La 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 westerly-driven 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 a return to ‘normal’ weather for NZ.
This simple process can have a 180-degree dramatic difference to your local weather.
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 patterns impacting us that helps bring changeable variety, it’s our mountains and ranges too.
You would all likely know that certain wind directions in your local part of NZ means you have a higher chance of sun or cloud, rain or dry, hot or cold. La Niña summers tend to increase the chances of easterlies in the top of NZ and nor’easters further south. So regions to the opposite of that will be hotter and drier. For example, Waikato, Manawatū, Horowhenua, Whanganui, through the Southern Alps, Central Otago and Northern Southland all have chances of a hotter, drier summer – but only when we have those east to northeast flows. When the westerlies return from time to time that means western areas get more clouds and lower temperatures. But some – like Central Otago and South Canterbury and Northern Southland – can be dry in both those setups, hence the increased risk of a drier summer there this year.
Finally, there is also the ‘wild card’ factor – a potential single weather event that entirely reverses all the long-range forecasts, not because the data was wrong but because NZ is simply so tiny compared to the ocean surrounding us. So one tropical cyclone can hit almost anywhere in NZ and reverse what was forecast for an entire month, maybe two if big enough. Stubborn high pressure zones can also get in the way north of NZ and block sub-tropical rainmakers for weeks, only to have big rainmakers just miss us to the west or east.
So, this summer be prepared for more changeable weather patterns, but also understand that La Niña is measured a long way from where some of us live, so any La Niña headline you see should be balanced with the fact NZ is basically two large mountainous islands plonked halfway in the Roaring Forties belt of westerly-driven weather.
