Indian Ocean Dipole

Australia: The Land Where Time Began

Indian Ocean Dipole - IOD

Until recently, southern Australia has been receiving much of its rain as winter rain from cold fronts spun off by the Antarctic Circumpolar Vortex, high speed winds that constantly circle Antarctica, deflecting any warmer winds from the north away from Antarctica. As the southern polar regions have been warming, the vortex continues spinning but has contracted further south, and now the cold fronts are not reaching far enough north to bring rain the southern Australia. So this area has depended more on the cloud belts coming down from the northwest in the negative phase of the Indian Ocean Dipole (IOD). This weather system has also began to change, there are now more positive than negative phases and the result is less rain has been brought to south-eastern Australia.

Relatively recently it has been found that the climate of southern Australia, such as Victoria, where the current drought is almost 15 tears long, was being affected not by the La Niña and El Niño, but by the Indian Ocean Dipole, another ocean/atmosphere combination in the Indian Ocean. Like the El Nino and La Nina systems in the Pacific, its negative and positive phases bring warm water to or away from the western coast of Australia. In the negative phase its winds move from west to east piling up the warm water along the coast and the moisture laden associated winds carry belts of cloud over southern Australia, as well as to the western coast. In the positive phase, the wind blows from east to west, and the warm water is moved to Africa, which then gets the benefits of plenty of rain.

It has been found that the where the Pacific Ocean meets the Indian Ocean in the islands of Indonesia, at what has been called the Indonesian Throughflow, the choke point, around the islands of Lombok, Omboi and Timor, the point were warm water is being transferred from the Pacific to the Indian Ocean in powerful currents involving huge amounts of water. The Leeuwin Current is the result of warm water being pushed through the chokepoint, moving down the west coast and continuing along the south coast and around Tasmania. The equatorial winds pile up large volumes of water along the western Pacific, and it is the resultant water pressure that forces the warm water through the throughflow. It is the Leeuwin Current that prevents Western Australia from becoming a very cold desert.

The IOD is the switch that is toggled between cool positive and warm negative phase by the competing pressures between the oceans. The water coming from the Pacific is competing with the local winds that produce large amounts of upwelling of cooler water, with less moisture being taken up by the atmosphere, so drying the climate of Western Australia, as well as the south-east that is beginning to depend more heavily on the rain from the north-west.

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