El Niño: extreme weather explained

If you thought the worst of El Niño was behind you, think again. The World Meteorological Organization announced Tuesday that this year's cycle will be one of the worst since 1950: surface temperatures in the east-central Pacific Ocean likely to exceed averages by two degrees. And El Niño is expected to strengthen - not weaken - as the year goes on, peaking between October and January 2016.

There is, after all, something devilish in the name "El Niño" - certainly more so than in its sister phenomenon, La Niña. In Spanish El Niño refers to the "first child," or Jesus. So it should be anything but devilish. But El Niño can simultaneously cause wetter than usual winters, with freak waves in California, droughts in South America, affect fish stocks, and wild fires in Indonesia and Australia. It can stretch as far as Japan in the North Pacific region.

"[El Niño] persists as an anomaly in the Equatorial region, but it's a large scale anomaly which spans a large part of the circumference of the globe," says Dr Lydia Gates, a researcher at the Deutscher Wetterdienst (DWD), Germany's meteorological service. "Events that start in the ocean and connect with the atmosphere travel extremely far because of wave patterns and other anomalies."

El Niño and La Niña are two parts of the El Niño Southern Oscillation (ENSO) cycle. In any given year, one or the other is in effect.

Broadly speaking, El Niño is characterized by rising temperatures of surface sea water in the tropical Pacific, while La Niña, "the female child," is characterized by cooling temperatures in the same area.

This change in sea surface temperature interacts with the atmosphere above the Pacific Ocean, and in an extreme year, we get what scientists call an event.

In some senses, it's an annual cycle - and in some years, the effects are barely significant at all. But every 3-7 years there is a spike when a large enough pool of warm water - in an El Niño - collects in the eastern Pacific, off the coast of Peru.

The tangible weather event that we can witness and feel starts to peak at the end of December - around Christmas - hence the reference to Jesus.

Rise - sustain - and fall

"The mature phase of an El Niño or a La Niña is in December, January and February. The build up comes before that, starting in March, April, May, and you start to see anomalous values of the sea surface temperature in the Pacific," says Gates.

The event starts in the eastern Pacific and branches out in a "tongue" formation along the Equator over to the west.

That occured in singificant ways in the years 1982, 1997, and is happening again in 2015. Some say the coming season may be as strong as the 1997 event, and last into April.

Gates says "there is a lot of heat trapped in the ocean," but she is cautious with the data as it stands.

"We can only really trust the July measurements - of what we're observing in sea surface temperature - and that is a little bit less than in 1997 right now," says Gates. "But given that as an initial condition [for a three-month forecast], the [prediction] models all seem to agree it will go into a very elevated state, like the top two of 1982 and 1997."

So what's happening?

There is no single event that creates an El Niño event. It's a combination of oceanic and atmospheric factors.

The warm water from an anomalous spike in the sea surface temperature in the eastern Pacific can take time collect as it moves from the west, but once there's a significant pool, the chances of an El Niño rise.

In a normal year, strong Trade winds would help drive the warm water back to the western Pacific by causing ocean currents to draw water from the deep to the surface. In an anomalous year, the Trade winds relax, allowing the warm water to collect in the eastern and central Pacific.

The Humboldt (or Peruvian) current

The Humboldt is a cold current and in a normal year pushes cold water from the depths of the Pacific Ocean to the surface. The process is called "upwelling."

Upwelling cools the sea surface temperature. It also brings rich nutrients closer to the surface of the water and therefore closer to the sun, allowing plankton to grow and making the waters off Peru and other South American countries fertile fishing grounds.

In a significant El Niño year, the weaker trade winds cause a depression of the thermocline - a thin layer of cold water at depth. And this depression reduces the effectiveness of upwelling and its cooling of the sea surface temperature.

As a result, the fishing grounds in the eastern Pacific are less plentiful, winters get wetter in California, and the western Pacific, is threatened by drought and wild fires.

Extremes within the extreme

All this would make little sense without the Walker cell.

The Walker cell is an east-west atmospheric circulation, spanning the Pacific region. The easterly trade winds form part of the cell.

It's a loop in which "you have a region where the air rises and a region where the air sinks again," says Gates. "Where the air rises you have more precipitation and where it sinks precipitation is subdued."

As Gates explains, the system rides with the sea surface temperature anomalies of the Pacific - so as the anomaly moves, the atmosphere moves, too, flipping weather events from one side of the Pacific to the other, and that on an extreme scale during an El Niño.

That's how El Niño causes both fires and floods.

All that's left is the question, why, if you live in Europe, and all this happens in the Pacific, should you care?

"The anomalies Europe sees are tiny compared to what happens in the tropics," says Gates. "But from a socio-economic point of view Europe has to worry, because [El Niño] affects global markets, food production and disaster management further down the track."