What is El Niño and what might it mean for Australia?
Australia's weather is influenced by many climate drivers. El Niño and La Niña 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 Niño–Southern Oscillation (ENSO) and are associated with a sustained period (many months) of warming (El Niño) or cooling (La Niña) in the central and eastern tropical Pacific. The ENSO cycle loosely operates over timescales from one to eight years.
Current ENSO forecasts indicate the likelihood that an El Niño will occur in 2014. Potential effects of El Niño on Australia include:
- Reduced rainfall
- Warmer temperatures
- Shift in temperature extremes
- Increased frost risk
- Reduced tropical cyclone numbers
- Later monsoon onset
- Increased fire danger in southeast Australia
- Decreased alpine snow depths
What causes an El Niño?
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. Typically, the equatorial trade winds blow from east to west across the Pacific Ocean. El Niño 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 Niño
The term El Niño describes a particular phase of the ENSO climate cycle. 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.
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 Niño and La Niña are not turned on and off like a switch. Rather, El Niño and La Niña are a function of the strength of departures from average in NINO3.4 and the SOI.
El Niño events are typically defined when SOI values fall below −8 and NINO3.4 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 Niño threshold levels may sometimes be associated with El Niño-like effects on weather.
Potential effects of El Niño
Reduced rainfall
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.
Nine of the ten driest winter–spring periods on record for eastern Australia occurred during El Niño years. 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 and 2006 all associated with El Niño.
Australian winter spring mean rainfall deciles averaged for twelve strong El Niño events.
Although 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. For example, during the very strong El Niño that occurred in 1997–98 impacts on rainfall were generally confined to coastal southeastern Australia and Tasmania, while the relatively weak event of 2002–03 saw widespread and significant drought.
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. El Niño is typically associated with sustained negative SOI values.
Warmer temperatures
El Niño 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 (2009 and 2002), spring (2006 and 2002), and summer (1982–83 and 1997–98) had all occurred during an El Niño. The warmth of recent El Niño events has been amplified by background warming trends which means than El Niño years have been tending to get warmer since the 1950s.
Australian winter–spring mean maximum temperature deciles averaged for twelve strong El Niño events.
Shift in temperature extremes
For temperature extremes, there are three different measures of heat that are relevant to El Niño: 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 El Niño 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 El Niño 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 El Niño years but there is a reduced frequency of prolonged warm spells. Further north, El Niño 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 El Niño years, decreased cloud cover often leads to cooler-than-average night-time temperatures during winter–spring, particularly across eastern Australia. For example, regions of southern New South Wales and northern Victoria can experience 15–30% more frost days during El Niño than the historical average; frost days which occur during spring can have significant impacts on agriculture. The Australian record cold temperature of −23.0 °C was observed at Charlotte Pass, New South Wales, on 29 June 1994 in an El Niño year.
Reduced tropical cyclone numbers
On average, there are fewer tropical cyclones in the Australian region during El Niño years. This is particularly true around Queensland, where cyclones are half as likely to cross the coast during El Niño 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 2–6 weeks later during El Niño years than in La Niña years. This means that rainfall in the northern tropics is typically well-below-average during the early part of the wet season for El Niño years, but close to average during the latter part of the wet season.
Australian mean rainfall deciles during October–December (left) and February–April (right) averaged for twelve strong El Niño events.
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 El Niño year, particularly when combined with a positive Indian Ocean Dipole (IOD) event. Some El Niño years have been followed by very severe summer fires, including Ash Wednesday (16 February 1983) and the 2002–03 and 2006–07 seasons.
However, not all major fires follow El Niño years. The spring bushfires in the Blue Mountains during October 2013 occurred during a neutral ENSO year, while Black Saturday (7 February 2009) in fact followed a weak La Niña (but notably, a positive IOD).
Decreased alpine snow depths
El Niño 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 Niño 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 Niño years; notably, snow depths never reached 100 cm in 1982 or 2006.
However, El Niño does not guarantee a poor snow season. Indeed, three El Niño years (1972, 1977 and 1991) actually had well-above-average peak snow depths. Cooler night-time temperatures and lower rainfall during El Niño 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.
Forecasting ENSO
The significant impacts that El Niño and La Niña 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 Niño or La Niña is 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 Niño requires ocean and atmospheric anomalies to come together and become self-reinforcing.
For the most recent information on the likelihood of El Niño or La Niña 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 Niño and La Niña, 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.
Further information
See our website for further information about average El Niño rainfall patterns and past events, as well as current ENSO conditions.
References
- Alexander B, Hayman P. 2008. Can we use forecasts of El Niño and La Niña for frost management in the Eastern and Southern grains belt? Proceedings of the 14th Australian Agronomy Conference, 21–25 September 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, 150–161.
- 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 variations of area burnt in Tasmanian bushfires: relationships with climate and predictability. International Journal of Wildland Fire, 16(4020), 540–546.
- Power S, Haylock M, Colman R, Wang X, 2006. The Predictability of Interdecadal Changes in ENSO Activity and ENSO Teleconnections. J. Climate, 19, 4755–4771.
- Williams AAJ, Karoly DJ, Tapper N. 2001. The sensitivity of Australian fire danger to climate change. Climatic Change, 49, 171–191.
Archive: Previous Climate updates
State of the Climate 2014
Climate change analysis requires datasets that not only cover a long span of time, but are also homogeneous through time. A homogeneous climate record is one in which all observed climate variations are due to the behaviour of the atmosphere, not other influences, such as changes in location, exposure of the observation site, instrumentation type or measuring procedure.
Dedicated datasets, carefully curated from weather station sites with long-records and subjected to complex quality control to address inconsistencies and errors, have been developed for this purpose.
Climate change tracker uses the Australian Climate Observations Reference Network – Surface Air Temperature (ACORN-SAT) dataset (temperature) and other high-quality datasets (rainfall, cloud amount and pan evaporation).
| Temperature | Rainfall | Pan evaporation | Cloud amount |
|---|---|---|---|
ACORN-SAT employs sophisticated analysis techniques and takes advantage of newly digitised observational data to provide a temporally homogenised daily temperature record. This dataset consists of a network of over 100 stations, with data for more than half of these stations starting in 1910. Daily, monthly, seasonal and annual data are available. |
Daily and monthly rainfall datasets are available, with data from 1900. The daily rainfall network is a subset of the monthly rainfall network and includes merged series (using combined observations from more than one station) and some shorter series. Consequently, monthly, seasonal and annual rainfall data are available for many more sites than for daily rainfall. |
The pan evaporation dataset contains homogenised observations from a network of 60 stations. Data are available at a monthly timescale, commencing from 1970. |
Homogenised total cloud amount data are available at a monthly timescale for a network of 165 stations. The dataset is based on observations of 9am and 3pm total cloud amount, with data at most sites available from 1957. |
Web sites with information about Australian and regional climate change impacts
The impacts of climate change are likely to be many and varied, and will be different for each region. The links below provide up-to-date information on projections of Australia's future climate and the likely impacts.
Information developed by the Australian Government Department of Climate Change and Energy Efficiency about the impacts of climate change on each state and territory, as well as on specific industries (for example agriculture).
Projections for the coming decades.
Understanding past and future climate.
The Intergovernmental Panel on Climate Change (IPCC) is the leading body for the assessment of climate change, established to provide the world with a clear scientific view on the current state of climate change and its potential environmental and socio-economic consequences.
Understanding how Australia's climate can vary, and how a changing climate will affect us, helps us to plan for the future. The links below provide some background information into the science of climate change and climate variability.
