Weather

Positive IOD declared as El Nino looms

Ben Domensino, 7 November 2018

A positive Indian Ocean Dipole event has been officially declared, while the Pacific Ocean continues to head towards El Nino.

According to the Bureau of Meteorology, the pattern of sea surface temperatures in the Indian Ocean has reflected a positive Indian Ocean Dipole (IOD) for the last two months. As of Wednesday, this was long enough for it to be officially declared as a positive IOD event.

A positive IOD increases the chance of below average rainfall across large areas of central and southern Australia and above average daytime temperatures in the nation's south and west.

However, IOD events usually decay rapidly in late spring when the monsoon arrives in the southern hemisphere. As a result, the recently declared positive IOD won't last much longer and will have little, if any, influence on Australia's weather beyond November.

On the other side of Australia, persistent warmer than usual water in the equatorial Pacific Ocean has kept Australia on El Nino Alert for the last few weeks. According to the Bureau, there is a 70 per cent chance that an El Nino event will develop in the next two months and possibly persist into early 2019.

El Nino events typically form and decay during autumn and have their greatest influence on Australian rainfall during winter and spring. If an El Nino develops in the coming months, which is later than usual, some of its main influences in the near-term would be:

  • a potential suppression of early-wet season rainfall in northeastern Australia
  • an increased likelihood of more intense heat extremes in southern Australia this summer
  • Potential for fewer tropical cyclones than usual in the Australian region between November and April

Despite the declaration of a positive Indian Ocean Dipole event on Wednesday, a moisture-laden northwest cloud band has produced decent rain and storms across a large area of the country during the last week. Some parts of central and southeastern Australia received their best rain in 8-12 months during the last couple of days.

 

While a positive Indian Ocean Dipole and El Nino increase the chance of below average rainfall across much of Australia, heavy rain events can still happen under these regimes.