The Indian Ocean Dipole (IOD) is a pattern of changing ocean temperatures. During the positive phase, the western part of the Indian Ocean is warmer than the eastern part. During the negative phase, the western part is colder than the eastern part.
Phenomenon
The Indian Ocean Dipole (IOD) is a regular change in ocean surface temperatures between "positive," "neutral," and "negative" phases. During a positive phase, ocean temperatures are higher than usual in the western Indian Ocean, leading to more rain in that area. At the same time, the eastern Indian Ocean becomes cooler, which often causes droughts in nearby regions like Indonesia and Australia. In the negative phase, the opposite happens: the eastern Indian Ocean warms and gets more rain, while the western Indian Ocean cools and becomes drier.
The IOD also influences the strength of monsoon rains over the Indian subcontinent. A major positive IOD occurred from 1997 to 1998 and again in 2006. The IOD is part of Earth's larger climate patterns and interacts with similar events, such as El Niño-Southern Oscillation (ENSO), in the Pacific Ocean.
Scientists in India first discovered the IOD in 1999 through research at the Indian Institute of Science. On average, there are four positive and negative IOD events every 30 years, with each event lasting about six months. However, between 1980 and 2009, there were 12 positive IOD events, and no negative events occurred from 1980 to 1992. Consecutive positive IOD events are very rare, with only two recorded: one in 1913–1914 and three in 2006–2008. These events happened before the Black Saturday bushfires in 2009. Models suggest such events might occur twice every 1,000 years. In 2007, a strong positive IOD happened at the same time as La Niña, a rare event that has only been recorded once before, in 1967. A strong negative IOD in October 2010, combined with a strong La Niña, caused major floods in Queensland and Victoria in 2010–2011.
In 2008, Nerilie Abram used coral records from the eastern and western Indian Ocean to create a special index called the Dipole Mode Index, which tracks IOD events back to 1846. This research showed that positive IOD events became more frequent and stronger during the 20th century.
Effect on Southeast Asian and Australian droughts
A positive IOD is linked to droughts in Southeast Asia and Australia. Very strong positive IOD events are likely to occur.
A 2009 study by Ummenhofer et al. at the University of New South Wales (UNSW) Climate Change Research Centre found a strong connection between the IOD and droughts in the southern part of Australia, especially the southeast. Every major drought in southern Australia since 1889 has happened during periods of positive or neutral IOD conditions, including the droughts from 1895–1902, 1937–1945, and 1995–2009.
The research explains that during a negative IOD phase, cool water in the western Indian Ocean and warm water near northwest Australia (Timor Sea) create winds that carry moisture from the ocean toward southern Australia, increasing rainfall. During a positive IOD phase, the ocean temperature pattern is opposite, which weakens the winds and reduces the amount of moisture carried across Australia. This results in below-average rainfall in the southeast during positive IOD periods.
The study also shows that the IOD has a greater impact on rainfall patterns in southeast Australia than the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean, as shown in recent studies.
Effect on rainfall across East Africa
A positive IOD is connected to more rain than usual during the East African Short Rains (EASR) from October to December. This increased rainfall happens because the western Indian Ocean has warmer ocean temperatures, and winds blow from west to east across the equator, bringing moisture to East Africa.
More rainfall during the EASR linked to a strong positive IOD can cause flooding in East Africa. In late 2019, rainfall in the region was 300% higher than normal. This led to widespread flooding in Djibouti, Ethiopia, Kenya, Uganda, Tanzania, Somalia, and South Sudan. Heavy rain and landslides during this time often cause damage and loss of life across the area.
Scientists predict that climate change will cause the western Indian Ocean to warm faster, increasing the frequency of positive IOD events. This is expected to lead to stronger rainfall during the short rain season in East Africa.
Effect on El Niño
A 2018 study by Hameed et al. at the University of Aizu used computer models to examine how a positive Indian Ocean Dipole (IOD) event affects wind patterns and sea surface temperatures (SST) in the Pacific Ocean. The researchers found that wind changes caused by the IOD can create sea surface temperature patterns similar to those seen during El Niño events. They also discovered that the IOD's influence on SST is strongest in the far-eastern Pacific region. Additionally, the study showed that the interaction between the IOD and El Niño-Southern Oscillation (ENSO) is important for creating Super El Niños.
2020 IOD positive cycle
A positive IOD cycle is connected to several cyclones that caused severe damage in East Africa in 2019, resulting in thousands of deaths. The 2018–2019 South-West Indian Ocean cyclone season was more active than usual, partly because ocean waters were warmer than normal. This contributed to the formation of cyclones like Cyclone Idai and other storms during that time. The positive IOD cycle also caused droughts and bushfires in Australia, as it brings dry air toward the region. In 2020, the same cycle led to heavy flooding in Jakarta by trapping moist air in the tropics instead of allowing it to move south. Additionally, the positive IOD cycle played a role in the 2019–2021 locust outbreak in East Africa.