A wet winter, a soggy spring: what is the negative Indian Ocean Dipole, and why is it so important?


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Nicky Wright, University of Sydney; Andréa S. Taschetto, UNSW, and Andrew King, The University of MelbourneThis month we’ve seen some crazy, devastating weather. Perth recorded its wettest July in decades, with 18 straight days of relentless rain. Overseas, parts of Europe and China have endured extensive flooding, with hundreds of lives lost and hundreds of thousands of people evacuated.

And last week, Australia’s Bureau of Meteorology officially declared there is a negative Indian Ocean Dipole — the first negative event in five years — known for bringing wet weather.

But what even is the Indian Ocean Dipole, and does it matter? Is it to blame for these events?

What is the Indian Ocean Dipole?

The Indian Ocean Dipole, or IOD, is a natural climate phenomenon that influences rainfall patterns around the Indian Ocean, including Australia. It’s brought about by the interactions between the currents along the sea surface and atmospheric circulation.

It can be thought of as the Indian Ocean’s cousin of the better known El Niño and La Niña in the Pacific. Essentially, for most of Australia, El Niño brings dry weather, while La Niña brings wet weather. The IOD has the same impact through its positive and negative phases, respectively.

Positive IODs are associated with an increased chance for dry weather in southern and southeast Australia. The devastating Black Summer bushfires in 2019–20 were linked to an extreme positive IOD, as well as human-caused climate change which exacerbated these conditions.

Negative IODs tend to be less frequent and not as strong as positive IOD events, but can still bring severe climate conditions, such as heavy rainfall and flooding, to parts of Australia.

The Indian Ocean Dipole (IOD) index, used to track the variability of the Indian Ocean Dipole. An event occurs after the index crosses the threshold for 8 weeks.
Bureau of Meteorology

The IOD is determined by the differences in sea surface temperature on either side of the Indian Ocean.

During a negative phase, waters in the eastern Indian Ocean (near Indonesia) are warmer than normal, and the western Indian Ocean (near Africa) are cooler than normal.




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This causes more moisture-filled air to flow towards Australia, favouring wind pattern changes in a way that promotes more rainfall to southern parts of Australia. This includes parts of Western Australia, South Australia, Victoria, NSW and the ACT.

Generally, IOD events start in late autumn or winter, and can last until the end of spring — abruptly ending with the onset of the northern Australian monsoon.

The negative phase of the Indian Ocean Dipole.
Bureau of Meteorology

Why should we care?

We probably have a wet few months ahead of us.

The negative IOD means the southern regions of Australia are likely to have a wet winter and spring. Indeed, the seasonal outlook indicates above average rainfall for much of the country in the next three months.

In southern Australia, a negative IOD also means we’re more likely to get cooler daytime temperatures and warmer nights. But just because we’re more likely to have a wetter few months doesn’t mean we necessarily will — every negative IOD event is different.

Rainfall outlooks for August–October suggest that large parts of Australia will likely experience above-median rainfall.
Bureau of Meteorology, CC BY

While the prospect of even more rain might dampen some spirits, there are reasons to be happy about this.

First of all, winter rainfall is typically good for farmers growing crops such as grain, and previous negative IOD years have come with record-breaking crop production.

In fact, negative IOD events are so important for Australia that their absence for prolonged periods has been blamed for historical multi-year droughts in the past century over southeast Australia.

Negative IOD years can also bring better snow seasons for Australians. However, the warming trend from human-caused climate change means this signal isn’t as clear as it was in the past.

A negative IOD may mean a better snow season in the High Country.
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It’s not all good news

This is the first official negative IOD event since 2016, a year that saw one of the strongest negative IOD events on record. It resulted in Australia’s second wettest winter on record and flooding in parts of NSW, Victoria, and South Australia.

The 2016 event was also linked to devastating drought in East Africa on the other side of the Indian Ocean, and heavy rainfall in Indonesia.

Thankfully, current forecasts indicate the negative IOD will be a little milder this time, so we hopefully won’t see any devastating events.

The number of Indian Ocean Dipole events (per 30 years) based on climate models.
Modified from Abram et al. (2020)

Is the negative IOD behind the recent wet weather?

