This year’s Indian monsoon withdrawal was the latest on record by more than a week, meaning a delayed start to Australia’s monsoon season is likely.
But another poor wet season isn’t necessarily set in stone, because when it comes to the atmosphere, nothing is quite that clear-cut.
Every year a band of tropical moisture moves up and down the globe as Earth’s tilt dictates the seasons. It goes by various names, but is often called the Inter-tropical Convergence Zone (ITCZ) and has the monsoon trough roughly embedded within it.
In the Northern Hemisphere summer it moves north, bringing the monsoon and rain to India and the northern tropics.
Then as we move into the Southern Hemisphere summer, it moves down and brings the Australian monsoon and northern wet season.
Greg Browning, tropical climatologist with the Bureau of Meteorology, said the monsoon usually started tracking south in September.
“It gets to a stage where there’s no longer those south-westerly winds occurring over India at all, and they call it the complete retreat of the monsoon,” he said.
But this year the India Meteorological Department announced the latest withdrawal of the monsoon on record.
“The south-west monsoon has withdrawn from some parts of Punjab, Haryana and north Rajasthan today, October 9, as against the normal date September 1,” it said.
“The most delayed withdrawal in the past years has been recorded in 1961 (October 1), followed by 2007 (September 30).”
Mr Browning said that delay would likely have follow-on effects for Australia.
“If the monsoon is later than usual, it stands to reason, and physics just dictates, that there will be a delay in that whole process of the monsoon tracking further south into the Southern Hemisphere.”
Why so late?
The finger is firmly being pointed at the positive Indian Ocean Dipole (IOD).
A positive IOD is a lot like an El Nino event, but it takes place over the Indian Ocean rather than the Pacific.
Mr Browning said this year’s could be the strongest on record.
“The really accurate sea surface temperature records don’t go back that far, but it looks like it’s certainly one of the strongest events on record.”
A positive IOD had a number of impacts, Mr Browning said.
“It enhances the rainfall in the western part of the Indian Ocean, and it’s basically contributed to a stronger monsoon than usual over India and hence that monsoon lasted a bit longer.”
Parts of Africa received above-average rainfall this year, and it was one of the strongest Indian monsoons in years, according to Mr Browning, up 10 per cent on the normal monsoon.
“Historically the other really late monsoon retreat was in 1961 and that was also associated with a positive IOD.”
But while the positive IOD brings wet conditions in the west, it is the opposite for us — the atmospheric circulation means we get descending dry air over Australia discouraging rain.
“It was basically a double whammy improving the monsoon conditions in the Northern Hemisphere and around India, and basically just making them a bit less favourable for us around Australia,” Mr Browning said.
Ocean temperatures have been down over north-western Australia.
“With those cooler waters, as well as the atmospheric circulation being more conducive to dry conditions, we’ve seen extended dry conditions.
“Particularly in New South Wales and Queensland, but broadly much of eastern Australia — the IOD is definitely not favourable for good rainfall for the eastern two-thirds of Australia.”
But a late start doesn’t guarantee a poor season
After last year’s generally poor wet season was followed by a typically dry dry season, a late start to this year’s wet will not be what many want to hear.
Thankfully, according to Mr Browning, there could be a silver lining on those clouds when they do eventually come.
As the monsoon moves south, it breaks down the IOD — the focus of the Sun’s energy further south warms the cool waters around northern Australia and the monsoon winds do the rest.
“While the Sun leads the way, it also will start to generate that monsoon flow in the Southern Hemisphere, and then we see westerly winds start dominating and they’ll basically break down the easterly winds that are associated with the positive IOD.”
Once the positive IOD is gone, which Mr Browning said we could expect by January at the latest, there are no other climate drivers on the horizon that are likely to reduce rainfall for the rest of summer.
“And that’s really the peak months of the wet season anyway — January, February, March — that’s when we get the real heavy rains,” he said.
“There is no reason at all why there can’t be just a normal season, and hopefully it can be a much better season than what we had last year.”
Last year was also a positive IOD, and even once it broke down, the Pacific Ocean remained warm, close to El Nino levels, which encouraged dry conditions.
“There will certainly be a lot of places that are struggling at the moment,” Mr Browning said.
“I know a lot of places across far northern Australia didn’t see good replenishment of the groundwater supplies, and that’s often the thing that gets them through the dry season.
“Having said that, a lot of northern Australia has actually seen an increasing trend of rainfall in recent decades.
“So you would hope that if it does go back to normal and we don’t have any of these adverse climate drivers, which looks like being the case, then we’re likely to see reasonable rains.”