What is El NiÃ±o and what might it mean for Australia?
Australia’s weather is influenced by many climate drivers. El Nino and La Nina have perhaps the strongest influence on year-to-year climate variability in Australia. They are a part of a natural cycle known as the El Nino Southern Oscillation (ENSO) and are associated with a sustained period (many months) of warming (El Nino) or cooling (La Nina) in the central and eastern tropical Pacific. The ENSO cycle loosely operates over timescales from one to eight years.
Potential effects of El Nino on Australia include:
What causes an El Nino?
An El Nino 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. Typically, the equatorial trade winds blow from east to west across the Pacific Ocean. El Nino events are associated with a weakening, or even reversal, of the prevailing trade winds.
Warming of ocean temperatures in the central and eastern Pacific causes this area to become more favourable for tropical rainfall and cloud development. 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.
Monitoring El Nino
The term El Nino describes a particular phase of the ENSO climate cycle. ENSO is a coupled atmosphere-ocean phenomenon, which means that the transition between La Nina, El Nino and neutral conditions (neither El Nino nor La Nina) is governed by interactions between the atmosphere and ocean circulation.
In the ocean, ENSO is most commonly monitored through observed sea surface temperatures within a boxed region of the central and eastern tropical Pacific known as NINO3.4. 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.
El Nino and La Nina are not turned on and off like a switch. Rather, El Nino and La Nina are a function of the strength of departures from average in NINO3.4 and the SOI.
El Nino events are typically defined when SOI values fall below 8 and NINO 3.4 temperatures are more than 0.8°C above average.
Events 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. 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 Nino threshold levels may sometimes be associated with El Nino-like effects on weather.
Potential effects of El Nino
The shift in rainfall away from the western Pacific, associated with El Nino, means that Australian rainfall is usually reduced through winter-spring, particularly across the eastern and northern parts of the continent.
Nine of the ten driest winter-spring periods on record for eastern Australia occurred during El Nino years. In the Murray-Darling Basin, winter-spring rainfall averaged over all El Nino events since 1900 was 28% lower than the long-term average, with the severe droughts of 1982, 1994, 2002 and 2006 all associated with El Nino
Although most major Australian droughts have been associated with El Nino, analysis of past El Nino events shows that widespread drought does not occur with every event, and the strength of an El Nino is not directly proportional to the rainfall impacts. For example, during the very strong El Nino that occurred in 1997â€“98 impacts on rainfall were generally confined to coastal southeastern Australia and Tasmania, while the relatively weak event of 2002â€“03 saw widespread and significant drough
ElÂ NiÃ±oÂ years tend to see warmer-than-average temperatures across most of southern Australia, particularly during the second half of the year. In general, decreased cloud cover results in warmer-than-average daytime temperatures, particularly in the spring and summer months. Higher temperatures exacerbate the effect of lower rainfall by increasing evaporative demand. Prior to 2013 (a neutral ENSO year), Australia’s two warmest years for seasonal daytime temperatures for winter (2009Â and 2002), spring (2006Â and 2002), and summer (1982â€“83Â and 1997â€“98) had all occurred during an ElÂ NiÃ±o. The warmth of recent ElÂ NiÃ±oÂ events has been amplified by background warming trends which means that ElÂ NiÃ±oÂ years have been tending to get warmer since the 1950s.
Shift in temperature extremes
For temperature extremes, there are three different measures of heat that are relevant to ElÂ NiÃ±o: wide-area heatwaves (as indicated by a very warm national area-average temperature); single-day extremes at specific point locations; and long-duration warm spells. The relationship of ElÂ NiÃ±o with each of these elements may be quite different, and location dependant.
During the warmer half of the year, there is a tendency for weather systems to be more mobile during ElÂ NiÃ±oÂ years, with fewer blocking (stationary) high pressure systems. This means that for southern coastal locations such as Adelaide and Melbourne, individual daily heat extremes tend to be of greater intensity (hotter) during ElÂ NiÃ±oÂ years but there is a reduced frequency of prolonged warm spells. Further north, ElÂ NiÃ±o is associated with both an increase in individual extreme hot days and multi-day warm spells.
Increased frost risk
Although maximum temperatures are generally warmer than average during ElÂ NiÃ±oÂ years, decreased cloud cover often leads to cooler-than-average night-time temperatures during winterâ€“spring, particularly across eastern Australia. For example, regions of southern New South Wales and northern Victoria can experience 15â€“30% more frost days during ElÂ NiÃ±o than the historical average; frost days which occur during spring can have significant impacts on agriculture. The Australian record cold temperature of âˆ’23.0Â Â°C was observed at CharlotteÂ Pass, New South Wales, on 29Â JuneÂ 1994 in an ElÂ NiÃ±oÂ year.
