Indian Ocean Dipole explained and outlook
By
Jason Nicholls, AccuWeather senior meteorologist
Published May 15, 2020 3:54 PM EDT
The Indian Ocean Dipole, or IOD, is an irregular oscillation of sea surface temperatures across the tropical Indian Ocean that tends to peak in boreal autumn. The IOD is characterized by three phases: neutral, positive and negative. A positive IOD features higher-than-normal water temperatures over the western tropical Indian Ocean, off Somalia, and lower-than-normal waters in the eastern tropical Indian Ocean, off Indonesia. A negative IOD features lower-than-normal water temperatures over the tropical western Indian Ocean, off Somalia, and warmer-than-normal waters in the eastern tropical Indian Ocean, off Indonesia.
Sea surface temperature anomalies during Indian Ocean Dipoles. Arrows indicate wind direction, white patches are areas with more clouds and rain. Marchant et al 2007, Author provided
Courtesy of Australia BOM
The above graphics illustrate the positive and negative phases of the Indian Ocean Dipole and some of their impacts. The impacts of the IOD are most significant for areas around the Indian Ocean, namely Africa, Australia and Asia. However, the IOD can influence typhoon tracks in the western Pacific and ENSO. ENSO can also influence the IOD such as a +IOD tends to coincide with El Nino and a -IOD tends to coincide with La Nina. This relationship does not always occur because there have been examples of a -IOD with El Nino and vice versa.
The IOD can influence ENSO as several papers have shown a teleconnection between the phase of the IOD and ENSO 14 months later. For example, a strong +IOD tends to favor the development of La Nina conditions in the Pacific 14 months later. This teleconnection worked following the two previously strong +IOD events in 1994 and 1997 where moderate to strong La Ninas occurred in the boreal winters of 1995/96 and 1998/99 respectively. A strong -IOD tends to favor the development of an El Nino 14 months past the peak of the IOD. The strongest +IOD on record occurred in 2019, and we are expecting La Nina conditions to develop in the Pacific heading into boreal winter.
Indian Ocean Dipole Mode Index DMI since 1982
The record strong +IOD in 2019 led to multiple cyclones ravaging East Africa and killing thousands aided by warmer-than-normal waters offshore. The strong +IOD was also a big factor in the Australian drought and wildfires during austral spring into early austral summer in 2019. So what is in store for the Indian Ocean Dipole this year?
To answer that question, we will first look at an important teleconnection which is the subtropical Indian Ocean Dipole. The subtropical Indian Ocean Dipole is the difference in water temperatures across the southern Indian Ocean between an area southeast of Madagascar and an area off western Australia. The SIOD typically peaks in austral summer, and this past austral summer, the SIOD was at its most strongly negative value since the austral summer/boreal winter of 1997/98.
South Indian Ocean dipole mode index
Research papers found a relationship between the SIOD and the tropical IOD where a -SIOD in austral summer tends to be followed by a -IOD in boreal autumn or austral spring. A +SIOD tends to be followed by a +IOD. Looking at a composite of water temperature anomalies during boreal autumn following a -SIOD in austral summer/boreal winter, we can see there is a pretty good teleconnection between the two dipoles.
The lower-level wind flow in the years following -SIOD austral summers also look like what is typical during a -IOD in boreal autumn. A noticeable strengthening of easterly flow occurs in the boreal autumn following a -SIOD which is typical with a -IOD.
As a result of this teleconnection and other signals, we do expect a moderate -IOD to develop across the tropical Indian Ocean later this boreal summer into autumn. Many climate models support the idea of a -IOD developing heading into boreal autumn.
Climate model summary from the Australia BOM
Sea surface temperature anomalies from the C3 multi-system seasonal forecast
What does a -IOD in 2020 mean? Across Australia, a -IOD should lead to a wetter austral spring into summer than what we had in 2019 which will mean a reduced chance of severe drought and a less active wildfire season than what we saw in 2019. The development of a -IOD will also mean a good chance of above-normal rainfall across Malaysia and Indonesia unlike the drought conditions of last year.
Depending on the timing of the -IOD, there could be a drier end to the southwest monsoon across India. A -IOD would also suggest a drier-than-normal northeast monsoon across southern India which typically runs from October to December. We should also see above-normal rainfall across southeastern China into Japan. A -IOD will also shift the main development area in the western Pacific westward heading into boreal autumn which may lead to more impacts on China, Taiwan and Vietnam late in the season.
