Higher risk of tropical cyclones, early monsoon amid negative Indian Ocean Dipole and possibly La Nina

Australia’s northern rainy season is expected to arrive earlier this year, according to the Bureau of Meteorology (BOM), bringing a higher likelihood of offshore tropical cyclones.

Much of southeastern Australia was hit by record flooding following La Nina, while the north of the continent saw decent rainfall totals in the past rainy season.

Now the weather bureau has officially declared another weather phenomenon – known as the Indian Ocean Negative Dipole – which is expected to add to the recent incessant rains on the east coast.

The weather system is also expected to bring wetter weather to the Northern Territory’s Top End and Far North Queensland. However, parts of northern Western Australia are likely to remain somewhat drier.

The Bureau of Meteorology predicts that large parts of northern Australia will see an early start to rainfall, but a later start in WA is possible.(Provided)

Australia’s northern rainy season typically runs from December to April, following an accumulation of hot and humid conditions from October.

The weather bureau measures how early or late the onset of rainfall is by the time it takes for rain gauges to register 50mm after the start of September. Typically, gauges reach the 50mm threshold in Darwin at the end of October.

“There’s a good chance of reaching 50mm from September 1 potentially early October or maybe even late September,” BOM lead forecaster Moses Raico said of this year’s conditions.

The culprits are broad climatic factors in the Pacific and Indian Oceans, including La Nina, responsible for much of the East Coast’s wet weather so far this year.

The BOM predicts a 50:50 chance of La Nina forming for a third consecutive summer, but Mr Raico said it was actually a negative Indian Ocean dipole that was more likely to form. influence rainfall in northern Australia.

He said it was when “the waters across the east side of the [Indian Ocean] basin are warmer than compared to what is happening on the west side”.

“The combination of these warmer waters, and if we also call in La Nina later, could help see wetter conditions than we expected, which would contribute to a greater chance than a sill precipitation is reached.”

A map showing regions that typically receive more precipitation during a negative IOD
Although every year is different, areas shaded in green generally receive wetter than normal weather during a negative Indian Ocean Dipole – particularly southeastern Australia.(ABC: Shakira Wilson)

Earlier rains, more potential cyclones

Every year, the monsoon comes around the same time. In Darwin, torrential rains usually begin between Christmas and New Years.

But Mr Raico said the combination of a La Nina and a negative Indian Ocean dipole could push the monsoons forward earlier in December.

“However, the exact timing, it’s a little hard to pin down,” he said.

“Certainly, if that were to happen and we see these warmer sea surface temperatures, that would generally translate to potentially above average tropical cyclones over the course of the season.”

Typically, a negative Indian Ocean Dipole and La Nina trigger above average rainfall in northern Australia during its rainy season.

But the BOM, in its report concluding the 2021-22 rainy season, acknowledges that this was not entirely true.

“La Nina 2021-2022 was conducive to an early start to the rainy season and above-average rainfall until January 2022,” BOM said.

“However, somewhat atypically for a La Nina, many parts of northern Australia were drier than average from February to April 2022.”

A man wearing thick black rimmed glasses looking at the camera with fishing boats moored behind him
Michael O’Brien says the forecast will benefit shrimp fishermen. (ABC News: Dane Hirst)

The rains are good news for some

The increase in wet weather could bring a premature end to the drought-ridden Northern Territory’s bumper tourist season.

But those who are likely to benefit are shrimp fishermen, whose catches benefit greatly from rainy conditions.

Australia Bay Seafoods managing director Michael O’Brien said 16 boats were already out in the Gulf of Carpentaria last week for the tiger prawn season.

“[The forecast] is great news for us because our banana [prawn] season, which is the first half of the year, is totally related to how much rain we get to drain the rivers,” he said.

Mr O’Brien said the rains washed out “the rivers with all the little prawns so they can come out and grow”.

“Our prawns will be bigger, which is good for everyone – good for the market, good for us and good for the consumer who is going to eat big prawns.”

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