Over the past year, Australia battled its largest bushfires on record, while parts of East Africa and the Middle East fought off locust swarms the size of entire cities.
While the factors behind each disaster are complex, they are both linked to a climate system known as the “Indian Ocean dipole”.
When the Indian Ocean dipole is in its positive phase, it can cause drought-like conditions in Australia, which raises the risk of fire. At the same time, it can cause more rainfall and storminess in the Arab Peninsula, which has been linked to deadly flooding and the rapid spread of locusts. In 2019, the dipole reached its most extreme positive level in 40 years.
A new study published in Nature Climate Change found that severe positive dipole events like the one seen in 2019 are likely to become more frequent as the world heats up.
“While the total number of positive Indian Ocean dipole events may not change much, the frequency of strong positive Indian Ocean dipoles is expected to increase, while the frequency of moderate events is expected to decrease under greenhouse warming,” Dr Wenju Cai, study lead author and a chief research scientist at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia, told The Independent.
The research uses a range of climate models to explore how future greenhouse gas emissions are likely to impact the frequency of positive Indian Ocean dipole events.
It makes projections for two possible future scenarios. In the first, future greenhouse gas emissions are moderate until 2100, (this scenario is called “RCP4.5”). In the second, future greenhouse gas emissions are extremely high (this scenario is called “RCP8.5”).
The results for both scenarios suggest that the overall number of positive Indian Ocean Dipoles may not change much in the coming decades. However, the number of strongly positive dipole events is expected to rise.
“Extreme impacts as seen in 2019 are likely to occur more often,” said Dr Cai.
The severe positive dipole event in 2019 affected weather significantly on both sides of the Indian Ocean.
The dipole has three phases: neutral, positive and negative. In a neutral dipole phase, waters around Australia and Papua New Guinea are warm, which causes air to rise up and fall as rain. Winds blow in a westerly direction. In this phase, the dipole does not have a strong effect on the weather in countries surrounding the Indian Ocean.
During a positive phase, however, the pattern reverses. Westerly winds weaken and, sometimes, easterly winds form — dragging warm water away from Australia and towards the Arab Peninsula and the Horn of Africa. The diagram below shows the impacts of this.
When rainfall moves away from Australia, it leaves the climate dry and raises the risk of fire.
From 2019-20, Australia faced its largest bushfires on record. The fires spread across 17m hectares in regions including New South Wales, Queensland and Victoria. The flames killed more than 30 people and destroyed more than 3,000 homes, official statistics show.
In addition to the severe positive Indian Ocean dipole event of 2019, other factors also played an important role in the historic fires. For example, a quick-fire study found that the fires were made 30 per cent more likely by human-caused climate change.
Around the same time that Australia was experiencing drought, East Africa was experiencing heavy rainfall. Across the Horn of Africa, rainfall was 300 per cent above average from October to November 2019.
As well as contributing to deadly floods, the deluge has been linked to the spread of locust swarms from the Arab Peninsula into East Africa and its surrounding regions from 2019 to 2020.
This is because the heavy rainfall caused lush vegetation to grow in the Arab Peninsula, which gave locusts a place to gather, eat and then multiply, scientists say. (However, there are other reasons why locusts were able to spread, such as a lack of early detection and removal.)
The climate crisis could lead to more frequent severe positive dipole events because it is causing sea surface temperatures to heat up at a faster rate on the African side of the Indian Ocean than on the Australian side, according to the new research.
This means that conditions needed for a severe positive dipole event to occur are happening more regularly, said Dr Cai.