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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.
El Niño typically means:
- 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 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
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, 2006 and 2015 all associated with El Niño.
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.
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 that El Niño 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 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.
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.
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.
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