Impacts of the event in Australia
Signs of a developing La Niña emerged during autumn 2010 as the Pacific cooled rapidly at the end of the 2009-10 El Niño. By July, La Niña conditions were established and most of Australia experienced significantly higher than average rainfall over the next eight months. Peaking between late 2010 and early 2011, this La Niña event was one of the strongest observed, in a record dating from the late 1800s. Record high rainfall occured across much of northern and eastern Australia during this event, leading to widespread flooding in many regions between September 2010 and February 2011. This event saw Australia experience its wettest September on record, the wettest “dry” season on record in northern and central Australia, and the wettest summer on record in Victoria. Severe Tropical Cyclone Yasi, possibly the strongest cyclone to make landfall in Queensland since the strong La Niña event of 1918, crossed the coast between Cairns and Townsville on the 3rd of February. The calendar year 2010 also ranked as Australia’s second wettest year on record.
Special Climate Statements
A number of the events mentioned above are described in more detail in Special Climate Statements.
Special Climate Statements are produced for events which are unusual in the context of the climatology of the affected region. They provide a detailed summary of the significant event, including figures, impacts and contributing weather patterns.
Climatic drivers of the events
What is the El Niño–Southern Oscillation (ENSO)?ENSO describes the state of the equatorial Pacific Ocean - the position of warm and cool water, the strength and direction of winds, atmospheric pressure gradients, and the location of convection (cloudiness and associated rainfall). ENSO has three phases; La Niña, El Niño, and neutral. A typical ENSO event begins in autumn, matures during winter and spring, then begins to decay in late summer, with the event generally ending in the autumn of the following year, although some events may last more than one year (e.g. the 1998-2001 La Niña lasted 35 months, from May 1998 to March 2001).
The Southern Oscillation refers to the shifting pattern of air pressure between the Asian and east Pacific regions known as the Walker Circulation. The strength and direction of this pattern is one indicator of La Niña and El Niño; it is measured by the Southern Oscillation Index (SOI). The SOI measures the difference in air pressure between Tahiti and Darwin (representing the central-east and west of the Pacific Ocean Basin respectively). The SOI is best represented by monthly or seasonal (or longer) averages since daily or weekly SOI values can fluctuate markedly due to short-lived, day to day weather patterns. Sustained positive SOI values are associated with La Niña events while sustained negative SOI values indicate an El Niño.
La NiñaDuring a La Niña event, as well as sustained positive SOI values, sea surface temperatures (SST) in the western Pacific, north of Australia, are often warmer than normal while the SST across the central and eastern tropical Pacific Ocean is cooler than normal; convection (cloudiness) over tropical Australia, Papua New-Guinea, and Indonesia is increased; and trade winds (easterlies) across the Pacific Ocean (but not necessarily in the Australian region) are stronger than normal.
Historically, La Niña events are associated with wetter than normal conditions across eastern and northern Australia. The warm water gathered closer to our east coast is a source of atmospheric convection and is associated with cloudiness and rainfall. Combined with stronger than normal trade winds, this provides more moisture to the atmosphere and directs the moisture towards eastern Australia. In the past, La Niña years have been correlated with an increased chance of wetter springs and higher numbers of tropical cyclones during the cyclone season (November to April).
El NiñoDuring El Niño events, as well as a sustained period of negative SOI, sea surface temperatures in the central and eastern tropical Pacific Ocean are warmer than normal; the focus of convection migrates from the Australian/Indonesian region eastward towards the central tropical Pacific Ocean; and trade winds (easterlies) are weaker than normal.
This results in less atmospheric moisture available for rain in Australia. In the past, El Niño years have been correlated with an increased chance of drier springs and a reduction in rainfall over eastern and northern Australia.
Due to the complexity of land-ocean interactions across the Pacific, no two La Niña events, and no two El Niño events, are exactly the same. Similarly, the SOI cannot accurately predict rainfall for Australia on an event by event basis. Rather, indicators of the El Niño-Southern Oscillation serve as a guide to the chance (probability or likelihood) of receiving more or less rainfall in a given season.
