Climate variability

Australia has a highly variable climate, with large changes from year to year superimposed on longer-term trends. This variability affects our environment and many aspects of our society, including our agricultural crops and cycles, and Indigenous communities across Australia.

Much of this variability is driven by broader influences in the global climate system.

El Niño–Southern Oscillation and Indian Ocean Dipole

The 2 most significant influences on seasonal to interannual climate variability in Australia are the El Niño–Southern Oscillation (ENSO), centred in the tropical Pacific Ocean, and the Indian Ocean Dipole (IOD) centred in the Indian Ocean.

An El Niño event occurs when waters in the equatorial central and eastern Pacific Ocean warm significantly above average. This results in a reduction in the atmospheric pressure gradient across the tropical Pacific and a weakening of the easterly trade winds there. El Niño events are associated with reduced rainfall in many parts of Australia; the largest impact is on winter and spring rainfall in eastern Australia.

The opposite phase of ENSO, known as La Niña, occurs when the equatorial central and eastern Pacific is abnormally cool. This is associated with above-average rainfall and lower summer temperatures in many parts of northern and eastern Australia.

El Niño and La Niña events have a typical lifecycle of 9–12 months, extending from the Southern Hemisphere winter to the following autumn, and each occurs on average 2 or 3 times per decade. La Niña events occasionally extend into a second year, but it is rare for El Niño events to do so. The strong La Niña event of 2010−11 was associated with major flooding in many parts of Australia, especially Queensland and Victoria. El Niño events have been associated with many of eastern Australia’s driest years, including 1982, 1994, 2002 and 2006.

The IOD is a phenomenon of the tropical Indian Ocean. In its positive phase, waters in the eastern equatorial Indian Ocean, in the region immediately west of Java and Sumatra, are cooler than average, and those off the east coast of Africa are warmer than average. In its negative phase, the reverse is true. The IOD is correlated with ENSO (a positive IOD is more likely during an El Niño event and rare during a La Niña event), but is separate from it.

Positive phases of the IOD are associated with lower winter and spring rainfall in many parts of Australia, and negative phases with high rainfall. In 2019, which was Australia’s driest year on record, one of the strongest known positive IOD events occurred; conversely, the strong negative IOD event of 2016 saw an exceptionally wet May–September over much of the country.

ENSO activity varies from decade to decade, with La Niña dominating in some periods (such as the 1950s and 1970s) and El Niño in others (such as the 1940s and 1990s). The past 50 years have seen a relatively high level of ENSO activity compared with most of the first half of the 20th century, with numerous strong El Niño and La Niña events. Indirect evidence (such as coral samples) indicates that large century-to-century changes in ENSO activity are not unusual. There is hence low confidence that the recent high levels of ENSO activity represent a long-term trend beyond the limits of natural variability (IPCC 2021). In addition, there is low confidence that the recent relatively high frequency of El Niño events centred in the central (as opposed to eastern) Pacific, which have a particularly strong impact on Australian rainfall, represents a long-term trend. There is also little evidence of long-term trends in the IOD since reliable instrumental records of sea surface temperature in the key regions of the Indian Ocean began in the 1950s.

Other climate influences

Other broadscale influences affect Australia’s climate on shorter timescales, typically of several weeks (Figure 15).

The Southern Annular Mode (SAM) is a phenomenon of the atmospheric circulation in the middle and higher latitudes of the Southern Hemisphere. In the negative phase of the SAM, westerly winds are displaced north of their usual position, bringing higher cool-season rainfall to areas exposed to the westerlies (such as western Tasmania, southern Victoria and south-west Western Australia), but warm, dry conditions to the east coast in the warmer months. In the positive phase, the reverse is broadly true. A persistent negative phase of the SAM in late 2019 contributed to the severe fire weather in eastern Australia during that period.

The SAM is the one broadscale climate driver that does show a significant trend. Positive phases of the SAM have become more common in recent decades, contributing to higher pressure over southern Australia and the declining trend in cool-season rainfall in the region. It is very likely that human influence, both through greenhouse gas emissions and stratospheric ozone depletion, has contributed to the observed SAM trend in summer, but attribution is less clear in other seasons (IPCC 2021).

The Madden–Julian Oscillation (MJO) is the most active convection influence, bringing rainfall, moving from west to east through the tropics on a typical cycle of 30–60 days. Its influence is greatest during the tropical wet season from November to April. In a normal wet season, there are 3 or 4 periods when the MJO is centred in Australian longitudes, typically leading to enhanced monsoon activity and heavy rain in northern Australia, as well as a higher risk of tropical cyclones in the Australian region. The MJO’s primary influence is in the tropics, but it can also affect weather outside the tropics.

A further source of natural climate variability is volcanic activity. On occasions, volcanic eruptions, particularly in tropical areas, eject enough material into the upper atmosphere to have a significant cooling impact on global temperatures for 1–3 years. The most recent eruption that was large enough to have a significant global climatic impact was Mount Pinatubo in the Philippines, which had a peak impact of approximately 0.4 °C on global temperatures following its 1991 eruption. Other eruptions since 1950 with a significant climatic impact include El Chichón (Mexico) in 1982, and Mount Agung (Indonesia) in 1963. The Pinatubo eruption was the most significant for global climate in the past 100 years, but some 19th century eruptions were larger than any observed since 1900. Volcanic activity in the past decade has only had local and short-lived climatic impacts, with no discernible signal in Australia.