It’s too early to tell, but most likely not.

While Perth is experiencing one of its wettest Julys on record, the southwest WA region has historically been weakly influenced by negative IODs.

Negative IODs tend to be associated with moist air flow and lower atmospheric pressure further north and east than Perth, such as Geraldton to Port Hedland.

Outside of Australia, there has been extensive flooding in China and across Germany, Belgium, and The Netherlands.

It’s still early days and more research is needed, but these events look like they might be linked to the Northern Hemisphere’s atmospheric jet stream, rather than the negative IOD.

The jet stream is like a narrow river of strong winds high up in the atmosphere, formed when cool and hot air meet. Changes in this jet stream can lead to extreme weather.

What about climate change?

The IOD — as well as El Niño and La Niña — are natural climate phenomena, and have been occurring for thousands of years, before humans started burning fossil fuels. But that doesn’t mean climate change today isn’t having an effect on the IOD.




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Scientific research is showing positive IODs — linked to drier conditions in eastern Australia — have become more common. And this is linked to human-caused climate change influencing ocean temperatures.

Climate models also suggest we may experience more positive IOD events in future, including increased chances of bushfires and drought in Australia, and fewer negative IOD events. This may mean we experience more droughts and less “drought-breaking” rains, but the jury’s still out.

When it comes to the recent, devastating floods overseas, scientists are still assessing how much of a role climate change played.

But in any case, we do know one thing for sure: rising global temperatures from climate change will cause more frequent and severe extreme events, including the short-duration heavy rainfalls associated with flooding, and heatwaves.

To avoid worse disasters in our future, we need to cut emissions drastically and urgently.




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The Conversation


Nicky Wright, Research Fellow, University of Sydney; Andréa S. Taschetto, Associate Professor, UNSW, and Andrew King, ARC DECRA fellow, The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

A rare natural phenomenon brings severe drought to Australia. Climate change is making it more common



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Nicky Wright, Australian National University; Bethany Ellis, Australian National University, and Nerilie Abram, Australian National University

Weather-wise, 2019 was a crazy way to end a decade. Fires spread through much of southeast Australia, fuelled by dry vegetation from the ongoing drought and fanned by hot, windy fire weather.

On the other side of the Indian Ocean, torrential rainfall and flooding devastated parts of eastern Africa. Communities there now face a locust plague and food shortages.

These intense events can partly be blamed on the extreme positive Indian Ocean Dipole, a climate phenomenon that unfolded in the second half of 2019.




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Why drought-busting rain depends on the tropical oceans


The Indian Ocean Dipole refers to the difference in sea surface temperature on either side of the Indian Ocean, which alters rainfall patterns in Australia and other nations in the region. The dipole is a lesser-known relative of the Pacific Ocean’s El Niño.

Climate drivers, such as the Indian Ocean Dipole, are an entirely natural phenomenon, but climate change is modifying the behaviour of these climate modes.

In research published today in Nature, we reconstructed Indian Ocean Dipole variability over the last millennium. We found “extreme positive” Indian Ocean Dipole events like last year’s are historically very rare, but becoming more common due to human-caused climate change. This is big news for a planet already struggling to contain global warming.

So what does this new side-effect of climate change mean for the future?

The Indian Ocean brings drought and flooding rain

First, let’s explore what a “positive” and “negative” Indian Ocean Dipole means.

During a “positive” Indian Ocean Dipole event, waters in the eastern Indian Ocean become cooler than normal, while waters in the western Indian Ocean become warmer than normal.

Warmer water causes rising warm, moist air, bringing intense rainfall and flooding to east Africa. At the same time, atmospheric moisture is reduced over the cool waters of the eastern Indian Ocean. This turns off one of Australia’s important rainfall sources.




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In fact, over the past century, positive Indian Ocean Dipoles have led to the worst droughts and bushfires in southeast Australia.

The Indian Ocean Dipole also has a negative phase, which is important to bring drought-breaking rain to Australia. But the positive phase is much stronger and has more intense climate impacts.

We’ve experienced extreme positive Indian Ocean Dipole events before. Reliable instrumental records of the phenomenon began in 1958, and since then a string of very strong positive Indian Ocean Dipoles have occurred in 1961, 1994, 1997 and now 2019.

