what is a ‘Negative Indian Ocean Dipole’ and why does it mean more rain for eastern Australia?

The Bureau of Meteorology recently announced that an Indian Ocean negative dipole event is underway.

But what does this mean and how does it affect Australia’s climate? Will we have a respite from the torrential rains of the past few months?

For many places on the east coast, the answer is no. Spring will not bring a clean break from this year’s very wet winter.

For many places on the east coast, this means spring will not bring a clean break from this year’s very wet winter.
Image AAP/Joel Carrett

Read more: Why is it so cold right now? And how long will it last? A climatologist explains

Warmer waters around Australia mean more rain

The Indian Ocean Negative Dipole, or IOD, was declared because ocean temperatures are warmer in the eastern tropical Indian Ocean than in the western Indian Ocean.

The Indian Ocean Dipole is a type of year-to-year climate variability, much like El Niño-Southern Oscillation, but in the Indian Ocean rather than the Pacific.

Once it enters a particular phase, it usually stays in that phase for a few months. This persistence of sea surface temperature patterns results in the predictability of Australian weather conditions.

The Bureau of Meteorology’s seasonal outlook suggests warmer ocean conditions are on the way in northern Australia.
meteorological office

When we have negative dipole conditions in the Indian Ocean, we tend to see more rain over southern and eastern Australia.

The relationship between the Indian Ocean Dipole and Australian rainfall is strongest in September and October, so we are likely to see wet conditions for at least the next few months.

The warm waters of the eastern Indian Ocean, as we are seeing right now, are increasing the occurrence of low pressure systems over southeastern Australia as well as the amount of moisture in the air .

This means there is a higher probability of rain in general and an increased probability of extreme rain events as well.

When will the rain stop?

Another wet outlook is alarming for many people in eastern Australia who have experienced wetter than normal conditions since 2020.

We have seen back-to-back La Niña events and a negative Indian Ocean Dipole during the winter of 2021.

The negative dipole of the Indian Ocean shows its influence in the seasonal forecasts of the Bureau of Meteorology. Wetter than normal conditions are forecast for the coming months across the eastern part of the continent.

There is a good chance that conditions will be wetter than average over most of Australia over the next few months.
meteorological office

It also happens to be the time of year when seasonal prospects tend to be the most skilful. In the summer, more rain falls in storms and small-scale systems that are harder to predict long in advance. In winter and spring, the outlook tends to be a bit sharper but isn’t always perfect.

Unfortunately, after two consecutive La Niña summers, it also seems increasingly likely that we will see another form of La Niña later this year.

Three La Niña events in a row is not unprecedented, but it is unusual. The increased risk of another La Niña increases the chances that wetter conditions will persist for a few more months at least.

Have we seen the end of the cold?

We saw a cold start to winter in southeastern Australia. Since then, temperatures have been below normal across much of northern Australia.

Nationally, we had the coldest July in a decade, but in the past this would have been an unremarkable event as it was only marginally cooler than historical averages.

As we leave the coldest part of winter, temperatures will inevitably rise, although we could still have another cold snap.

The Bureau’s seasonal outlook suggests warmer-than-average daytime temperatures are likely for northern and southern coastal areas. Other areas may be cooler than normal as increased cloud cover and rain are likely to suppress temperatures.

Daytime temperatures will likely be lower than average in areas that are expected to be humid.
meteorological office
But minimum temperatures are more likely to hold as cloud cover reduces the likelihood of cold nights.
meteorological office

On the other hand, minimum temperatures are expected to be above normal across the country. Increased cloud cover and rain tend to be associated with warmer nights, as clouds prevent the ground from cooling quickly overnight. This reduces the risk of freezing.

Is the constant rain the result of climate change?

For parts of New South Wales, the news of wet conditions being on the cards couldn’t come at a worse time. Sydney and other parts of the east coast have already received record rain totals so far this year, including July. Watersheds remain saturated, so further rainfall could well lead to more flooding.

Australia has highly variable rainfall and we have seen multi-year periods of persistent wet conditions before – notably in the mid-1970s and 2010-2012.

There is no strong rainfall trend in flood-hit areas and it has only been three years since we had the driest year on record in New South Wales.

The jury is still out on how climate change is affecting rainfall over most of Australia.

It is essential that we build a better understanding of changes in precipitation under global warming so that we can better plan our future climate.

Read more: ‘One of the most extreme disasters in Australian colonial history’: Climate scientists talk about floods and our future risks