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El Nino and positive IOD looming

Ben Domensino, Wednesday October 10, 2018 - 12:26 EDT

Australia may be affected by El Nino and a positive Indian Ocean Dipole during the coming months. So, what does this mean in terms for Australia in terms of weather?

Australia was placed on El Nino Alert by the Bureau of Meteorology this week. This is the one step short of the Bureau declaring that an El Nino event is underway and indicates a 70 per cent chance of El Nino occurring during the coming months.

Meanwhile, an index called the Indian Ocean Dipole has been in a positive phase for the last month and looks set to continue on this trend during the rest of spring.

Both El Nino and a positive Indian Ocean Dipole are the names given to distinct patterns of sea surface temperature anomalies in equatorial Pacific and Indian Oceans, respectively. These particular patterns usually cause below average rain and above average temperatures across large areas of Australia, however their influence on rainfall and temperature varies throughout the year.

El Nino's greatest influence on Australian rainfall occurs during winter and spring. Its impact on rain in eastern and southern Australia then diminishes from about November onwards and during summer, El Nino's main effects on Australian weather include:

  • A suppression of early wet-season rainfall in Australia's tropics, particularly over the Top End and northern Queensland
  • An increased likelihood of above-average temperatures in southern Australia, including more intense heat on the hottest days of the season along the nation's south coast and more intense and prolonged heat over Australia's southern inland
  • Fewer tropical cyclones between November and April, although not necessarily weaker cyclones
  • Increased bushfire risk in parts of southeastern Australia

A positive Indian Ocean Dipole causes much of the same weather as El Nino and when they occur at the same time, these effects can be exacerbated. However, like El Nino, the Indian Ocean Dipole has its greatest influence on Australia's weather during winter and spring.

If the currently positive Indian Ocean Dipole persist into late spring, as it's expected to,  the likelihood of suppressed rainfall and above average daytime temperatures will be increased across much of the country. Indian Ocean Dipole events rapidly decay when the monsoon develops over northern Australia around the end of spring or early summer.

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