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Weather Blogs / Global weather
Indian Ocean Dipole explained and outlook
By Jason Nicholls, AccuWeather senior meteorologist
Published May 15, 2020 3:54 PM EDT
The Indian Ocean Dipole, or IOD, is an irregular oscillation of sea surface temperatures across the tropical Indian Ocean that tends to peak in boreal autumn. The IOD is characterized by three phases: neutral, positive and negative. A positive IOD features higher-than-normal water temperatures over the western tropical Indian Ocean, off Somalia, and lower-than-normal waters in the eastern tropical Indian Ocean, off Indonesia. A negative IOD features lower-than-normal water temperatures over the tropical western Indian Ocean, off Somalia, and warmer-than-normal waters in the eastern tropical Indian Ocean, off Indonesia.
Sea surface temperature anomalies during Indian Ocean Dipoles. Arrows indicate wind direction, white patches are areas with more clouds and rain. Marchant et al 2007, Author provided
Courtesy of Australia BOM
The above graphics illustrate the positive and negative phases of the Indian Ocean Dipole and some of their impacts. The impacts of the IOD are most significant for areas around the Indian Ocean, namely Africa, Australia and Asia. However, the IOD can influence typhoon tracks in the western Pacific and ENSO. ENSO can also influence the IOD such as a +IOD tends to coincide with El Nino and a -IOD tends to coincide with La Nina. This relationship does not always occur because there have been examples of a -IOD with El Nino and vice versa.
The IOD can influence ENSO as several papers have shown a teleconnection between the phase of the IOD and ENSO 14 months later. For example, a strong +IOD tends to favor the development of La Nina conditions in the Pacific 14 months later. This teleconnection worked following the two previously strong +IOD events in 1994 and 1997 where moderate to strong La Ninas occurred in the boreal winters of 1995/96 and 1998/99 respectively. A strong -IOD tends to favor the development of an El Nino 14 months past the peak of the IOD. The strongest +IOD on record occurred in 2019, and we are expecting La Nina conditions to develop in the Pacific heading into boreal winter.
Indian Ocean Dipole Mode Index DMI since 1982
The record strong +IOD in 2019 led to multiple cyclones ravaging East Africa and killing thousands aided by warmer-than-normal waters offshore. The strong +IOD was also a big factor in the Australian drought and wildfires during austral spring into early austral summer in 2019. So what is in store for the Indian Ocean Dipole this year?
To answer that question, we will first look at an important teleconnection which is the subtropical Indian Ocean Dipole. The subtropical Indian Ocean Dipole is the difference in water temperatures across the southern Indian Ocean between an area southeast of Madagascar and an area off western Australia. The SIOD typically peaks in austral summer, and this past austral summer, the SIOD was at its most strongly negative value since the austral summer/boreal winter of 1997/98.
South Indian Ocean dipole mode index
Research papers found a relationship between the SIOD and the tropical IOD where a -SIOD in austral summer tends to be followed by a -IOD in boreal autumn or austral spring. A +SIOD tends to be followed by a +IOD. Looking at a composite of water temperature anomalies during boreal autumn following a -SIOD in austral summer/boreal winter, we can see there is a pretty good teleconnection between the two dipoles.
The lower-level wind flow in the years following -SIOD austral summers also look like what is typical during a -IOD in boreal autumn. A noticeable strengthening of easterly flow occurs in the boreal autumn following a -SIOD which is typical with a -IOD.
As a result of this teleconnection and other signals, we do expect a moderate -IOD to develop across the tropical Indian Ocean later this boreal summer into autumn. Many climate models support the idea of a -IOD developing heading into boreal autumn.
Climate model summary from the Australia BOM
Sea surface temperature anomalies from the C3 multi-system seasonal forecast
What does a -IOD in 2020 mean? Across Australia, a -IOD should lead to a wetter austral spring into summer than what we had in 2019 which will mean a reduced chance of severe drought and a less active wildfire season than what we saw in 2019. The development of a -IOD will also mean a good chance of above-normal rainfall across Malaysia and Indonesia unlike the drought conditions of last year.
Composite of -IOD years
Depending on the timing of the -IOD, there could be a drier end to the southwest monsoon across India. A -IOD would also suggest a drier-than-normal northeast monsoon across southern India which typically runs from October to December. We should also see above-normal rainfall across southeastern China into Japan. A -IOD will also shift the main development area in the western Pacific westward heading into boreal autumn which may lead to more impacts on China, Taiwan and Vietnam late in the season.
Report a Typo