The South Australian State Emergency Service has begun evacuating people from low lying areas in Old Noarlunga and other parts of the Onkaparinga River basin, as well as parts of the Adelaide Hills and Fleurieu Peninsula following widespread flooding caused by heavy rainfall in the area. No casualties have been reported in the floods, but many homes are without power, and some have been inundated by water over two meters deep.
Flooding at Bridgewater Football Club, South Australia. Malcolm Sutton/ABC News.
South Australia has been suffering an exceptionally wet winter, driven by a climatic event called a Negative Indian Ocean Dipole Phase. Indian Ocean Dipole Phases are similar to the El Niño/La Niña climatic oscillation that affect the Pacific Ocean. Under normal circumstances equatorial waters off the east coast of Africa and west coast of Indonesia are roughly similar in temperature, however during a Negative Indian Ocean Dipole Phase the waters off the coast of Indonesia become significantly warmer. As the prevailing currents in the area flow west to east, this warm water is then pushed onto the shallower continental shelf of north Australia, where it warms the air over the sea more rapidly, leading to increased evaporation (which fuels rain) and a drop in air pressure over the east Indian Ocean and west Pacific. This in turn drives air currents over the Indian Ocean to flow more strongly west to east, leading to higher rates of cooling off the coast of Africa (where more water is drawn up from the cool sea depths) and more warming off the coast of Indonesia, fuelling a feedback cycle that tends to remain through the winter season in any year when it forms. This leads to a particularly wet winter across much of Australia, as well as a potentially damaging heatwave in the north, while much of East Africa is at risk of drought (during a Positive Indian Ocean Dipole Phase the reverse happens, with drought in Australia and flooding in East Africa).
Areas of warming and cooling and air flow during a Negative Indian Ocean Dipole Phase. Australian Bureau of Meteorology.
However, as well as the more direct influence of rainfall, the recent flooding in South Australia has also been linked to increasing urbanisation in many areas, particularly low-lying river floodplains and coastal flats. These areas are naturally prone to flooding, placing any structures built on them at increased risk of flooding. In addition, the covering of absorbent soil surfaces with water-resilient concrete of asphalt surfaces prevents water from soaking into the ground, leading to further problems as this water moves over the surface.
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