The Dipole Mode Index is used to track variability of the Indian Ocean Dipole.
Author provided

But this instrumental record is very short, and it’s tainted by the external influence of climate change.

This means it’s impossible to tell from instrumental records alone how extreme Indian Ocean Dipoles can be, and whether human-caused climate change is influencing the phenomenon.

Diving into the past with corals

To uncover just how the Indian Ocean Dipole has changed, we looked back through the last millennium using natural records: “cores” taken from nine coral skeletons (one modern, eight fossilised).

These coral samples were collected just off of Sumatra, Indonesia, so they’re perfectly located for us to reconstruct the distinct ocean cooling that characterises positive Indian Ocean Dipole events.

Scientists drilling into corals to study past climate. Corals are like trees, and grow a band for every year they live.
Jason Turl, Author provided

Corals grow a lot like trees. For every year they live they produce a growth band, and individual corals can live for more than 100 years. Measuring the oxygen in these growth bands gives us a detailed history of the water temperature the coral grew in, and the amount of rainfall over the reef.

In other words, the signature of extreme events like past positive Indian Ocean Dipoles is written in the coral skeleton.

Altogether, our coral-based reconstruction of the Indian Ocean Dipole spans 500 years between 1240 and 2019. There are gaps in the timeline, but we have the best picture so far of how exactly the Indian Ocean Dipole has varied in the past.

How unusual was the 2019 Indian Ocean Dipole event?

Extreme events like the 2019 Indian Ocean Dipole have historically been very rare.

We found only ten extreme positive Indian Ocean Dipole events in the entire record. Four occurred in the past 60 years, but only six occurred in the remaining 440 years before then. This adds more weight to evidence that positive Indian Ocean Dipole events have been occurring more often in recent decades, and becoming more intense.




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But another finding from the reconstruction surprised – and worried – us. Events like 2019 aren’t the worst of what the Indian Ocean Dipole can throw at us.

Of the extreme events we found in our reconstruction, one of them, in 1675, was much stronger than anything we’ve seen in observations from the last 60 years.

The 1675 event was around 30–40% stronger than what we saw in 1997 (around the same magnitude as 2019). Historical accounts from Asia show this event was disastrous, and the severe drought it caused led to crop failures, widespread famine and mortality, and incited war.

The wiggles that make up 500 years of reconstructed Indian Ocean Dipole variability. The red triangles show when extreme positive events occurred.
Author provided

As far as we can tell, this event shows just how extreme Indian Ocean Dipole variability can be, even without any additional prompting from external forces like human-caused climate change.

Why should we care?

Indian Ocean Dipole variability will continue to episodically bring extreme climate conditions to our region.

Drilling through fossilised coral layers to look into the past.
Nerilie Abram, Author provided

But previous studies, as well as ours, have shown human-caused climate change has shortened the gaps between these episodes, and this trend will continue. This is because climate change is causing the western side of the Indian Ocean to warm faster than in the east, making it easier for positive Indian Ocean Dipole events to establish.

In other words, drought-causing positive Indian Ocean Dipole events will become more frequent as our climate continues to warm.

In fact, climate model projections indicate extreme positive Indian Ocean Dipole events will occur three times more often this century than last, if high greenhouse gas emissions continue.




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This means events like last year will almost certainly unfold again soon, and we’re upping the odds of even worse events that, through the fossil coral data, we now know are possible.

Knowing we haven’t yet seen the worst of the Indian Ocean Dipole is important in planning for future climate risks. Future extremes from the Indian Ocean will act on top of long-term warming, giving a double-whammy effect to their impacts in Australia, like the record-breaking heat and drought of 2019.

But perhaps most importantly, rapidly cutting greenhouse gas emissions will limit how often positive Indian Ocean Dipole events occur in future.The Conversation

Nicky Wright, Research Fellow, Australian National University; Bethany Ellis, PhD Candidate, Australian National University, and Nerilie Abram, Professor; ARC Future Fellow; Chief Investigator for the ARC Centre of Excellence for Climate Extremes, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.