The Indian Ocean Dipole (IOD) is an atmosphere-ocean coupled weather cycle, defined by a difference in sea-surface temperatures across the tropical Indian Ocean. It was first identified by climate scientists in 1999. 1 2
The IOD is one of several naturally occurring weather cycles that play an important role in Earth’s climate system around the equator and in the subtropical southern hemisphere. The others include the El Niño-Southern Oscillation (ENSO) in the tropical Pacific, the travelling Madden-Julian Oscillation in the Indo-Pacific, and the Southern Annular Mode, or Antarctic oscillation in the Southern Ocean.
NOTE: In climate science, a ‘dipole’ is a weather cycle with two main opposing centers of action.
The meteorological impacts of the IOD – notably flooding and drought – are especially pronounced in East Africa, Southeast Asia and Australia. But its influence is felt on the climate of all countries on the rim of the Indian Ocean, including India, Pakistan, Bangladesh and Myanmar, as well as countries of north eastern Asia located under the pathways of Rossby waves emanating from the tropical Indian Ocean. 3
The Indian Ocean Dipole is not thought to be directly linked to climate change, although some climatologists claim that the positive IOD event that contributed to the Australian bushfires in 2019-20 is likely to occur three times more often than it has in the past.
So, as with ENSO, while much uncertainty remains over why the Indian Ocean Dipole changes as it does, it seems that unless we curb our use of fossil fuels, the effects of IOD, ENSO and the Southern Annular Mode are likely to intensify around the world.
- How Does the Indian Ocean Dipole Work?
- IOD in Neutral Phase
- IOD in Negative Phase
- IOD in Positive Phase
- How Long Does an IOD Episode Last?
- How Often is the Indian Ocean Dipole in Neutral Mode?
- How Do Scientists Distinguish the Different Phases of the Indian Ocean Dipole?
- What Causes the Indian Ocean Dipole?
- How Does the Indian Ocean Dipole Affect Global Weather?
- Is the Indian Ocean Dipole Affected by Global Warming?
How Does the Indian Ocean Dipole Work?
Like its neighbor, ENSO, the Indian Ocean Dipole oscillates between three basic phases – negative, neutral, and positive – depending on sea temperature. Beginning around May or June, the IOD peaks between August and October and then rapidly declines with the arrival of the southern hemisphere monsoon.
The IOD is all about sea surface temperatures (SSTs) in different parts of the Indian Ocean. It’s the warm surface water at one end of the ocean (compared to cooler water at the other end) that drives the whole IOD weather cycle.
IOD in Neutral Phase
When the Indian Ocean Dipole is in neutral mode, warm water from the Pacific flows between the islands of Indonesia under the influence of the Pacific’s prevailing easterly winds. This warms the eastern surface waters of the Indian Ocean, causing normal rainfall in the area as the ocean water evaporates into the atmosphere. As this water vapor rises, cooler air from the central and western Indian Ocean moves in to replace it, resulting in a circular flow of air, as shown below.
As well as normal rainfall, a neutral IOD also leads to normal temperatures, and so there is little change to climate conditions across the tropical Indian Ocean.
IOD in Negative Phase
When the Indian Ocean Dipole is in negative mode, sea surface temperatures in the eastern zone are warmer than average, while in the western zone SSTs are cooler than average. This leads to stronger currents of warm air (convection currents) rising from the surface of the sea which causes westerly surface winds to intensify along the equator. This stronger circular air flow reinforces the difference between sea surface temperatures in the eastern and western zones of the Indian Ocean.
A negative IOD phase typically results in higher-than-average rainfall over SE Asia and the southern two thirds of Australia. Conversely, countries in East Africa usually experience drier and hotter conditions than normal with an increased risk of drought across the area.
EXPLAINER: What is Blue Carbon?
IOD in Positive Phase
When the Indian Ocean Dipole is in positive mode, it means that SSTs in the eastern zone are cooler than those in the western part of the ocean. As a result, the usual westerly winds weaken and go into reverse, as they seek to replace the warm air rising from the western end of the Indian Ocean. A new circular air flow is established with cooler air descending in the eastern zone and turning west along the equator.
This new easterly wind causes more warm water to shift towards Africa, and at the same time allows cold nutrient-rich water to upwell from the deep ocean in the east. Both of these things reinforce the temperature difference across the tropical Indian Ocean, with cooler-than-normal water in the east and warmer water in the west.
A positive IOP phase typically results in abnormally high rainfall across East Africa with a risk of floods, and abnormally dry conditions over SE Asia and Australia, with an increased risk of wildfires.
