Thursday, December 7, 2023

A nice bit of chaos as we wait for El Niño

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NZ is actually in quite a positive phase of weather from a farming and growing point of view, says weatherman Phil Duncan.
WeatherWatch forecasters say there’s been a notable reduction in frosts this year, especially coastal areas.
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By Philip Duncan, WeatherWatch

El Niño is still developing. That’s the latest in the August update from the Bureau of Meteorology and also NIWA. 

This means New Zealand and Australia remain in a “neutral” weather pattern. Typically La Niña creates more easterlies for NZ and Australia and low pressure in the Tasman Sea, while El Niño creates more westerlies for our two nations and an uptick in high pressure in the Tasman Sea. 

A neutral pattern means we have no driver from any particular direction, so we can have a bit of an oxymoron where a neutral pattern = chaotic. 

El Niño also tends to bring cooler sea surface temperatures to the Tasman Sea, and while that has partially happened further away from NZ, locally we still have a marine heatwave. 

Some don’t understand how much impact that has on our weather, but if you add just a degree or two to the sea it can turn small lows into bigger storms, it can turn showers into thunderstorms or flash flooding and it can keep NZ slightly milder around coastal fringes at night. We’ve certainly noticed a reduction in frosts this year, especially coastal areas. 

While our pattern is neutral and chaotic, we may still notice some patterns forming. El Niño is not officially here yet on our side of the world, but there are hints that it is showing up in our atmosphere with an increase in high pressure over Australia and more westerlies in NZ. You could also argue that’s similar to an early spring weather pattern.

But even when El Niño is officially declared here, it may not be a “normal” one. That’s because right now the sea surface temperatures not only around NZ, but also in the western equatorial Pacific, are above normal. 

Usually they are normal to below normal in El Niño and that’s part of what links our atmosphere up with it. So this warmer water on our side of the world is acting a bit like the “one step backwards” for the El Niño that is trying to form.

Put simply, NZ is actually in quite a positive phase of weather from a farming and growing point of view. We no longer appear stuck in a rut as we were earlier this year with the persistent flood events hammering the same eastern and northern areas. 

The east and north of the South Island lean drier than usual during an El Niño spring, while a big chunk of the North Island leans drier. Those areas wetter than normal will be Southland, the West Coast and potentially parts of Auckland and Northland, which get a lot of showery westerlies in spring – but they tend to fade more in summer. Image Credit: NIWA

We’re seeing longer dry spells and temperatures generally above normal. Rainfall is continuing, and our latest ClimateWatch update at talks about how rainfall will continue on for most regions heading into spring – but perhaps a little below normal for the North Island and upper South Island. That may mean a softer, more gentle transition from being super wet to drying out.

Upcoming weather highlights

• Monday and Tuesday kick off with another colder front in the South Island

• NZ generally leaning drier than average this week

• More high pressure for the North Island

• High pressure from Australia dominates NZ mid-week

• A sub-tropical low late week may bring easterlies to northern NZ

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