For many of us it’s hard to get our heads around what La Niña actually means for New Zealand and lately there have been many news stories at WeatherWatch.co.nz and across the media with several different predictions.

WeatherWatch.co.nz has taken the main points to explain why some news headlines have warned of “Looming La Niña droughts” while others say “Looming La Niña rains”.

Firstly it’s important to understand what La Niña is overall:

La Niña is when the cold pool of water in the eastern Pacific intensifies.  This is mostly EAST of the International Dateline and around the equator – so well away from New Zealand.

During El Niño (the opposite of La Niña) the warm water spreads from the west Pacific and the Indian Ocean and into to the east Pacific.  (So, the opposite of what we currently have now).  It takes the rain with it, causing extensive drought in the western Pacific and brings rainfall to the normally dry eastern Pacific.

To help understand what is currently happening to the sea temperatures during this La Niña event look at this current map provided by Australia’s Bureau of Meteorology. 

You can see the cooler than average waters east of the International Date Line and near the equator stretching towards South America (marked in blue).

But you can also see the warmer than average waters in our part of the world (pink and red).  Stretching westwards from the International Date Line to the Coral Sea in north east Australia and into the Indian Ocean (west of Australia).  It’s this pink and red area that creates more lows which in turn creates more rain.  It also elevates the risks of tropical cyclones in the south west Pacific around New Caledonia for example.

What do warmer waters bring?  
Well warm waters fuel rain making lows.  This is why during El Niño (the opposite of what we have at the moment) we often get droughts right over New Zealand – because the fuel for rain bearing lows shifts away from us and towards South America.

With the current warmer than average waters north of us we’ve already seen major flooding in Queensland and New South Wales plus a tropical depression over Vanuatu around 10 days ago.  New Zealand has missed out on rain due to persistent highs blocking these lows from reaching us. 

So why the confusing headlines lately?

Well, quite simply, La Niña creates a number of different scenarios for New Zealand’s various regions.  Forget about what La Niña does for other countries – let’s just focus on us…

A strong La Niña can bring to New Zealand:

• More rain, humidity and cloud to northern and eastern NZ.
• More north easterly winds 
• Droughts to western and southern parts of the country
• More heat (Dry and scorching hot in the south and west, sticky, humid and muggy in the north)

This means we can have news stories about La Niña that have headlines like:

• La Niña brings cooler seas to Pacific (around the equator, well away from NZ)
• La Niña brings warmer seas to South West Pacific (Coral Sea, Vanuatu, Fiji etc)
• La Niña means more tropical cyclones in South West Pacific (eg, New Caledonia)
• La Niña brings hotter weather to NZ
• La Niña may bring droughts to parts of NZ (Central Otago, South Canterbury – which we’ve been seeing this past month)
• La Niña may bring heavy rain to parts of NZ (Northland, Coromandel Peninsula, Auckland, Bay of Plenty, East Cape)

So, you can see why it’s a little confusing for some!

– Philip Duncan, WeatherWatch.co.nz

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Extra info from NASA:

The Effects of La Niña
Changes in global atmospheric circulation patterns accompany La Niña and are responsible for weather extremes in various parts of the world that are typically opposite to those associated with El Niño.

These patterns result from colder than normal ocean temperatures inhibiting the formation of rain-producing clouds over the eastern equatorial Pacific region while at the same time enhancing rainfall over the western equatorial Pacific region (Indonesia, Malaysia and northern Australia.) These patterns affect the position and intensity (weakening) of jet streams and the behavior of storms outside of the tropics in both the Northern and Southern hemispheres.