A positive IOD phase is signalled by cooler-than-normal water in the tropical eastern Indian Ocean and warmer than average water in the west. Conversely, a negative IOD phase is indicated by warmer-than-normal water in the tropical eastern Indian Ocean and cooler than average water in the west.
How Long Does an IOD Episode Last?
On average, positive and negative phases of the Indian Ocean Dipole occur about once every three to five years. Typically, they begin in autumn or winter (of the southern hemisphere), and return to neutral in November and December when the Australian summer monsoon starts.
How Often is the Indian Ocean Dipole in Neutral Mode?
Since 1960, when reliable records of the IOD began, to 2016 there have been 37 neutral years, with 11 negative and 12 positive IOD events. 4
Positive IOD Years
1961, 1963, 1972, 1982, 1983, 1994, 1997, 2006, 2012, 2015, 2018, 2019.
Negative IOD Years
1960, 1964, 1974, 1981, 1989, 1992, 1996, 1998, 2010, 2014, 2016.
Neutral IOD Years
1962, 1965-71, 1973, 1975-1980, 1984-1988, 1990, 1991, 1993, 1995, 1999-2005, 2007-9, 2011, 2013, 2017.
EXPLAINER: Ocean Acidification from CO2 Overdose.
How Do Scientists Distinguish the Different Phases of the Indian Ocean Dipole?
Oceanographers usually measure the Indian Ocean Dipole using an index, known as the Dipole Mode Index, or DMI. The strength of the Indian Ocean Dipole is reflected by the difference in sea surface temperature between the western equatorial Indian Ocean (50°E to 70°E and 10°S to 10°N) and the south eastern equatorial Indian Ocean (90°E to 110°E and 10°S to 0°S).
When assessing the IOD, researchers consider sustained recordings above plus 0.4 °C as typical of a positive IOD, and values below minus 0.4 °C as typical of a negative IOD.
What Causes the Indian Ocean Dipole?
We know that sea surface temperature, wind and atmospheric pressure are the three main meteorological elements that determine whether IOD enters a neutral, positive or negative phase. However, what causes these elements to shift from neutral to negative, or from neutral to positive, or from positive to negative remains a matter of debate. The causes of the Indian Ocean Dipole, like those of other regional weather systems, are unknown.
How Does the Indian Ocean Dipole Affect Global Weather?
The year 2019 was a perfect illustration of how the Indian Ocean Dipole affects global weather during a positive phase. In fact, it was the strongest positive phase for six decades. A positive IOD brings cooler-than-normal sea surface conditions west of Indonesia, and warmer-than-normal conditions in the western Indian Ocean. A negative IOD brings the opposite pattern.
2019: A Perfect Example of a Positive IOD
In 2019, as we know, the positive IOD resulted in extremely dry conditions across Indonesia, as well as record-breaking wildfires across eastern and southeastern Australia.
At the same time, in East Africa, the IOD led to a series of destructive cyclones beginning with Cyclone Idai. In addition, heavy rains devastated parts of the continent, with the Horn of Africa experiencing three times the normal rainfall. Ethiopia, Kenya, Uganda, Rwanda, Burundi, Tanzania, Somalia and South Sudan were all hard hit, with flash floods and mudslides uprooting communities across the region.
The UN Office for the Coordination of Humanitarian Affairs (OCHA) reported that as many as 2.8 million people were affected, with 300 deaths.
2016: A Perfect Example of a Negative IOD
In contrast, the year 2016, was a perfect example of how the Indian Ocean Dipole influences global weather during a negative phase. This strong IOD led to a major East African drought, with some regions reporting less than half the normal rainfall. According to the Inter-Agency Working Group (IAWG), more than 15 million people in Somalia, Ethiopia and Kenya were hit by devastating drought, lack of food and drinkable water. 5
At the other end of the Indian Ocean, Australia recorded over twice the average spring rainfall over the Northern Territory, inland NSW and Queensland, northern and western Victoria and inland South Australia. The result was significant flooding in parts of western Queensland, central NSW, western Victoria, and areas around Adelaide in South Australia.
The IOD also affects the weather in India. For example, a positive IOD with cooler sea temperatures in the south-eastern equatorial Indian Ocean causes the formation of more tropical cyclones in the Arabian Sea off India’s west coast, and also higher-than-average rainfall over central India. Conversely, a negative IOD, leads to drier conditions in the center of the country combined with greater cyclogenesis in the Bay of Bengal, off India’s east coast.
EXPLAINER: Effects of Global Warming on the Oceans.
Is the Indian Ocean Dipole Affected by Global Warming?
