SUPPLEMENTARY FEEDING
La Niña explained By Phil Duncan
The team at WeatherWatch look ahead to see what farmers may have in store for the summer growing and harvesting season.
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a Niña this and La Niña that – it’s sometimes easy to think that when La Niña is forming, it’s like a switch and it’s either turned ‘on’ or ‘off’ – it’s not. While the actual setup of La Niña is, in essence, simple, the way it impacts New Zealand is not. So without breaking your mind, we thought it would be good to share with you how La Niña can impact NZ – but also, why your local weather may sometimes do the opposite of what you might expect. To keep it simple, La Niña is measured in the equatorial Pacific Ocean and NZ is a long way from the tropics, in fact, we’re halfway to Antarctica. So just like if there was an earthquake at the tropics, it needs to be a big one to impact NZ with any risks. Same goes for La Niña – if it’s weak and spluttering, then the usual Roaring Forties belt of windy weather in the Southern Ocean will still carry on impacting NZ. Here’s the analogy which I think best explains La Niña and why it sometimes impacts NZ – and sometimes doesn’t. Imagine NZ (both the North and South Islands) are one big traffic island and we have two highways of weather merging over us. The first is the main highway of weather traffic – the Roaring Forties. The entire South Island is in the Roaring Forties belt of westerlydriven weather and it goes up as far north as Whanganui. This is like the ‘State Highway One’ of weather in NZ; it dominates most of our weather and it has the busiest lanes of weather traffic in our part of the world. But during La Niña seasons another lane feeds in from the north, merging with the Roaring Forties over the NZ area. This means we can have a La Niña setup one week – full of cloud in the north, higher humidity and east to north east winds nationwide – followed one week later by a burst of windy westerlies, which kicks that La Niña pattern to the kerb, putting a pause on La Niña weather conditions in NZ and
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A great example of how La Niña and our usual westerly patterns can merge right over the top of New Zealand. A really clear example of why La Niña is not a one-sizefits-all forecast.
a return to ‘normal’ weather for NZ. This simple process can have a 180-degree dramatic difference to your local weather.
“Imagine NZ (both the North and South Islands) are one big traffic island and we have two highways of weather merging over us.”
Let’s look at Hawke’s Bay and Gisborne, for example. While all of NZ is forecast to be warmer than average this month, these regions are at the lower end of that scale due to more La Niña easterlies. Winds off the sea aren’t as warm as winds coming from inland where daytime heating adds more oomph to it. But reverse that with a westerly
and these eastern regions can go from cloudy, drizzly and a cool easterly, to sunny, hot and dry with a temperature in the 30s. What many would consider the opposite of La Niña weather. So for La Niña to really override that ‘State Highway One’ of windy westerly weather, it needs to be a very powerful, dominant La Niña. If you think back to the start of this year, La Niña didn’t end the droughts in northern NZ, they actually got worse as northern rain failed to arrive. Why did it fail? Because La Niña wasn’t very dominant, powerful or long-lasting. This summer La Niña is again leaning towards forming, but is also looking short-lived (likely faded by March) and so once again may not have the big impact some Kiwis may expect, because – as a reminder – it’s not a switch that is either “on” or “off” for our weather. It’s simply another lane of weather traffic to factor into our usual chaotic weather here in the South Pacific. It’s also not just these two weather
DAIRY FARMER
December 2021
