Negative Indian Ocean Dipole

Indian Ocean Dipole (IOD) Index. Used to track the volatility of the Indian Ocean Dipole. The event occurs after the index crosses the threshold for 8 weeks. Credit: Bureau of Meteorology

This month we had a crazy and devastating weather.Perth The wettest July In decades, it has been raining for 18 consecutive days.Overseas, parts of Europe and China have endured Large-scale floodHundreds of people were killed and hundreds of thousands were evacuated.

And last week, the Bureau of Meteorology officially declaration There is the Negative Indian Ocean Dipole Phenomenon, the first negative event in five years known to bring rain.

But what is the Indian Ocean Dipole and is it important? Is it because of these events?

What is the Indian Ocean Dipole?

Indian Ocean Dipole (IOD) is natural climate Affecting phenomena Rainfall pattern Around the Indian Ocean, including Australia. It is brought about by the interaction between the flow along the surface of the sea and the atmospheric circulation.

This can be thought of as the Indian Ocean cousins ​​of El Nino and La Nino, well known in the Pacific Ocean. Basically, in most parts of Australia, El Nino brings dry weather and La Niña brings rain. IOD has the same effect through the positive and negative phases, respectively.

Positive IODs are associated with an increased likelihood of dry weather in southern and southeastern Australia.Catastrophic Black summer wildfire In 2019-20, it was associated with extremely positive IODs as well as artificial ones. Climate change Which one Deterioration These conditions.

Negative IODs tend to be less frequent and less intense than positive IOD events, but they can still bring harsh climatic conditions such as heavy rains and floods to parts of Australia.

IOD is determined by the difference in sea surface temperature on both sides of the Indian Ocean.

In the negative phase, the water in the eastern Indian Ocean (near Indonesia) is warmer than usual, and the western Indian Ocean (near Africa) is cooler than usual.

This allows more moist air to flow towards Australia, facilitating changes in wind patterns in a way that promotes more rainfall to southern Australia. This includes Western Australia, South Australia, Victoria, New South Wales, and parts of ACT.

In general, IOD events begin in late fall or winter and can last until the end of spring, ending abruptly at the beginning of the monsoon in northern Australia.

Why do we need to care?

We are probably wet a few months before us.

Negative IODs mean that winter and spring are likely to rain in the southern regions of Australia.Indeed, the seasonal outlook shows many above-average rainfalls in the country. Next 3 months..

In southern Australia, negative IODs also mean that temperatures are likely to be cooler during the day and warmer at night. However, just because it is likely to rain for several months does not mean that it will. All negative IOD events are different.

The possibility of more rain can weaken some spirits, but there are reasons to be happy with this.

First of all, winter rainfall is usually suitable for farmers who grow crops such as grains. Record crop production..

In fact, negative IOD events are so important to Australia that they have been absent for a long time. because of A historic multi-year drought of the last century in southeastern Australia.

Negative IOD years can also bring better snow seasons for Australians. However, warming trends due to anthropogenic climate change mean that this signal is not as clear as it used to be.


The August-October rainfall outlook suggests that most of Australia is likely to experience above-median rainfall. Credits: Bureau of Meteorology, CC BY

That’s not all good news

This is the first official negative IOD event since 2016 and the year we saw one of the most powerful negative IOD events on record.It’s australia The second rainiest winter In part of the record and flood NSW, Victoria, And South Australia.

2016 events were also linked to catastrophic ones East African drought The other side of the Indian Ocean, and Heavy rain in Indonesia..

Thankfully, the current forecast shows that this time the negative IOD will be a bit milder, so I hope there are no catastrophic events.

Is there a negative IOD behind the recent rain?

It’s too early to tell, but probably not.

in the meantime Perth Experienced one of the rainiest Julys on record, and the southwestern part of Western Australia has historically Weak influence Due to negative IOD.

Negative IODs tend to be associated with moist airflow and even lower atmospheric pressure North and east From Geraldton to Port Hedland, rather than Perth.

Other than Australia, large-scale floods have occurred in China, Germany, Belgium and the Netherlands.

Although still in its infancy and needs further investigation, it seems that these events may be related to events in the Northern Hemisphere. Atmospheric jet stream, Not a negative IOD.

A jet stream is like a narrow river with strong winds in the high atmosphere, formed when cold and hot air meet. This change in jet stream can lead to extreme weather events.

Rainy Winter, Sticky Spring: What is a Negative Indian Ocean Dipole and Why Is It So Important?

Number of Indian Ocean Dipole events based on climate model (per 30 years). Credit: Abram et al. (2020)

How about climate change?

IOD, like El Nino and La Nino, is a natural climatic phenomenon that has occurred for thousands of years before humans began burning fossil fuels. But that doesn’t mean that climate change today isn’t affecting IOD.

Scientific research shows positive IODs associated with dry conditions in eastern Australia More general..And this is related to the effects of anthropogenic climate change Ocean temperature.

Climate models also suggest what we may experience More positive IOD event In the future, this will include increasing the likelihood of wildfires and droughts in Australia and reducing negative IOD events. This may mean that there is a lot of drought and less rain that “breaks the drought”, but the jury hasn’t come out yet.

Regarding the recent catastrophic floods abroad, scientists Still under evaluation The magnitude of the role that climate change has played.

But in any case, we do know one thing. Rising global temperatures due to climate change will cause more frequent and serious extreme events, such as short-term heavy rains associated with floods. heat wave..

In order to avoid future disasters, it is necessary to reduce emissions significantly and urgently.

Why drought-preventing rain depends on tropical waters

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Quote: Rainy winter, sticky spring: Negative Indian Ocean dipole (July 27, 2021) https: // Obtained from July 27, 2021

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