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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 ourclimate drivers’.

  • One of our strongest climate drivers is the El Niño–Southern Oscillation, orENSO”.

  • 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 thenormalphase because were in this state more than

  • half of the time.

  • While a neutral phase may bring morenormalweather to Australia, droughts and floods

  • are certainly still possible.

  • When we move into a La Niña, it’s a bit like the neutraphase 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 were 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 followsshifting 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.

Australia’s highly variable climate is influenced by the broad patterns in the oceans around

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Understanding ENSO

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    James posted on 2015/06/21
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