THE CLIMATE KELPIE BLOG: What goes around – may bring rain to northern Australia

Posted by BCG on 10th August 2020

The Madden-Julian Oscillation (MJO) is a global circulation system that can bring big rains to northern Australia during the wet season, but it doesn’t always deliver.

Essentially, it is a very large-scale wave in the atmosphere that can bring a ‘pulse’ of cloud and rain. It travels from west to east circling the globe near the equator every 30 to 60 days. This can lead to a major fluctuation in tropical weather on weekly to monthly timescales.

As the wave passes to the north of Australia it brings a change in the direction of the prevailing winds (favouring westerlies) and a change in the amount of cloudiness and rainfall across northern Australia, especially from November to April.

While it mainly influences the climate in northern Australia the MJO can also influence temperatures in southern and eastern Australia during spring and early summer, often resulting in heat waves across southern parts of South Australia and Victoria, and even Tasmania, about one week after the dry phase passes through northern Australia.

During our winter the MJO becomes weaker and its main influence shifts to the northern hemisphere, although it does influence night-time minimum temperatures in Queensland.

The Bureau of Meteorology track the influence of the Madden Julian Oscillation as it passes across the north of Australia using a range of tools including a radiosonde, launched here by Dr Matthew Wheeler off the coast of northern Australia. Source: Bureau of Meteorology.

It’s just a phase

Australian Bureau of Meteorology researchers, Dr Matthew Wheeler and Dr Harry Hendon, have developed an index to plot the MJO’s strength and progress around the globe, which has now been adopted throughout the world.

“The index describes the MJO using eight numbered phases plus a ninth ‘weak’ or inactive phase,” says Dr Wheeler.

The biggest impact on rainfall in Australia’s north occurs during Phases 5 and 6 of the MJO (Figure 1).

Figure 1. The chance of weekly rainfall exceeding the median increases for northern Australia during Phase 5 and 6 of the Madden-Julian Oscillation (MJO) when it passes to the north of the continent (probabilities for December to February shown). Arrows show wind anomalies at the 850 hPa level (about 1 km above the surface). Source: Bureau of Meteorology.

“By analysing global observations of wind and cloudiness we can plot the daily progress of the MJO as a line moving in an anticlockwise direction on the diagram (Figure 2).

The numbered phases on the diagram allow us to visualise the location of the MJO’s rainy signal. Phase 3 and 4 are over the Indian Ocean, Phases 5 and 6 influence northern Australia and Phases 7 and 8 are over the Pacific Ocean.”

“The point on the diagram is plotted close to the centre when the MJO is weak and further away from the centre when it is stronger.”

Every cycle of the MJO looks different, with some events having more impact on northern Australia than others.

“We still don’t have a complete theory of what controls or influences the speed of movement of the MJO from west to east or how far south it will have an impact, but fortunately, our computer models are getting better at predicting this varying influence.” says Dr Wheeler.

“We can predict the movement of the strength and phase of the MJO very accurately with at least a two-week lead time.”

Figure 2. The Madden-Julian Oscillation (MJO) index diagram shows the global position and strength of the MJO. In this example the MJO was relatively weak during December 2019 (red line) and grew stronger in January 2020 (green line). The strong MJO signal played a role generating good rainfall totals as it passed northern Australia (Phases 5 and 6) during January 2020. Source: Bureau of Meteorology.

Monsoon

The MJO is not the only influence on Australia’s monsoon season, but it can have an impact on both the location and intensity of the monsoon trough.

“When the rainy phase of the MJO approaches northern Australia from the Indian Ocean, the monsoon trough tends to shift southwards from the Timor Sea to lie over the Australian landmass bringing more rain over northern Australia,” says Dr Wheeler.

“Then, about a week or so later as the rainy phase of the MJO shifts to the Pacific Ocean, the monsoon trough usually shifts back to the north again and the rain over northern Australia eases.”

During last summer (2019-2020) the monsoon arrived in Darwin on January 18, which is 24 days after the long-term mean date of December 25. This delay can be attributed to the late persistence of the positive Indian Ocean Dipole (IOD).

While the positive IOD was in place the MJO was in or near the weak phase, but once the positive IOD broke down, the MJO strengthened.

“Thankfully for northern Australia this coincided with the MJO’s passage across our continent, bringing welcome rain to many locations” Dr Wheeler said.

Because of the importance of the MJO for northern Australia’s climate and agriculture, its status is monitored and a discussion of its recent and expected impacts for the coming weeks is available on the Bureau’s Madden-Julian Oscillation page, which is updated weekly.

Contact

agriculture@bom.gov.au

Bureau of Meteorology: Madden-Julian Oscillation

The climate dogs: Meet Mojo and the other Climate Dogs (Enso, Indy, Ridgy, Sam and Eastie) on the Climate Kelpie website to find out more about the major drivers of Australia’s climate and weather.

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