16 January 2008

How the El Nino cycle works

Cosmos Online
Climate change is a major factor in Australia's ongoing drought – but cycles of wet and dry have long been driven by the fickle moods of the El Niño Southern Oscillation. Here we explain the science of that weather system.
How the El Nino cycle works

Credit: NASA

It is a natural part of the climate cycle, but El Niño’s unpredictability and link to reduced rainfall means that in Australia, it is feared as a harbinger of drought.

As far back as the 1800s, Peruvian fishermen were taking advantage of the upwelling of cold, nutrient-rich waters along their coast, which gave rise to rich fishing grounds.

They noticed that, periodically at about Christmas time, a warm ocean current, from the opposite direction in the tropics, would sometimes replace this, leading to a collapse in fishing stocks. They dubbed this El Niño, or the little boy, because of its link to Christmas.

Ongoing impact

At a similar time, early Australian settlers, accustomed to predictable British rains, were caught off guard by the harsh vagaries of the Australian climate. As poet and novelist Dorothea MacKellar wrote, Australia is quintessentially a land “of droughts and flooding rains”.

Gradually meteorologists came to the realisation that Australia was often thrown into drought at around the same time Peru experienced its El Niño.

“El Niño… is by far the largest natural climate variation which impacts on Australia,” says David Jones, head of the Climate Analysis Section at Australia’s Bureau of Meteorology (BOM) in Melbourne. Realising the ongoing impact that El Niño has on Australian weather, meteorologists like Jones have been working on computer models that predict impending El Niño events.

Many years of research since MacKellar made her observation have revealed that in most years (non-El Niño years), the Humboldt Current brings cold water northwards from the Southern Ocean, along South America’s west coast. The cool water is then driven westwards by the equatorial trade winds. As it travels across the Pacific, it is slowly warmed up eight to 10 °C by the tropical sun. By the time it reaches northeastern Australia it’s heated to a balmy 28 to 30 °C.

Trade winds

Simultaneously, trade winds bring warm, moist air to Indonesia and Australia, where it rises, yielding rain. This is called the Walker Circulation, and as the air rises over Australia it creates a region of low pressure.

However, during an El Niño event, the flow of cold water up the South American coast weakens and the east-central Pacific becomes warmer. A major El Niño event can cause the temperature differential between the east and west sides of the Pacific to disappear altogether. Up in the atmosphere, this causes the trade winds and the Walker Circulation to weaken. Instead of warm moist air rising over Australia, El Niño brings dry, sinking air in its place.

El Niño and its antithesis La Niña (the periodic opposite extreme) are collectively called ENSO – El Niño Southern Oscillation (ENSO). The vagaries of the ENSO explain Australia’s unpredictable weather patterns. Every three to eight years, according to the Bureau of Meteorology, we move into an El Niño phase, which might typically last for a year or a year and a half.

“[El Niño] is not a scourge, and as far as Australia is concerned, it shouldn’t be thought of as a synonym for drought, although it’s often linked to reduced rainfall in eastern and northern Australia,” claims the BOM.

Likelihood of drought

The development of an El Niño event does not necessarily mean there will be a drought – but it does make it a lot more likely.

“Major droughts of about 12 months occurred in 2002/03 and 2006/07 as a result of two separate El Niño events, so they played a vital role,” says Grant Beard, a climatologist at the BOM. “Critically, there was no wet period between the two to compensate; generally speaking water storages continued to dwindle from 2003 to 2005.”

El Niño also brings warmer daytime temperatures, which adds a double bite to a drought, by increasing evaporation off an already parched landscape. Another negative consequence for farmers is that, El Niño reduces cloud cover, resulting in rapid cooling at night and more frequent frosts.

Rural productivity, such as wheat yields, especially in Queensland and New South Wales, are closely tied to the fluctuations of ENSO. But, El Niño events can also wreak havoc in many other countries too, and it’s linked to the periodic failure of the Indian monsoon, flooding in South America and bushfires in Indonesia.

Vital predictions

Long-term predictions of approaching El Niño-induced drought are therefore vital for farmers wanting to make decisions such as whether to sow crops. If Australia is to experience an El Niño event in a given year it usually becomes clear between March and June, characterised by weakening of the Walker Circulation, which causes higher than average air pressure over Australia.

Other clues include changes in water temperature, ocean currents and salinity, which are detected by an armada of measuring buoys and orbiting satellites. The BOM’s David Jones explains that these observations can be compared with data from past years to find events that were similar to help in making forecasts.

The BOM relies on both their own models to forecast ENSO events and forecasts from other countries. The latest computer model, developed in conjunction with Australian government research body, the CSIRO, is called the Predictive Ocean Atmosphere Model for Australia (POAMA).

“More sophisticated computer models of the ocean coupled to the atmosphere, together with the steady advance in computer power mean that skilful predictions of ENSO are available to around nine months in advance,” says Beard. The models will become more precise with better resolution, as computers get larger and faster, says the researchers. But the big question for the future is how will ENSO be affected by a warming climate?

Decline in water reserves

According to Beard, the widespread decline in Australia’s water reserves over the past five to seven years has involved numerous factors, including the two El Niño events, natural variability and climate change. “It’s impossible to disentangle these down to specific numbers, but the climate change signal will not reverse,” said Beard.

Neville Nicholls from Monash University, in Melbourne, who is an author on recent U.N. Intergovernmental Panel on Climate Change (IPCC) reports, and a former group leader at BOM’s Climate Forecasting Group, said that predicting the impact of climate on the ENSO is complex and beyond the capabilities of current models.

“However, all the models and physical reasoning and empirical studies all indicate that future droughts will be warmer even if they are no drier – we have already started to see this happening,” says Nicholls. “The warmer conditions will presumably lead to a greater demand for water.”

So Australia will have to get used to living with worsening droughts and the unpredictable cycles of wet and dry are here to stay. As the BOM explains: “El Niño is not a freak of climate, it’s not a rogue weather phenomenon, and it isn’t in any way abnormal”.

Hilary Jones is a science writer based in Adelaide.

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