What is the Indian Ocean Dipole (IOD)?The Indian Ocean Dipole (IOD) is a coupled ocean and atmosphere phenomenon in the equatorial Indian Ocean characterised by a difference in SSTs between the western dipole (in the Arabian Sea) and the eastern dipole (in the eastern Indian Ocean around Indonesia). The IOD affects the climate of Australia and other countries that surround the Indian Ocean Basin.
A positive IOD period is characterised by cooler than normal water in the tropical eastern Indian Ocean and warmer than normal water in the tropical western Indian Ocean. Positive IOD events are associated with a decrease in rainfall over parts of central and southern Australia.
A negative IOD period is characterised by warmer than normal water in the tropical eastern Indian Ocean and cooler than normal water in the tropical western Indian Ocean. Negative IOD events are associated with an increase in rainfall over parts of southern Australia.
While the IOD operates in the Indian Ocean, it has some relationship to ENSO in the Pacific. Positive and negative IOD events can influence the effect of ENSO on Australian rainfall. For example, having both a negative IOD and La Niña conditions has historically further increased the likelihood of heavy rainfall across the continent.
December SST anomalies - NASA's AMSR-E
Where to get information
The ENSO Wrap-UpThe National Climate Centre, at the Bureau of Meteorology, monitors a number of climate indicators including the temperature of water at the sea surface and at depth, the SOI index, cloudiness and winds to form a picture of current ENSO conditions and likely developments. A summary of the current ENSO state and outlook for the months ahead is published regularly in the ENSO Wrap-Up.
ENSO Model SummariesAs well as the ENSO Wrap–Up, the National Climate Centre publishes a summary of outlooks produced by several international dynamical climate models, including the Bureau’s own POAMA model. The ENSO Model Summary may be read here.
Indices of ENSO – NINO regions and monitoring graphsClimatologists often use several NINO indices to monitor the Pacific Ocean. These indices refer to the difference from the long term mean of the SST in several regions located along the equatorial Pacific. These regions are called NINO1 and NINO2 (which both lie on the South American coast), NINO3 (which occupies the eastern equatorial Pacific), and NINO4 (which occupies central equatorial Pacific) and NINO3.4 (which partially overlaps both the NINO3 and NINO4 areas). Generally, the National Climate Centre uses NINO3.4 as the most informative index for the Australian region.
Australian climatologists often cite monthly NINO values above +0.8 as typical of El Niño conditions, with values of below –0.8 as of a La Niña. These values are approximately one standard deviation from the long term mean (i.e., around 70% of monthly NINO3.4 values, for example, lie between –0.8 and +0.8).
A number of ENSO related indices are tracked on the Bureau’s ENSO Monitoring Graphs:
NINO1, NINO2, NINO3, NINO3.4, NINO4 and IOD weekly values
SOI monthly value and the 5-month weighted mean
Typical La Niña and El Niño Rainfall PatternsThe average impact of La Niña and El Niño events on Australian rainfall patterns can be explored by looking at past events. Rainfall over twelve of the strongest "classic" or "canonical" events (having the typical autumn to autumn pattern of evolution and decay) was used to form a composite of average rainfall impacts in both La Niña and El Niño events. Included is a table of the winter-spring SOI for each event, commentary and composite maps showing the evolution of Australian rainfall patterns during a "classic" event.
Average El Niño rainfall patterns
Average La Niña rainfall patterns
Past La Niña and El Niño EventsA case by case analysis has been produced for each of the of La Niña and El Niño events effecting Australian since 1900. Follow the links below to read summaries of individual events and their impacts.
El Niño events since 1900
La Niña events since 1900
Typical Rainfall Patterns in IOD EventsThe average impact of IOD events on Australian rainfall patterns has also been described by a composite of past events; these were formed using eleven years since 1958 which are general accepted as positive IOD years, and ten years in the same period generally accepted as negative IOD years. Composite maps showing the evolution of Australian rainfall patterns during a positive and negative event, and the interaction IOD events with La Niña and El Niño events are included in the pages below:
Australian rainfall patterns during positive IOD years
Australian rainfall patterns during negative IOD years.
Winter/Spring rainfall deciles for ENSO events
Winter/Spring rainfall deciles for IOD events