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Introduction (continued)

The influence of the oceans

El Nio-Southern Oscillation

Australian climate is influenced by fluctuations that occur in the Pacific, Indian and Southern Oceans. The effects of each of these wax and wane, adding to and subtracting from each other, making each year different from the last.

El Nio is an event that has become well known for its influence on weather patterns across much of the world. It arises on average every two to seven years.

Pacific Ocean trade winds propel surface water generally in a westerly direction along the equator. This permits warm water to accumulate in the western equatorial Pacific, to the north-east of Australia, heating air in contact with it. The warm, moist air produces clouds and rain. During El Nio events, however, trade winds weaken. Warm water in the western Pacific off the Australian coast is displaced to the central Pacific. Clouds over the western tropical Pacific decrease and parts of Australia may experience drought. Simultaneously, parts of northern and southern America experience above-average rainfall and sometimes floods.

The strength of El Nio is assessed partly by a measure known as the Southern Oscillation Index (SOI). The index is determined by the difference in atmospheric surface pressure between Tahiti and Darwin. Pressure seesaws between the two locations, normally being higher in Tahiti than in Darwin. During El Nio, atmospheric pressure is higher than normal at Darwin and lower than normal at Tahiti, taking the SOI to negative values.

La Nia, the counterpart to El Nio, often brings above-average rainfall to parts of Australia. An early sign of La Nia is warming of the ocean north of New Guinea, a region known as the western equatorial Pacific warm pool. Meanwhile, in the equatorial eastern Pacific Ocean, the ocean surface cools. Atmospheric pressure in Tahiti rises relative to that in Darwin and easterly trade winds intensify across the central Pacific.

The western equatorial Pacific warm pool heats the air in contact with it. The warm air rises, lifting tonnes of moisture that condenses in the atmosphere forming massive cloud banks. These clouds bring rain to eastern Australia and parts of South-East Asia. La Nia's warmer seas usually generate more tropical cyclones around Australia.

Indian Ocean dipole

Another oceanic phenomenon that affects Australia's rainfall is the Indian Ocean Dipole. One form of the Dipole consists of a warm water region in the area around Indonesia and New Guinea, and a relatively colder region in the central Indian Ocean west of Australia. The warm region is fairly common, particularly in La Nia years.

The Indian Ocean Dipole gives rise to rain-producing systems that extend across the Australian continent from north-west to south-east. These north-west cloud bands are the principal means by which rain occurs in the dry centre, although most of the associated rain actually falls in the south-east of the country.

Antarctic Circumpolar Wave

The Antarctic Circumpolar Wave is a recently discovered phenomenon of the Southern Ocean. Within this Ocean, a massive current transports water, taking eight or nine years for a complete, clockwise rotation of Antarctica (Figure 5).

Embedded in this current and carried along with it are two large regions of relatively warm water, thousands of kilometres across, alternating with two equally large regions of relatively cold water. As these relatively warm and cold regions pass south of Australia, they often flood the Great Australian Bight and surround Tasmania. Consequently, when the southern Australian States experience winds off the Southern Ocean, these winds are slightly warmer than average when a warm region is present, and they also contain slightly more moisture than usual. Hence, the passage of warm regions are likely to cause winters to be warmer and wetter than average, and cold regions are likely to make them cooler and drier.

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