Nina events with the 2010-12 La Nina event bringing widespread flooding across Australia.

 

SOURCE: BOM

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