Dan Corbett: El Niño and what it might mean for spring and summer

Analysis: 1News meteorologist Dan Corbett takes a look at how our weather patterns are changing heading into the warmer months, and what parts of the country might be in for a pleasant summer.

A new season is upon us, and it could bring some big changes to our weather.

Spring as well as autumn are known as the seasons of transition, when the type of weather we typically get changes from cold rainy southerlies to warm northerly beach days (in spring).

The other big player in our spring and summer weather this year will be El Niño.

You may have heard about it earlier in the winter but not much has been said about it since. You will hear plenty of it over the coming weeks and months.

El Niño and La Niña are cyclical changes in the currents of the Pacific Ocean which in turn change the distribution of warmer and cooler water across the basin.

This change in sea temperature affects the way clouds and ultimately weather patterns behave across not just the Pacific but much of the globe.

El Niño is the warm phase because it creates a large mass of warmer-than-normal sea water off the equatorial Pacific Ocean.

On the opposite side of the Pacific Ocean near Papua New Guinea and Australia the sea tends to be cooler than normal in an El Niño phase.

Think of the analogy of a bathtub of very warm water. The steam rises up from the warm tub and fogs up the bathroom. That rising steam is similar to the rising motion of air above warm pools of air. More rising air creates more cloud and ultimately weather systems.

On the opposite side of the coin think of that bathtub but this time it is loaded up with ice cubes. Not much steam will rise from that bathtub. This is just like the cooler pool of sea water which tends not to favour cloud growth and instead helps create areas of fine weather and high pressure.

In a nutshell El Niño will produce warmer seas in the east, cooler in the west, clouds growing across central and eastern equatorial regions and drier weather with high pressure in the western equatorial regions.

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It is the opposite in the La Niña phase with cooler seas in the east and warmer in the west. This gives rise to more storms and rain over the west central Pacific and fine and much drier in the east.

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Last summer we were working through the third year of La Niña.

We all remember the non-summer we had up north and the extreme flooding events that hit parts of the North Island.

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Basically, El Niño was still developing. It is a bit like building a house - it does not go up overnight. Your first need a foundation, then the walls followed by the interior fit out.

In winter we also have something else rather big and imposing influencing our weather - Old Man Winter and the polar vortex.

It sends us all the cold outbreaks from the Southern Ocean during our winters. However, as the higher sun angle returns to the Southern Hemisphere over the coming weeks of spring, that will fade away until next year.

The first signs of El Niño, or the foundation, are the warming seas in the eastern equatorial regions.

This started to happen in autumn but more was needed to change the weather patterns to those more typical during El Niño events.

The walls and ultimately the roof of our house needed to go on.

This has been happening during the last month or so and will continue to ramp up in the coming months.

The extra parts of the El Niño house happen when the clouds in the atmosphere start to react to the warming seas. This has been starting to happen over the last few weeks. The late winter anti-cyclone has been larger than normal over Australia which has helped bring a drier than normal winter for many parts of Australia.

Take for instance Sydney, which will end up with roughly a third of its normal winter rainfall.

The drying has already begun across Australia with soil moisture levels running below average.

After significant grass growth from the record rainfall of the previous years in La Niña a shift to very dry and an El Niño summer could be a recipe for a significant bush fire risk in many parts of Australia.

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The clouds have also been building in the warmer than normal seas of the central Pacific.

The typical El Niño pattern tends to dry out Australia with a big dominant high which also stays a bit further north during summer.

Last summer with La Niña the highs bathed the South Island in sunshine while the north and east of the North Island coped the humid tropical air and rainmakers.

These should stay further north this summer but down in the deep south you might not catch a breath between passing fronts and spells of rain.

It is worth noting the last El Niño we had was during the summer of 2019.

Those down south may remember constant rain that saw flooding and high lake levels in December 2019 in Queenstown and Wānaka.

One thing that many people also forget is the jet stream in El Niño events can become more active, meaning some storms can pack a punch.

It's important to note seasonal forecasts only give a flavour of what is to come because the computer model resolutions are a lot more coarse.

There is also a wildcard that is coming into play more and more these days - weather extremes and long-term warming (global warming).

The long-term warming has not just warmed the seas and land but it has also added more moisture into the atmosphere, in other words, more rain in the clouds.

That is why when it rains lately it can be much more intense and lead to flash flooding more quickly.

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The other factor that often gets missed is when the weather patterns get stuck because of blocking highs and cut off lows in the upper atmosphere.

These weather patterns can happen more often with a warming atmosphere but can be hard to predict beyond a few days in advance.

Yes, there is plenty of time to get in some decent weather for all of us but it is safe to say that what our weather will be like is more likely going to be different from last spring and summer.

The El Niño pattern could bring more fine dry weather to northern and eastern parts of the country - say from Northland, Auckland, Gisborne, Hawke’s Bay and down to Canterbury.

The rest of the North Island should fare pretty well too but the far south-west like the West Coast of the South Island and deep south may see more weather systems pass by than last year.

This could in turn bring more wind and rain at times.

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If you are more interested in moisture for paddocks and orchards than just a fine beach day, it will mean drier than normal conditions the further north and east you are in the country.

In some previous El Niño's drought has even been on the cards but it is too early to say that just yet. The warmer seas and the wild card element could still bring some rainmakers further north, even compared to typical El Niño’s.

Here comes spring and El Niño.