EXPLAINER

What might La Nina mean for you?

La Nina brings cooler and wetter weather to Australia. Picture: Shutterstock
La Nina brings cooler and wetter weather to Australia. Picture: Shutterstock

The Bureau of Meteorology has declared that there is "an active La Nina phase" occurring.

With memories of last year's bushfires still raw in our minds, we may be relieved because it means more rainfall.

But why?

Some real basics first

La Nina is pronounced La Neen-ya. It means "girl" in Spanish but has also come to mean a particular weather phenomenon which has a big impact on our weather in Australia.

Its counterpart, El Nino, is pronounced El Neen-yo and means boy. It's a different part of the same phenomenon over the Pacific but which affects our weather in a big way.

What is La Nina

La Nina is a cooling of the surface of the Pacific Ocean across its middle and eastern parts towards north and south America and a warming of the western parts nearer Australia.

El Nino, its counterpart, is a warming of the surface. The two switch between each other every eight or so years as part of a natural phenomenon.

The two oscillate between each other and they have different effects. El Nino is associated with hotter summers in Australia. La Nina with cooler and wetter ones.

Let's stick with La Nina

When different parts of the surface of the ocean have different temperatures, air rushes between the two - in other words, winds pick up.

One of the ways the Bureau knows there's a La Nina event is that wind-speed between Darwin and Tahiti is rising.

These winds from the cooler than usual eastern Pacific warm up the water in the western part of the Pacific - the section north and east of Australia.

So the sequence is: cool water in the Pacific towards America causes wind which causes warm water near Australia.

And that means more wind and rain in Australia.

"The six wettest winter-spring periods on record for eastern Australia occurred during La Nina years," the BoM said.

But too much rain?

Maybe.

The presence of La Nina increases the chance of widespread flooding.

La Nina has happened 18 times since 1900 and twelve of those times resulted in floods in parts of Australia.

In the Murray-Darling Basin, spring and summer rainfall during those 18 La Ninas was 22 per cent higher than the average.

The bad floods of 1955, 1988, 1998 and 2010 were all associated with La Nina.

So what does this spring and summer look like?

The last La Nina went from 2010 to 2012 (they are events which can stretch over a few years). The result was one of the wettest periods on record in Australia. There was very intense flooding.

But the Bureau of Meteorology thinks it's too early to say how severe the rains will be this time. Each one is different.

The BoM certainly expects above average rainfall but probably not flooding as bad as a decade ago.

Good news for firefighters

The meteorologists reckon the extra rain should dampen the bushfire season.

"We can't avoid some fires in any summer now," the Bureau of Meteorology's manager of climate operations, Andrew Watkins, said.

"It reduces the bushfire potential a little but it doesn't extinguish it completely."

"The La Nina, though, with a bit of extra rainfall, hopefully will keep those fires a little smaller and a little shorter than what we've seen in recent years."

It's not just rain

The Bureau says that La Nina brings more tropical cyclones in the north of Australia. Twice as many reach land as in more usual years.

"The first cyclone to cross the Australian coast also tends to occur earlier in the season. The only years with multiple severe tropical cyclone landfalls in Queensland have been La Nina years," it said.

"This means an increased likelihood of major damage and flooding related to strong winds, high seas and heavy rains from tropical cyclones."

The date of the monsoon season in tropical Australia also comes forward. It's generally two weeks earlier in La Nina years.

"This means that rainfall in the northern tropics is typically above-average during the early part of the wet season for La Nina years but only slightly above average during the latter part of the wet season."

Is it anything to do with global warming?

La Nina and its counterpart El Nino are not caused by global warming. They occur naturally and have done so for thousands of years, according to the official National Oceanic and Atmospheric Administration in the United States.

La Nina and El Nino are a shifting of heat. They involve hotter parts of the ocean in one place and cooler parts elsewhere, not a general increase or decrease of temperature.

But there still may be a connection, though it is far from certain.

One study suggests that global warming could mean that the effects of El Nino and La Nina would be spread over a much wider area, particularly in Australia.

How long will La Nina last?

It is common for La Ninas to last for more than a year, occasionally between two and three years. They can last for less but the longest lasted for 33 months.

This story What might La Nina mean for you? first appeared on The Canberra Times.

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