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:
This means we can have news stories about La Niña that have headlines like:
So, you can see why it’s a little confusing for some!
– Philip Duncan, WeatherWatch.co.nz
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.
on 7/12/2010 7:24pm
Now that’s a good story. Good luck trying to get it into the mass hysteria media!
on 7/12/2010 8:19pm
Oddly, no one in the mainstream media picked it up!
on 7/12/2010 10:37am
Even the data is confusing. This SST data from the US National Oceanic and Atmospheric Administration http://www.esrl.noaa.gov/psd/map/images/sst/sst.anom.month.gif shows colder than usual sea surface temperatures around and north of New Zealand in the 30 days to December 4th. Does that mean the higher than normal rainfall in the North and East is not as likely as it looked from the October data?
on 7/12/2010 6:20pm
that SST map you have on this page, that looks like an average for a long period
the current SST map has a persistant tongue of cooler than normal water to the east and north of NZ, but a warmer tasman. Normaly in a la nina the coral sea and area to the north of NZ would be warmer than normal, but the ocean is a bit decoupled from the climate at the moment, hopefully it will catch up.
one reason for the colder than normal water to the east and north could be that for a long time that was warmer than normal (for the past couple of years) but has cycled to be colder…bad timing though to help re enforce la nina (i.e its hard to get all the ducks to line up), and also could be due to persistant cooler SE winds to the north and east of NZ (due to the highs over NZ)
on 7/12/2010 8:46am
Good explanation Phil. As much as the media try to dumb things down for us, some things defy simple explanations. Climate change being another example.
on 7/12/2010 12:46am
was 07-08 summer not a la nina that was very hot and dry throughout much of the north island ???
on 7/12/2010 1:21am
Not all La Nina’s are the same – and because we are just a small dot in the Pacific Ocean we can’t always say with 100% confidence that "this will happen". This link explains what happened over that period – http://www.niwa.co.nz/news-and-publications/publications/all/cs/seasonal/sclimsum_08_summer – hope that help.