Indian Ocean Dipole

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Mentawai Islands dropped about 4 °C during the height of a positive phase of the Indian Ocean Dipole in November 1997. During these events unusually strong winds from the east push warm surface water towards Africa, allowing cold water to upwell
along the Sumatran coast. In this image blue areas are cooler than normal, while red areas are warmer than normal.

The Indian Ocean Dipole (IOD), also known as the Indian Niño, is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern part of the ocean.

Phenomenon

The IOD involves an aperiodic oscillation of sea-surface temperatures (SST), between "positive", "neutral" and "negative" phases. A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region,[dubious – discuss] with a corresponding cooling of waters in the eastern Indian Ocean—which tends to cause droughts in adjacent land areas of Indonesia and Australia. The negative phase of the IOD brings about the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.

The IOD also affects the strength of

El Niño-Southern Oscillation (ENSO) in the Pacific Ocean
.

The IOD phenomenon was first identified by climate researchers in 1999.[1][2]

An average of four each positive-negative IOD events occur during each 30-year period with each event lasting around six months. However, there were 12 positive IODs between 1980 and 2009, and no negative events between 1980 and 1992. The occurrence of consecutive positive IOD events is extremely rare with only two such events recorded, 1913–1914 and the three consecutive events from 2006 to 2008 which preceded the

2011 Victorian floods
.

In 2008, Nerilie Abram used coral records from the eastern and western Indian Ocean to construct a coral Dipole Mode Index extending back to 1846 AD.[8] This extended perspective on IOD behaviour suggested that positive IOD events increased in strength and frequency during the 20th century.[9]

Effect on Southeast Asian and Australian droughts

A positive IOD is associated with droughts in Southeast Asia[10],[11] and Australia. Extreme positive-IOD events are expected.[12]

A 2009 study by Ummenhofer et al. at the University of New South Wales (UNSW) Climate Change Research Centre has demonstrated a significant correlation between the IOD and drought in the southern half of Australia, in particular the south-east. Every major southern drought since 1889 has coincided with positive-neutral IOD fluctuations including the 1895–1902, 1937–1945 and the 1995–2009 droughts.[13]

The research shows that when the IOD is in its negative phase, with cool western Indian Ocean water and warm water off northwest Australia (Timor Sea), winds are generated that pick up moisture from the ocean and then sweep down towards southern Australia to deliver higher rainfall. In the IOD-positive phase, the pattern of ocean temperatures is reversed, weakening the winds and reducing the amount of moisture picked up and transported across Australia. The consequence is that rainfall in the south-east is well below average during periods of a positive IOD.

The study also shows that the IOD has a much more significant effect on the rainfall patterns in south-east Australia than the

El Niño-Southern Oscillation (ENSO) in the Pacific Ocean as already shown in several recent studies.[14][15][16]

Effect on rainfall across East Africa

A positive IOD is linked to higher than average rainfall during the East African Short Rains (EASR) between October and December.[17] Higher rainfall during the EASR are associated with warm sea-surface temperatures (SST) in the western Indian Ocean and low level westerlies across the equatorial region of the ocean which brings moisture over the East Africa region.[17]

The increased rainfall associated with a positive IOD has been found to result in increased flooding over East Africa during the EASR period. During a particularly strong positive IOD at the end of 2019, average rainfall over East Africa was 300% higher than normal.[18] This higher than average rainfall has resulted in a high prevalence of flooding in the countries of Djibouti, Ethiopia, Kenya, Uganda, Tanzania, Somalia and South Sudan.[19] Torrential rainfall and increased risk of landslides over the region during this period often results in widespread destruction and loss of life.[20][21][22][23]

It is expected that the Western Indian ocean will warm at accelerated rates due to climate change [24][25] leading to an increasing occurrence of positive IODs.[26] This is likely to result in the increasing intensity of rainfall during the short rain period over East Africa.[27]

Effect on El Niño

A 2018 study by Hameed et al. at the University of Aizu simulated the impact of a positive IOD event on Pacific surface wind and SST variations.[28] They show that IOD-induced surface wind anomalies can produce El Nino-like SST anomalies, with the IOD's impact on SST being the strongest in the far-eastern Pacific. They further demonstrated that IOD-ENSO interaction is a key for the generation of Super El Ninos.[29]

2020 IOD positive cycle

A positive IOD cycle is related to multiple cyclones that ravaged East Africa in 2019, killing thousands. The

2019–21 East Africa locust infestation.[30][31]

See also

References

  1. ^ Saji et al. 1999
  2. S2CID 205033630.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  3. doi:10.1029/2008GL037038
    .
  4. ^ Cooper, Dani (March 25, 2009). "Bushfire origins lie in Indian Ocean". Australian Broadcasting Corporation. Retrieved December 22, 2009.
  5. ^ Perry, Michael (February 5, 2009). "Indian Ocean linked to Australian droughts". Reuters. Retrieved December 22, 2009.
  6. ^ Rosebro, Jack (February 12, 2009). "Australia Reels From Split Weather System". Green Car Congress. Retrieved December 22, 2009.
  7. ^ "Seasonal Prediction: ENSO forecast, Indian Ocean forecast, Regional forecast". Low-latitude Climate Prediction Research. JAMSTEC.
  8. ^ "Coral Dipole Mode Index, World Data Center for Paleoclimatology".
  9. doi:10.1038/ngeo357
    .
  10. ^ Tan, Audrey (2019-08-22). "Dry spell likely caused by climate phenomenon". The New Paper. Retrieved 2019-09-12.
  11. ^ Tan, Audrey (2019-08-22). "Dry spell in Singapore likely to last several months". The Straits Times. Retrieved 2019-09-12.
  12. S2CID 4458688
    .
  13. doi:10.1029/2008GL036801
    .
  14. doi:10.2151/jmsj.81.169
    .
  15. S2CID 17013509
    .
  16. doi:10.1038/NGEO760
    .
  17. ^
    ISSN 0894-8755
    .
  18. ^ "East Africa Food Security Outlook: High food assistance needs persist, but food security in the Horn is likely to improve in 2020, November 2019 - South Sudan". ReliefWeb. Retrieved 2020-01-10.
  19. ^ "The climate phenomenon linking floods and bushfires". 2019-12-07. Retrieved 2020-01-10.
  20. ^ "Flooding in western Uganda kills more than a dozen". www.aljazeera.com. Retrieved 2020-01-10.
  21. ^ "Risk of more flooding and landslides as rains batter East Africa". www.aljazeera.com. Retrieved 2020-01-10.
  22. ^ "Kenya floods: More rain expected in region". www.aljazeera.com. Retrieved 2020-01-10.
  23. ^ "East Africa floods". BBC News. Retrieved 2020-01-10.
  24. ISSN 1432-0894
    .
  25. ISSN 0894-8755
    .
  26. PMID 29650992
    .
  27. PMID 31015416
    .
  28. PMID 29955048
    .
  29. S2CID 128595874
    .
  30. ^ "Indian Ocean Dipole: What is it and why is it linked to floods and bushfires?". BBC News. 2019-12-07. Retrieved 2021-06-02.
  31. ^ "Billions of locusts swarm through East Africa after year of extreme weather". France 24. 2020-01-24. Retrieved 2021-06-02.

Further reading

External links
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