Oceans, winds and Australia’s rainfall

In Year 7 you learned that El Niño years tend to be dry in eastern Australia and La Niña years wet. This page explains why, and introduces the other ocean–atmosphere patterns that forecasters watch.

The Walker circulation

Normally the trade winds blow from east to west across the tropical Pacific. They drag warm surface water west, so a deep "warm pool" builds up near Indonesia and northern Australia, while cold water wells up from below near South America. Above the warm pool, moist air rises, forming towering thunderclouds and heavy rain. The air flows east high up, sinks over the cool eastern Pacific, and returns as the trade winds. This loop is the Walker circulation, named after Gilbert Walker.

The ocean and air hold each other in place: the temperature difference across the Pacific drives the trade winds, and the trade winds keep the temperature difference going. That is a positive feedback, so a small nudge can tip the system one way or the other:

• El Niño: the trade winds weaken, warm water spreads back east, and the rising air and rain shift to the central Pacific. Air sinks over Australia instead, so eastern Australia tends to be drier and warmer, with more heatwaves and fire risk.
• La Niña: the trade winds strengthen, even more warm water piles up near Australia, and eastern and northern Australia tend to be wetter, with more floods and often more tropical cyclones.

Together these are called ENSO (El Niño–Southern Oscillation). Each phase usually builds in winter, peaks around summer and fades by autumn, and they recur irregularly every two to seven years. The Bureau of Meteorology tracks ENSO with sea-surface temperatures in the central Pacific, the trade winds, cloud, and the Southern Oscillation Index (SOI), which compares the air pressure at Tahiti with Darwin. A sustained SOI below about −7 points to El Niño; above about +7, La Niña.

The Indian Ocean Dipole

The Indian Ocean has its own seesaw, the Indian Ocean Dipole (IOD), which mostly matters from about May to November.

• Positive IOD: cooler than normal water near Indonesia and warmer water off Africa. Less moisture flows over Australia, so southern and central Australia tend to have a dry winter and spring. In 2019 the IOD reached its strongest positive value on record, and 2019 became Australia's driest and hottest year, setting up the Black Summer fires.
• Negative IOD: warm water builds up near Indonesia. Moist air is carried south-east across the continent by cold fronts and northwest cloudbands, bringing above-average rain to much of southern Australia. A negative IOD together with La Niña (as in 2010 and 2022) often means widespread flooding.

The Southern Annular Mode

Around Antarctica a belt of strong westerly winds carries the cold fronts that bring winter rain to southern Australia. The Southern Annular Mode (SAM) describes whether this belt has shifted towards Antarctica (positive SAM) or towards Australia (negative SAM). In winter, a positive SAM keeps the fronts further south, so south-west WA, Victoria and western Tasmania are drier. In spring and summer, a positive SAM brings more moist easterly winds to the NSW coast. Both the ozone hole and rising greenhouse gases have pushed the SAM towards its positive phase, one reason for falling winter rain in the far south-west.

A shifting pattern

Australia's rainfall has always been the most variable of any inhabited continent, but the long-term pattern is shifting. According to the CSIRO and Bureau of Meteorology State of the Climate 2024 report:

• April–October rainfall in the south-west of WA has fallen by about 16% since 1970, and in south-east Australia by about 9% since the late 1990s.
• Northern Australia has become wetter since the 1970s.
• Short bursts of very heavy rain are becoming more intense, which raises the risk of flash flooding, even in places where total rainfall is falling.

Long before European records, First Nations peoples tracked these rhythms through seasonal calendars built on generations of observation. The D'harawal calendar of the Sydney region, for example, recognises six seasons, each marked by signals such as the flowering of particular plants and the behaviour of animals, rather than by fixed dates. The Bureau of Meteorology now publishes Indigenous seasonal calendars alongside its own climate information, and researchers are studying how these signals are shifting as the climate warms.