Indian Ocean Dipole

Indian Ocean Dipole

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The Indian Ocean Dipole (IOD) is an irregular oscillation of sea-surface temperatures in which the western Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

 becomes alternately warmer and then colder than the eastern part of the ocean.

The phenomenon


The IOD involves an aperiodic oscillation of sea-surface temperatures, 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, with a corresponding cooling of waters in the eastern Indian Ocean—which tends to cause droughts in adjacent land areas of Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...

 and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. 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
El Niño-Southern Oscillation
El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years...

 (ENSO) in the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

.

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
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...

 period, 6500 years ago.

An average of four each positive/negative IOD events occur during each 30 year period with each event lasting around six months. However, there have been 12 positive IODs since 1980 and no negative events from 1992 until a strong negative event in late 2010. The occurrence of consecutive positive IOD events are extremely rare with only two such events recorded, 1913–1914 and the three consecutive events from 2006-2008 which preceded the Black Saturday bushfires. Modelling indicates that consecutive positive events occur twice over a 1,000 year period. The positive IOD in 2007 evolved together with La Niña
La Niña
La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3–5 °C...

 which is a very rare phenomenon that has happened only once in the available historical records (in 1967). A strong negative IOD developed in October 2010, which, coupled with a strong and concurrent La Niña, caused the 2010-2011 Queensland Floods and the 2011 Victorian floods
2011 Victorian floods
High intensity rainfall between 12–14 January 2011 caused major flooding across much of the western and central parts of the Australian state of Victoria...

.

Effect on Australian Droughts


A 2009 study by Ummenhofer et al. at the University of New South Wales
University of New South Wales
The University of New South Wales , is a research-focused university based in Kensington, a suburb in Sydney, New South Wales, Australia...

 (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
Federation Drought
In Australia, the Federation Drought is the name given to a prolonged period of drought that occurred around the time of Federation in 1901.Though often thought of as a long drought, until the record dry year of 1902 the period was actually one of a number of very dry spells intercepted with wetter...

, 1937–1945 and the current 1995-present
2000s Australian drought
The 2000s drought in Australia is the worst since settlement. This drought began in 1995 and continued until late 2009....

 droughts.

The research shows that when the IOD is in its negative phase, with cool Indian Ocean water west of Australia and warm Timor Sea
Timor Sea
The Timor Sea is a relatively shallow sea bounded to the north by the island of Timor, to the east by the Arafura Sea, to the south by Australia and to the west by the Indian Ocean....

 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
El Niño-Southern Oscillation
El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years...

(ENSO) in the Pacific Ocean as already shown in several recent studies.

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