Published by Australian Academy of Science
El Niņo – riding the climate roller coaster Box 1 | The Walker circulation and weather forecasting

Gilbert Walker was an early 20th century British climatologist who studied air circulations over the Pacific ocean (later called the Walker circulation) that resulted from abnormally high pressures in Australia and low pressures in Argentina, or vice versa. For a long while, Walker's observations were looked upon more as a curiosity than as vital climate information. But developments over the past 25 years, particularly the use of satellite data collection and super-computers, have given his work new meaning. Fluctuations in the Walker circulation, usually with a time scale of 2 to 7 years, are known as the Southern Oscillation.

Australia experiences variable rainfall

The variability of the rainfall is a particularly important characteristic of the Australian climate. It has shaped Australia’s flora and fauna as well as its primary industry and way of life. The variability of rainfall is a consequence of Australia’s geographical location at the western edge of the largest ocean in the world, whose sheer size and water temperature distribution determine the nature of much of the Walker circulation. Fluctuations in the Walker circulation increase the variability of the Australian rainfall. The Walker circulation also has a major effect on the frequency and location of tropical cyclones and on annual rainfall pattern over the wider Pacific region.

Long-range weather forecasting

With satellite-based observations available, investigators have more closely studied the Walker circulation and the associated El Niño phenomenon. The approach to long-range weather forecasting has changed significantly over the past 25 years. Scientists now look at irregularities in the temperature of the surface of the ocean as a potential cause of the irregularities of the temperature of the atmosphere. At the same time, other scientists found that certain repetitive patterns of atmospheric flow are related to each other in different parts of the world.

These observations led to a better understanding about how the occurrence of unusually warm or cold ocean waters – and of unusually high or low atmospheric pressures – could be interconnected in worldwide climate systems. Knowledge about these links and about the behaviour of parts of these global systems helps forecasters to make better long-range predictions. This is partly because the features change slowly and with some regularity. This approach of studying interconnections between the atmosphere and the ocean may represent the beginning of a revolutionary stage in long-range forecasting.

In the last 10 years scientists have applied numerical weather prediction models to long-range forecasting. These models are not concerned with the predicting the details of weather 20 or 30 days in advance – but with predicting out-of-the-ordinary events in the global weather system. The reliability of these long-range forecasts, like that of short- and medium-range projections, has improved substantially in recent years.

Other boxes

Box 2. Modelling climate