A classic example of ‘blessings’ from an extended weather event in one country turning ino a ‘curse’ for another, half a world away, is just emerging in the Northern and Southern hemispheres.

It also goes to show how weather/climate phenomena treat the geographical limits determined by national/international aspirations with scant regard.

Fundamental driver

The reference here is to the latest weather development, in which a delayed South-West monsoon in India (Northern Hemisphere) is delaying the monsoon season in Australia (Southern Hemisphere).

The fundamental driver of all monsoon systems is the solar heating of the land during the Spring season (February-March in India) which helps establish a land-sea temperature difference.

Land heats up faster than water, causing the air to expand and lower the pressure, triggering winds from the seas to the land (wind flows from a high-pressure area to a lower-pressure area).

This moisture is rained out through the process of convection (cloud-building) over the monsoonal regions, mainly India and South-East Asia, during the summer.

Due to the change in season, peak solar heating moves towards the Equator (as it does currently) and then into the Southern Hemisphere, thereby heating the adjacent ocean more than the Asian land.

As a consequence, the winds reverse, and the monsoon rainfall moves to the opposite hemisphere during the Austral summer (summer in the South Hemisphere, December-February).

The villain: A positive IOD

The villain of the piece is mainly believed to be the intense positive phase of the Indian Ocean Dipole (IOD), which drove the Indian monsoon to a peak this year, even spilling it into October.

The positive IOD, which warms the western basin of the Indian Ocean relative to the East, boosts an Indian monsoon and is considered the nemesis of the Australian summer monsoon.

The 2019 monsoon in India started its withdrawal on October 9, against the normal date of September 1, and is the most delayed in recorded history.

The previous latest date was October 1, 1961 — also a strong positive IOD year — and much later than the average date of September 1, the BoM noted.

The Indian monsoon was the strongest in recent years with a surplus of 10 per cent in 2019, per the India Meteorological Department, which is in part due to the contribution of the positive IOD.

Climate models currently indicate that positive IOD may persist longer than typical events do, and the delayed transition of the monsoon trough from the northern hemisphere to the southern hemisphere may be a related factor.

Delay in onset

A positive IOD, which persists later than usual, may also contribute to a delay of the onset of the Australian monsoon this season.

This is because the withdrawal itself is a long-drawn process, followed by the North-East monsoon over India, only after which the monsoon trough crosses into the Southern Hemisphere.

The cumulative delay has a cascading impact on the summer monsoon for Australia, according to an update by the Australian Bureau of Meteorology (BoM).

A strong positive Indian Ocean Dipole continues to influence Australian and global climate, the BoM said, adding that the El Nino–Southern Oscillation (ENSO) remains neutral.

In fact, the current IOD event has strengthened significantly over the past month. The latest weekly value of +2.15 deg C is the strongest positive weekly value since at least 2001.

It was in 2001 that the Bureau's weekly dataset commenced, but the BoM suspects that this value could possibly be the most robust since 1997, when strong monthly values were recorded.

Over the past month, strong easterly trade winds across the tropical Indian Ocean have aided the upwelling of cooler water in the eastern Indian Ocean (closer to Australia).

Relentless run

At the same time, very warm waters off the Horn of Africa (West Indian Ocean) have caused an even greater temperature gradient across the basin.

Given the strength of the trade winds, the IOD may strengthen further over the next fortnight, the BoM reckoned, pointing to its relentless run since May this year.

However, international climate models surveyed by the Bureau indicate that the positive IOD is unlikely to persist far into the Australian summer.

The IOD breakdown occurs when the monsoon trough moves into the southern hemisphere in early December (this is when the North-East monsoon in India draws to a close).

With the monsoon trough having a record-late retreat from India this year, the shift into the Southern Hemisphere may also be later than usual, the Australian Bureau said.

Impact on Australian agriculture

The impact of a delayed monsoon may have implications for Australian summer crops, with the planted area expected to fall 28 per cent in 2019–20 to around 758,000 hectares.

This reflects low levels of soil moisture and an outlook for unfavourable seasonal conditions during Spring in Queensland and northern New South Wales. Summer crop production is forecast to fall by 20 per cent to 2.1 million tonnes, the Australian Agriculture Department said.

Typically, a positive IOD brings below-average winter-spring rainfall to southern and central Australia, with warmer days for the south of two-thirds of the country.

Positive IOD events are often associated with a more severe fire season for South-East Australia. In the tropical Pacific Ocean, the El Nino–Southern Oscillation (ENSO) remains neutral.

Climate models forecast a neutral ENSO for the remainder of 2019 and into the first quarter of 2020, the Australian Bureau said.

When ENSO is neutral, it has little effect on Australian and global climate, meaning other influences are more likely to dominate.

The Bureau has also released its Australian tropical cyclone outlook for the season, which runs from November 2019 to the end of April 2020.

The outlook predicts that fewer than the long-term average of 11 cyclones will form in the Australian region this coming season.