Will delayed, weak Australian monsoon impact 2020 Indian monsoon?

Bushfire-ravaged and drought-stricken Australia has seen the monsoon set in at least a month behind schedule, but the humidity levels have not measured up typical of a 'classic monsoon.'

The 'wet season' (2019–20) has seen the latest monsoon onset since records commenced in 1957-58. The previous latest onset at Darwin Airport (where, by convention, the onset is measured) was January 25, 1973.

Two flavours of monsoon

Australia and India host two flavours of the monsoon on either side of the Equator, one following the other. But is there an emerging chicken and egg story here? Which comes first, the Australian or the Indian? A section of the scientific community believes that the delayed withdrawal of the 2019 Indian monsoon, the most in recorded history, in the North of the Equator, has had a lag effect on the monsoon to its South (Australia).

So, will the delayed monsoon onset in Australia also trap the impending 2020 Indian monsoon in a vicious circle, delaying its onset? Here's where the chicken-and-egg question becomes relevant.

Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology, and an Adjunct Associate Professor at the University of Pune, puts the issue in perspective.

"The strong IOD (Indian Ocean Dipole) conditions during late 2019 might have played a role in a late withdrawal of 2019 summer monsoon and also delayed onset of the Australian monsoon. However, we do not have clues on how it might affect the Indian summer monsoon in the following year," he told BusinessLine .

Confounding onset matrix

The previous latest onset of the Australian monsoon was January 25, 1973. But the most delayed onset of the Indian monsoon occurred in the preceding season (1972), on June 18 (against the normal June 1).

According to Koll, the Indian Ocean responds quickly to the ENSO (El Nino-La Nina) conditions in the Pacific. Generally, El Nino-like conditions can trigger a positive IOD. "However, model forecasts from various agencies are not showing any early signals for an El Nino or La Nina to develop this summer."

During the positive IOD phase, the lower atmospheric pressure and ascending motion of air over the West Indian Ocean is matched by the reverse - descending air and higher pressure in the East. For Australia, this meant dryness, heat, prolonged drought and calamitous bushfires.

The withdrawal of the Indian monsoon is attributed to the extended 2019-20 IOD in the western basin of the Indian Ocean, among the strongest on record, far away to the North-West of Australia.

Elevated bushfire risk

This had locked up the winds and moisture for far too long over that part of the Indian Ocean, drying up Australia for an extended period, triggering heat waves and unprecedented bushfires.

An Australian Bureau of Meteorology (BoM) update said the recent heavy rain over New South Wales and Queensland has eased the dryness in some areas, but regions further inland require several months of above average rainfall to bring them out of drought.

Days and nights are likely to be warmer than average from March to June, increasing the chance of heatwaves and elevated bushfire risk in the coming months, the BoM said.

While monsoonal flow is currently affecting parts of North Australia, a tropical low in the northern parts of West Australia had not featured the high levels of humidity typical of a 'classic monsoon'.

In fact, dry air wrapping around the low had caused only small rainfall totals over much of the North-West of the country in recent days.

Below average autumn rainfall

This includes Darwin, which has seen little rain, despite the technical monsoon onset wind criteria having been met there since February 2 (normal onset by December-end).

Australia's northern wet season extends from about October to April in the far North of the Northern Territory, but generally starts later and ends earlier elsewhere.

Active monsoon periods may occur at any time during this period; however, the initial monsoon onset, as defined by the reversal of the winds, normally occurs in late December around Darwin.

A BoM outlook suggested that most of the southern half of Australia is unlikely to see significantly above or below average rainfall during March to May (autumn). However, below average rainfall is slightly more likely in the Far-East and above average rainfall is slightly more likely in parts of southern parts of West Australia.

In the North, above average rainfall is likely during March across much of the tropics. But for parts of Queensland, below average rainfall is likely from March to May.

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