In general, decreased cloud cover results in warmer-than-average daytime temperatures, particularly in the spring and summer months. El Niño is typically associated with sustained negative SOI values.Įl Niño years tend to see warmer-than-average temperatures across most of southern Australia, particularly during the second half of the year. Growing season (April–November) rainfall anomalies for eastern Australian plotted against the SOI averaged for April–November for all years from 1900 to 2013, showing the varied effect of both strong and weak El Niño events on rainfall. For example, during the very strong El Niño that occurred inġ997–98 impacts on rainfall were generally confined to coastal southeastern Australia and Tasmania, while the relatively weak event ofĢ002–03 saw widespread and significant drought. In the Murray–Darling Basin, winter–spring rainfall averaged over all El Niño events since 1900 was 28% lower than the long-term average, with the severe droughts of 1982, 1994, 2002, 20 all associated with El Niño.Īustralian winter spring mean rainfall deciles averaged for fourteen strong El Niño events.Īlthough most major Australian droughts have been associated with El Niño, analysis of past El Niño events shows that widespread drought does not occur with every event, and the strength of an El Niño is not directly proportional to the rainfall impacts. Nine of the ten driest winter–spring periods on record for eastern Australia occurred during El Niño years. The shift in rainfall away from the western Pacific, associated with El Niño, means that Australian rainfall is usually reduced through winter–spring, particularly across the eastern and northern parts of the continent. Potential effects of El Niño Reduced rainfall The strength of an event is not always reflected in the strength of its effects on weather, and events which don't quite reach El Niño threshold levels may sometimes be associated with El Niño-like effects on weather. Rather, El Niño and La Niña are a function of the strength of departures from average in NINO3.4 and the SOI.Įl Niño events are typically defined when SOI values fall below −8 and NINO3.4 temperatures are more than 0.8 ☌ above average.Įvents that maintain close to these threshold values are generally classified as moderate to weak, while those that greatly exceed them are referred to as strong. In the atmosphere, ENSO is monitored via the Southern Oscillation Index (SOI), a measure of atmospheric circulation that takes the difference of atmospheric pressure between Darwin and Tahiti.Įl Niño and La Niña are not turned on and off like a switch. In the ocean, ENSO is most commonly monitored through observed sea surface temperatures within a region of the central and eastern tropical Pacific known as NINO3.4. ENSO is a coupled atmosphere-ocean phenomenon, which means that the transition between La Niña, El Niño and neutral conditions (neither El Niño nor La Niña) is governed by interactions between the atmosphere and ocean circulation.
The term El Niño describes a particular phase of the ENSO climate cycle. As a result, the heavy rainfall that usually occurs to the north of Australia moves to the central and eastern parts of the Pacific basin. Warming of ocean temperatures in the central and eastern Pacific causes this area to become more favourable for tropical rainfall and cloud development. El Niño events are associated with a weakening, or even reversal, of the prevailing trade winds. Typically, the equatorial trade winds blow from east to west across the Pacific Ocean. An El Niño occurs when sea surface temperatures in the central and eastern tropical Pacific Ocean become substantially warmer than average, and this causes a shift in atmospheric circulation.
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