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Indian Ocean Dipole

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Water temperatures around the Mentawai Islands dropped about 4° Celsius during the height of the Indian Ocean Dipole in November of 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 colder than normal, while red areas are warmer than normal.

The Indian Ocean Dipole (IOD) is an oceanographic phenomenon affecting climate in the Indian Ocean region.

The phenomenon

The IOD involves an aperiodic oscillation of sea-surface temperatures, between "positive" and "negative" phases. A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, 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 monsoons over the Indian subcontinent. A significant positive IOD occurred in 1997-8, with another in 2006. The IOD is one aspect of the general cycle of global climate, interacting with similar phenomena like the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean. The positive IOD in 2007 evolved together with La Niña which is a very rare phenomenon that happened only once in the available historical records (in 1967). Also the occurrence of consecutive positive IOD events are extremely rare with only one such precedence within the records (during 1913–14).

The IOD phenomenon was first identified by climate researchers in 1999. Yet evidence from fossil coral reefs demonstrates that the IOD has functioned since at least the middle of the Holocene period, 6500 years ago.

Effect on Australian Droughts

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.[1]

The research shows that when the IOD is in its negative phase, with cool Indian Ocean water west of Australia and warm Timor Sea water to the north, 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.

Further reading

  • Abram, Nerilie J. (2007). "Seasonal characteristics of the Indian Ocean dipole during the Holocene epoch". Nature. 445 (7125): 299–302. doi:10.1038/nature05477. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Ashok, Karumuri (2001). "Impact of the Indian Ocean Dipole on the Relationship between the Indian Monsoon Rainfall and ENSO". Geophysical Research Letters. 28 (23): 4499–4502. doi:10.1029/2001GL013294. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Li, Tim (2003). "A Theory for the Indian Ocean Dipole–Zonal Mode". Journal of the Atmospheric Sciences. 60 (17): 2119–2135. doi:10.1175/1520-0469(2003)060<2119:ATFTIO>2.0.CO;2. {{cite journal}}: Cite has empty unknown parameter: |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Rao, S. A. (2002). "Interannual variability in the subsurface Indian Ocean with special emphasis on the Indian Ocean Dipole". Deep Sea Research-II. 49 (7–8): 1549–1572. doi:10.1016/S0967-0645(01)00158-8. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Saji, N. H. (1999). "A dipole mode in the tropical Indian Ocean". Nature. 401 (6751): 360–363. doi:10.1038/43854. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Behera, S. K. (2008). "Unusual IOD event of 2007". Geophysical Research Letters. 35: L14S11. doi:10.1029/2008GL034122. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

References

  1. ^ Ummenhofer, Caroline C. (2009). "What causes southeast Australia's worst droughts?". Geophysical Research Letters. 36 (L04706). doi:10.1029//2008GL036801. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
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