Understanding ENSO

Understanding ENSO

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Australia’s highly variable climate is influenced
by the broad patterns in the oceans around it, and the atmosphere above it. Some of these patterns are not only more obvious
than others, but also predictable. We call these our ‘climate drivers’. One of our strongest climate drivers is the
El Niño–Southern Oscillation, or “ENSO”. ENSO is a natural cycle in Pacific Ocean temperatures,
winds and cloud. This influences climate right around the globe. In Australia, ENSO is often behind our climate
extremes, from devastating floods to searing droughts. ENSO naturally swings between three key phases;
La Niña, Neutral and El Niño. A typical ENSO phase starts in the first half
of the year and lasts until the following autumn. Sometimes we can get the same phase for two
or more years in a row. On average, it takes about four years to swing
from El Niño to La Niña and back again. So what are these ENSO phases, and how do
they impact Australia’s climate? Well during the neutral phase, steady trade
winds blow across the tropical Pacific from the east to west. These winds pile up warm water in the western
Pacific. In contrast, water temperatures to the east are lower as the trade winds cause
cool water to be drawn up from the deep. The temperature difference across the tropical
Pacific Ocean causes air to rise to Australia’s north, and descend near South America. This
creates a huge connected cycle called the Walker Circulation. We consider neutral to be the “normal”
phase because we’re in this state more than half of the time. While a neutral phase may bring more ‘normal’
weather to Australia, droughts and floods are certainly still possible. When we move into a La Niña, it’s a bit
like the neutra’ phase has gone into overdrive. The trade winds blow harder, expanding the
warm pool on the Australian side of the tropical Pacific, and cooling the oceans towards South
America. This increases the east to west temperature
difference, and makes the Walker circulation even stronger and the trade winds blow even
harder again. This is called a feedback loop, and once it
starts we’re locked into a La Niña until at least the following autumn. With the higher ocean temperatures, we get
greater evaporation, more cloud and more rain in the western Pacific. For Australia, this means a higher risk of
widespread flooding, lower daytime temperatures, and more tropical cyclones. On the other end of the scale we have El Niño,
which is almost the direct opposite of La Niña. During El Niño, the trade winds actually
weaken, or reverse, allowing warmer waters to drift back towards the east. The change in the ocean temperature patterns
mean the Walker circulation breaks down, resulting in even weaker trade winds, and even more
warming in the east. Once this feedback starts, El Niño has set
in. With the warm water shifting east, the evaporation,
cloud and rain follows – shifting away from Australia. That means a greater risk of drought for northern
and eastern Australia, higher temperatures and more heatwaves, clearer nights and a longer
frost season, and fewer tropical cyclones. While there are scientific definitions for
El Niño and La Niña, in reality, no two events, and no two sets of impacts, are exactly
the same. We also know some impacts will emerge as an
ENSO event is developing, and some will persist even if an El Niño or La Niña never fully
forms. The Bureau updates the status of its ENSO
tracker whenever an event may be on the horizon, so you can keep well ahead of the game. Understanding ENSO is a big part of understanding
our climate, so stay up to date with our fortnightly ENSO Wrap Ups and of course, watch our monthly
Climate Outlook videos.

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