Saturday, December 2, 2023

Deciphering patterns between patches of chaos

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One big high pressure bubble after another drips out of the Indian Ocean / southern Australia area and moves over New Zealand.
Rainfall from the first half of May shows signs of a more classic westerly leaning rainfall pattern, away from the wet easterly pattern of La Niña and early 2023. Photo: Nur Andi Ravsanjani Gusma
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By Phil Duncan, WeatherWatch senior forecaster

When I was a teenager and falling further in love with weather forecasting, Bob McDavitt, a MetService weather ambassador at the time, taught me something valuable which took me years to really appreciate: “Weather is a mixture of pattern and chaos.”

It’s not something that teenage Phil got his head around, but over the years (decades now actually!) I’ve really seen what Bob was talking about. The year 2023 is absolutely kicking off with a mixture of pattern and chaos.  

The pattern? The regular huge high pressure zones coming out of Australia. Like a leaky faucet with very slow falling drips, we’re seeing one big high pressure bubble after another drip out of the Indian Ocean / southern Australia area and move over New Zealand. 

But the chaos is occurring in between these highs – with lows forming in the sub-tropics, tropical cyclones earlier this year, squash zones each month and the now over-used term “atmospheric river”. 

Basically, the highs both bring us a regular pattern and contribute to the chaos in between.  

May so far has been caught up in this chaos, and sometimes it feels like we’re just being bombarded by storm after storm. But when you look at the monthly and weekly historic weather maps you can start to see the current pattern emerging. La Niña feels like it’s lingering in New Zealand because it’s milder and wetter – but the NIWA rain map for the first 17 days of May paints a picture of rain shifting to the western side of NZ and eastern areas getting lower totals. That’s not just a sign of classic autumn, but it’s also perhaps the precursor to the likely building El Niño.

El Niño is measured at the equator and if you look at sea surface temperatures towards South America they are warming up significantly. While the sea surface is currently showing strong signs of a powerful El Niño building, the atmosphere is not. It’s a bit like boiling water on a stove – the element surface heats up fast, but the water above is still cold. That’s what we’re seeing in the atmosphere right now – sea temperatures rising, but the atmosphere yet to catch on.

Scientists still expect El Niño to officially show up in the coming few months – and if you look at Australia’s weather pattern they’re already looking more like El Niño with exceptionally long dry spells, plenty of windy and showery sou’westers and some big frosts inland (around Canberra and even as far north as the highlands of Queensland).

El Niño’s pattern tends to be more high pressure in the Tasman Sea and more sou’westers into NZ – which can lead to inland and eastern droughts. The chaos from El Niño can be when it dredges up Southern Ocean storms and Tasman Sea squalls.

Upcoming highlights

• A windier week with westerly to southwesterly winds to gales in exposed places

• Snow on the ranges, especially the South Island

• High pressure briefly brushes northern NZ this week

• More westerly driven wind/rain/showers this weekend/start of next week

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