A combination of heavy monsoon rain falling on already saturated ground, which has caused many rivers to burst their banks, is the straightforward explanation for the catastrophic flooding seen across vast areas of Queensland. Just before Christmas, the Australian Government's Bureau of Meteorology warned Queenslanders to prepare for heavy flooding during the holiday period, due to the heavily saturated ground and rainfall that was predicted in a range of 300mm to 600mm.
This part of Australia experiences tropical weather that is heavily influenced by the two opposing oscillating ocean currents: La Niña and El Niño. At present, La Niña is strong – stronger than for about 20 years, according to UK Met Office scientists. This is associated with heavier-than-usual rainfall in eastern Australia.
The Australian Bureau of Meteorology's seasonal rainfall outlook on 17 December, predicted that parts of eastern Australia were likely to have above-average rainfall in the following three-month period. This was in part, it said, because of the strength of La Niña across the Pacific Ocean.
La Niña ( "the little girl") and El Niño ("the little boy") are the two opposing parts of the southern oscillation index, a climatic phenomenon that is calculated using the barometric pressure difference between Tahiti and Darwin.
The index is also associated with the flow of cold and warm currents between Australia and South America. The current La Niña phase is usually linked with earlier-than-normal starts to the northern monsoon season in Australia, and heavier bouts of severe rain. The El Niño phase is often accompanied by droughts in Australia.