Reduced tropical cyclone numbers
On average, there are fewer tropical cyclones in the Australian region during ElÂ NiÃ±oÂ years. This is particularly true around Queensland, where cyclones are half as likely to cross the coast during ElÂ NiÃ±oÂ years compared to neutral years. This means a decreased likelihood of major damage and flooding related to strong winds, high seas and heavy rains associated with tropical cyclones.
Later monsoon onset
The date of the monsoon onset in tropical Australia is generally 2â€“6 weeks later during El Nino years than in La Nina years. This means that rainfall in the northern tropics is typically well-below-average during the early part of the wet season for El Nino years, but close to average during the latter part of the wet season.
Increased fire danger in southeast Australia
As a result of decreased rainfall and increased maximum temperatures, the frequency of high fire danger ratings and risk of a significant fire danger season in southeast Australia are significantly higher following an ElÂ NiÃ±oÂ year, particularly when combined with a positive Indian Ocean DipoleÂ (IOD) event. Some ElÂ NiÃ±o years have been followed by very severe summer fires, including AshÂ Wednesday (16Â FebruaryÂ 1983) and the 2002â€“03Â and 2006â€“07Â seasons.
However, not all major fires follow ElÂ NiÃ±oÂ years. The spring bushfires in the BlueÂ Mountains during OctoberÂ 2013 occurred during a neutral ENSOÂ year, while Black Saturday (7Â FebruaryÂ 2009) in fact followed a weak LaÂ NiÃ±a (but notably, a positiveÂ IOD).
Decreased alpine snow depths
El Nino years (as well as positive IOD years) tend to have lower snow depths in Australia’s alpine regions. On average, the peak snow depth measured at Spencer’s Creek is 35 cm lower during El Nino years, and the season length (i.e. the period of time with snow depths greater than 100 cm) is 2.5 weeks shorter. The four lowest peak snow depths on record were all measured during El Nino years; notably, snow depths never reached 100 cm in 1982 or 2006.
However, El Nino does not guarantee a poor snow season. Indeed, three El Nino years (1972, 1977 and 1991) actually had well-above-average peak snow depths. Cooler night-time temperatures and lower rainfall during El Nino years can often mean that the snow which does fall is retained and can aid artificial snowmaking which many resorts use to supplement the natural snow they receive.
The significant impacts that El Nino and La Nina can have across Australia and the wider globe make the ability to forecast these events important for agriculture, businesses and communities. The Bureau of Meteorology routinely issue seasonal forecasts which include ENSO outlooks for the next several months. While the skill of these longer-range outlooks varies with the time of year and decreases the further into the future they go, the outlooks can provide useful information about when an El Nino or La Nina likely to occur and how long it might last.
Forecasts of the likelihood of ENSO events take into account temperature patterns across the tropical Pacific Ocean, both at the surface and in the sub-surface, variations in trade wind strength and atmospheric pressure, and ocean currents. The atmospheric and oceanic conditions are analysed by climate models designed for long-range seasonal outlooks. Ultimately, the occurrence of an El Nino requires ocean and atmospheric anomalies to come together and become self-reinforcing.
For the most recent information on the likelihood of El Nino or La Nina events, visit the Bureau’s ENSO Wrap-Up and ENSO tracker web pages, both updated every fortnight. For a summary of climate model outlooks for El Nino and La Nina, our Climate Model Summary page surveys eight international models, and is updated on the 16th of every month. You can sign up to email alerts for all these products.
- Alexander B, Hayman P. 2008. Can we use forecasts of El Nino and La Nina for frost management in the Eastern and Southern grains belt? Proceedings of the 14th Australian Agronomy Conference, 21â€“25Â SeptemberÂ 2008, Adelaide, South Australia.
- Cai W, Cowan T, Raupach M. 2009. Positive Indian Ocean Dipole events precondition southeast Australia bushfires. Geophys. Res. Lett., 36, L19710.
- Drosdowsky W, Wheeler MC. 2014. Predicting the Onset of the North Australian Wet Season with the POAMA Dynamical Prediction System. Wea. Forecasting, 29, 150â€“161.
- Kuleshov Y, Qi L, Fawcett R, Jones D. 2008. On tropical cyclone activity in the Southern Hemisphere: Trends and the ENSO connection. Geophys. Res. Lett., 35, L14S08.
- Nicholls N, Lucas C. 2007. Interannual variation of area burnt in Tasmanian bushfires: relationships with climate and predictability. International Journal of Wildland Fire, 16(4020), 540â€“546.
- Power S, Haylock M, Colman R, Wang X, 2006. The Predictability of Interdecadal Changes in ENSO Activity and ENSO Teleconnections. J. Climate, 19, 4755â€“4771.
- Williams AAJ, Karoly DJ, Tapper N. 2001. The sensitivity of Australian fire danger to climate change. Climatic Change, 49, 171â€“191.