Indian Ocean Dipole decays, to hasten monsoon over fire-ravaged Australia

The Indian Ocean Dipole (IOD) has returned to neutral in what is a headline event, signalling the potential arrival of Australia’s delayed annual monsoon and some desperately awaited relief for the bush fire-ravaged country.

The Australian Bureau of Meteorology announced mid-morning today (Tuesday) that an aggressive positive phase of the IOD may have collapsed and that the Indian Ocean returned to neutral after one of the strongest positive IOD events to impact Australia in recent history.

The IOD is expected to remain neutral in the coming months, meaning that it will have little influence on the Australian and global climate. Closer home, the decay of the IOD event should help taper off the easterly flows across the South Indian peninsula and associated hit-or-miss thunderstorms across the geography.

Bushfire weather

However, the IOD’s legacy of widespread warm and dry conditions (ascending air, lower pressure, cloud and rain over the West Indian Ocean matched only by descending air, higher pressure, hot and dry air, nil clouds/rain to the East, the near Australian coast) during the second half of 2019 primed the Australian landscape for bushfire weather and heat waves this summer.

In the Pacific Ocean, although indicators suggest neutral (neither El Nino or La Nina) conditions, the tropical ocean near and to the West of the Date Line remains warmer than average, potentially drawing some moisture away from Australia. Most climate models indicate the Equatorial Pacific will remain neutral until at least March-May (end of the Southern Hemisphere autumn), meaning it will have limited influence on Australian and global climate. When the IOD and Pacific conditions are neutral, Australia’s climate can be impacted by more local or short-term climate drivers.

Meanwhile tropical cyclone Blake, which formed off the Kimberley coast of West Australia yesterday (Monday), presented a striking contrast to the raging bush fires to the South-East of the vast island-continent. Blake is the first tropical cyclone to form in the Australian region for the 2019-20 tropical cyclone season. Blake is not forecast to intensify beyond category 1 strength and is expected to make landfall along the northern West Australia coast during the course of the day today.

A second tropical system currently over the Arafura Sea, north of the Northern Australian Territory, is forecast to develop to tropical cyclone intensity in the coming days, potentially making landfall on the North Coast. Unfortunately, both the cyclones have chosen to form on the wrong side of the fire-stricken country.

Monsoon onset

In another good augury, the Australian Bureau of Meteorology observed that tropical cyclone Blake and the fresh low-pressure area to the North have both developed in a monsoon trough located off the North-West coast of Australia. Depending on the movement and development of these tropical systems, the monsoon trough may be dragged far enough South that the monsoon onset at Darwin takes place in the coming week.

The monsoon onset, which typically occurs over Darwin during the last week of the year, has been delayed in 2019-20 likely due to the influence of a positive Indian Ocean Dipole (IOD). The IOD has now returned to neutral values, meaning it's no longer actively reducing the chance of monsoon onset.

With the decay of the positive IOD, the Bureau's climate outlook no longer has its drying influences as a factor. Outlooks currently indicate an increased chance of average to above-average rainfall across parts of North Australia in the coming weeks and months. More broadly, if an active monsoon develops over North Australia, it increases the chance of rainfall, not only across northern Australia, but for regions further South. This is because the influx of moisture associated with the monsoon can be transported by weather systems from North to South and potentially generate periods of more intense rainfall.

The decay of the IOD may also open the doors for a helpful Madden-Julian Oscillation (MJO) wave to settle over the Australian latitudes and track eastwards during the next fortnight. The MJO wave travels periodically from the West to the East over the Indian Ocean and is a harbinger of clouds, wet weather, heavy rain and even storms. At this time of the year, an MJO pulse over Australian longitudes is typically associated with the development of monsoonal westerly winds and above-average cloudiness and rainfall for much of North Australia and parts of the Maritime Continent, to Australia's North. The focus of enhanced weather over Australia is typically greatest for the North-West of the continent in this scenario, but heavy rainfall for Cape York Peninsula is also possible.

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