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Forecast accumulated rain over Australia this week.Credit:Weatherzone

But La Nina is not the only climate pattern affecting our rainfall at the moment. Meet the lesser-known Madden-Julian Oscillation,or MJO,which is also currently active. This eastern-moving zone of wind and cloud circles the tropical areas of the globe,and when active,increases our chances of above-average rainfall.

Imagine the MJO as a “pulse” of storms – a collection of stormy weather that moves together – which travels around the equator,explains Melbourne University climate scientist Dr Andrew King. This pulse appears in the west Indian Ocean and moves over the “Maritime Continent”,the meteorological term for the area over Indonesia,Papua New Guinea,Malaysia and the Philippines.

“It has a greater effect across northern Australia,but it can affect southern Australia too,” King said.

“It sets off atmospheric waves that can influence our weather,even as far south as Victoria. When it’s active over Indonesia it tends to mean wetter weather in Australia.”

In its most recent update,the Bureau of Meteorology said most models indicated the Madden-Julian Oscillation would strengthen over the Maritime Continent in the coming week and maintain moderate strength as it moved east towards the western Pacific.

Unlike a standing pattern such as La Nina,the MJO is a travelling pattern that moves eastward,about 15 to 30km/h. Each cycle lasts between 30 and 60 days.

“While active in the Pacific at this time of the year,the MJO increases the chance of above average rainfall over the eastern half of Australia,” the bureau said.

Victoria is bracing for storms and flooding on Wednesday and Thursday,with some parts of the state forecast to cop up to 100 millimetres of rain.

Sydney has already broken all previous records for rainfall,and the official gauge at Observatory Hill reached a cumulative annual rainfall total of 2194.6 millimetres last week.

As well as La Nina and the MJO,there are other climate patterns also affecting Australia’s weather.

Indian Ocean Dipole: For most of Australia,El Nino brings dry weather,while La Nina brings wet weather.

There is asimilar climate driver in the Indian Ocean – the Indian Ocean Dipole – which has the same impact through its “positive” and “negative” phases,respectively.

A negative IOD typically increases the chance of above average spring rainfall for most of the eastern two thirds of Australia. This is the second successive year for a negative IOD,which refers to the patterns of sea surface temperature.

When La Nina and negative IOD conditions combine,the likelihood of above-average rainfall over Australia increases further,particularly for the eastern half of the continent.

Southern Annular Mode: In the region between Tasmania and Antarctica,strong westerly winds blow almost continuously and are associated with storms and cold fronts that move from west to east,bringing rainfall to southern Australia.

TheSouthern Annular Mode,or SAM,refers to how far north and south these westerly winds move. It has three phases:neutral,positive and negative.

A positive SAM in summer brings an increased chance of rainfall,though this varies greatly by region.

It is currently positive and likely to remain generally positive throughout spring into early summer.

Climate change continues to influence the Australian and global climate. Australia’s climate has warmed by around 1.47 degrees in the period between 1910 and 2020.

There has also been a trend towards a greater proportion of rainfall in high intensity,short duration bursts,especially across northern Australia.

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