Rain is vital for life, its absence can cause droughts and desolation, but as we have seen in the last tragic weeks a prolonged deluge can cause floods and devastation.

A major influence on our climate/weather in Queensland is the Pacific Ocean. The Pacific is a massive body of water and the large-scale interaction between atmosphere and ocean causes patterns and fluctuation of regional and global weather. Phases of drought and flood are associated with the Southern Oscillation, in which air pressure is shifted between the western and eastern Pacific Ocean.

The ocean currents, wind and weather of the Pacific follow a pattern that is known as the Walker circulation. This is named after Sir Gilbert Walker, a Director-General of British observatories in India who identified a number of relationships between seasonal climate variations in Asia and the Pacific region.

Generally the cold water of the Humbolt current flows northward along the west coast of South America and there is also an upwelling of cold water along the Peruvian coast. The cold water moves westward towards the equator. As it moves west, it is heated by the sun and as a result the western Pacific Ocean is usually 3 to 8 degrees C warmer than the eastern Pacific.

The prevailing winds across the Pacific Ocean are the easterly trade winds, which bring warm moist air towards the western Pacific. As the winds travel across the warm sea, the moist air rises, this produces cumulonimbus clouds, rain and low pressure. When this air reaches a high level of the atmosphere, it moves eastward where it sinks over the eastern Pacific producing high pressure and dry conditions. Consequently the air pressure of the western Pacific is generally lower the air pressure in the eastern Pacific.

Periodically this usual circulation pattern fluctuates. When the cold currents and upwellings of the South American coast are replaced by warmer water, a sequence of changes called El Nino occurs. The name, which refers to the Christ child, was first given by Peruvian fishermen when low nutrient warm currents replaced nutrient rich cold currents producing a decline in fish stocks. By warming the eastern Pacific Ocean, El Nino weakens trade winds and swings them east, cools the ocean temperature of the western Pacific, creates low pressure and wetter conditions in South America and high pressure and drier conditions in Australia and Asia.

The opposite phase of El Nino is La Nina, when the eastern Pacific Ocean cools and the usual Walker circulation pattern intensifies producing high rainfall in Australia. The recent floods in Queensland were caused by an extremely strong La Nina event.

The fluctuations of the Walker circulation changes are measured by the Southern Oscillation Index (SOI). This is calculated by the average difference in air pressure between Tahiti and Darwin. Typical Walker circulation that is close to the long term average is zero, while El Nino episodes are strongly negative and La Nina episodes are strongly positive.

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