Sea surface temperatures (SSTs) have been rising in the Indian Ocean for decades due to the effect of heat-trapping greenhouse gases in the atmosphere. However, some studies show that a significant inter-hemispheric gradient in Indian Ocean SST trends developed around 2000, not because of greenhouse gas emissions but because of an increase in Indonesian Throughflow (ITF) transport from the Pacific into the Indian Ocean, caused by stronger Pacific trade winds. 6
Fast forward to the extreme positive phase of the Indian Ocean Dipole which occurred in 2019. This unusually intense IOD event – in concert with an El Niño phase in the Pacific – is believed to have contributed to the catastrophic Australian bushfires of 2019-2020.
According to one study, the intensity of the IOD phase was triggered by a major inter-hemispheric sea level pressure gradient (IHPG) between low pressure over the South China Sea and the Philippine Sea (in the north) versus high pressure over Australia (in the south). 7
However, at present, scientists have not yet identified a clear link or correlation between global warming and the Indian Ocean Dipole – nor, for that matter between global warming and the El Niño-Southern Oscillation. What is clear, is that both IOD and ENSO are now operating in a much warmer world. Extreme weather events are becoming the new norm.
Ocean warming is becoming more intense and is spreading to deeper parts of the open sea. Marine heatwaves are now more common, and their effects on the mass bleaching of coral reefs is well-documented. The atmosphere is also warming strongly and is leading to more intense heatwaves on land.
It is well-documented that elevated temperatures in the circumpolar region have resulted in tinder-dry forests which were a pre-condition for the extraordinary Arctic fires of 2019 and the unprecedented US West Coast fires of 2020. The same can surely be said of the unprecedented Australian bushfires of 2019-2020, especially since the drought-like conditions across Australia began at least as far back as 2017, when the Indian Ocean Dipole was in neutral mode.
It’s worth noting, that when a positive IOD coincides with an El Niño, the two climate drivers reinforce each other, boosting their drying impacts over Australia and Indonesia.
In the same way, if a negative IOD coincides with a La Niña event, it boosts the likelihood and intensity of above-average rainfall in the region. 8 These mutually supportive events demonstrate that extreme weather usually has more than one cause.
So the answer to the question, does climate change affect the Indian Ocean Dipole? is almost certainly Yes. 9 But not in a linear, direct way. So, if temperatures rise by X amount, it doesn’t mean that the IOD intensifies by the same amount, or anything like that.
What is probably happening, is that global warming is setting the stage. It creates the wet or dry conditions which are then acted upon by the IOD and ENSO. As a result, the meteorological effects become more severe. Thus, for example, scientists expect that the Western Indian ocean will warm at accelerated rates because of climate change leading to more frequent positive IODs.
In the end, whether or not the IOD is affected by global warming, it seems obvious that climate action is urgently needed to decarbonize our energy system and dial down the warming that is aggravating the IOD and other weather cycles in the tropics.
DON’T MISS: What is the Root Cause of Climate Change?
- “A dipole mode in the tropical Indian Ocean.” Saji, N. H., B. N. Goswami, P. N. Vinayachandran, T. Yamagata, 1999: Nature, 401, 360-363.
- “Coupled ocean-atmosphere dynamics in the Indian Ocean during 1997-98.” Webster, P. J., A. M. Moore, J. P. Loschnigg, R. R. Leben, 1999. Nature, 401, 356-360.
- “Causes and Predictability of the Negative Indian Ocean Dipole and Its Impact on La Niña During 2016.” Lim, EP., Hendon, H.H. Sci Rep 7, 12619 (2017).
- “Indian Ocean influences on Australian climate.” Bureau of Meteorology.
- “An extreme negative Indian Ocean Dipole event in 2016: dynamics and predictability.” Lu, B., Ren, HL., Scaife, A.A. et al. Clim Dyn 51, 89–100 (2018).
- “Interhemispheric SST Gradient Trends in the Indian Ocean prior to and during the Recent Global Warming Hiatus.” Lu Dong; Michael J. McPhaden. J. Climate (2016) 29 (24): 9077–9095.
- “What Caused the Extreme Indian Ocean Dipole Event in 2019?” Lu, Bo; Ren, Hong-Li. Geophysical Research Letters, Volume 47, Issue 11. June 2020.
- For more on this convergence, see: “Two-year consecutive concurrences of positive Indian Ocean Dipole and Central Pacific El Niño preconditioned the 2019/2020 Australian “black summer” bushfires.” Wang, G., Cai, W. Geosci. Lett. 7, 19 (2020).
- “Indian Ocean Dipole linked to global warming in new research by Australian scientists.”