While traveling in Australia you may wonder why all those forecasters are so focused on El Niño (and what is “Southern Oscillation” anyways?). Australia has one of the most distinct climates on the planet, which is due in large part to it’s unique geography. The continent is the driest on earth, and even the temperate regions are prone to drought and brush fires. Australia has a high variability of year-to-year rainfall, which is influenced by Southern Oscillation and El Niño. El Niño – Southern Oscillation (ENSO) causes significant seasonal anomalies all over the globe, but Australia is particularly affected. Australia can experience severe droughts interspersed with extensive wet periods, and possibly increased cyclones, heat waves, and frosts.
Since Australia is so affected by variable weather, much work goes into researching Australia’s weather patterns and climate. The Australian Government’s Bureau of Meteorology maintains an extensive network of field offices and remains the primary source of forecasts and warnings to Australians. The Bureau provides a wide variety of user-friendly resources, but my personal favorite is their forecasts:
This is a screen-shot of my dashboard weather widget taken during my stay in Australia. The Australian BoM provides the forecasts, and while Thursday and Friday were clear, or “Fine,” Wednesday was “Mostly fine.”
Aside from the curious descriptions of the weather itself, the Bureau of Meteorology does extensive research in a variety of areas, including Southern Oscillation and El Niño.
Southern Oscillation and El Niño
Southern Oscillation refers to shift in pressure patterns across the South Pacific. This shift (or oscillation) tends to occur every two to five years, but the reasons are not fully understood. The shift in pressure alters wind patterns, which influences the temperature of ocean water across significant regions of the Pacific. This warming of the Pacific during Southern Oscillation is known colloquially as “El Niño.” El Niño can also cause changes in the Jet Stream, altering weather patterns around the globe.
Map showing departures from average ocean surface temperatures in November 1997, at the height of the 1997/1998 El Niño.
El Niño once referred to the warming ocean off the coast of South America, today, the term refers to sustained warming over a large part of the Southern Pacific Ocean, and the terms “Southern Oscillation” and “El Niño” are used more or less interchangeably. Over the course of about a year, this warming can spread westward across the equatorial Pacific (see illustration above).
Along with the warming ocean, atmospheric changes can affect weather patterns across the Pacific Basin. Atmospheric pressure decreases over the eastern Pacific and increases over Australia and the Indian Ocean, causing the surface winds along the equator to reverse. In the regions where the pressure decreases, excess rainfall occurs; in the regions where the pressure increases, drought is common.
El Niño generally lasts about 12 to 18 months. Unfortunately, El Niño events do not follow a regular pattern which makes it difficult to predict storms. The Southern Oscillation Index, an index of changes in ocean temperature, is used to measure the strength of El Niño events. El Niño events are also unique in their impact on rainfall patterns, spatial extent, and life span.
Southern Oscillation Index
The Southern Oscillation Index (SOI) is a tool used to predict and measure the strength of ENSO events. The SOI is calculated from the difference in pressure from Tahiti to Darwin, Australia. Sustained negative SOI values are associated with ENSO events. The most recent strong ENSO event was in 1997-1998.
Australia and El Niño
To illustrate the general effect of ENSO on Australian rainfall, the Australian Bureau of Meteorology has taken rainfall data from the last twelve strongest ENSO events, showing the canonical pattern of El Niño development and decay from autumn to autumn.