1News weather expert Dan Corbett explains what’s caused record high temperatures followed by flooding and snow across the country.
This week has been another active and boisterous time in weather terms.
One day earlier this week, it was near all-time record wind gusts, the next it was record September warmth. Then came the flooding in the deep south, oh, and some snow warnings to top it all off.
It is almost like this spring has taken an injection of steroids.
Now, some might just say it’s just spring. But what exactly is spring? What makes it so different from the other seasons?
A quick refresher
Spring, also known as the season of transition or change, is the time when we have the greatest contrasts of warm and cold.
At the start of the season, the direct rays of sun are just passing south across the equator into the Southern Hemisphere.
Meanwhile, that big guy to the south over Antarctica – ‘Old Man Winter’ and his polar vortex - is coming to the end of a busy season.
Those first hints of warmth and, more importantly, daylight over the polar region begin the shift to the boisterous spring phase. Think of it like when they spray water on the Wicked Witch in the Wizard of Oz and she screams “I’m melting!”
That is what happens to the big lump of cold. It starts to break down, but as it does it flicks out the odd arm or leg (lobes) that surge north.
At the same time, the increasing warmth over the tropical regions of our hemisphere from the increasing sun angle really start to heat things up.
That’s where it all starts to get interesting. When that building warmth and surge of cold meet, it creates a massive contrast or gradient. This in turn starts making the active spring weather.
We often talk about wind in spring but when you think of wind, it is just the movement of air as the result of a temperature gradient. The bigger the gradient, the stronger the wind.
So that is spring and active weather. But this year we have had an added boost to make the season feel supercharged.
El Niño is hitting its straps
The last few weeks have also seen El Niño finally start to find its stride across the Pacific.
El Niño affects the distribution of warm and cold water across the equatorial Pacific which, in turn, affects weather globally.
You might be wondering how something out in the Pacific has an effect on our weather in New Zealand.
As El Niño affects the sea temperatures across the Pacific, it also influences where the areas of lift are and where those clouds and storms are located.
A case in point: Last year, during La Niña, the significant areas of lift for cloud and storm development were across the western Pacific, in particular Northern Australia into the Coral Sea and southwest Pacific Ocean.
On the other side of the Pacific, the opposite happened and there was tremendous warmth high up in the atmosphere and downward motion. This in turn inhibited clouds and storms. In other words, fine weather.
Fast forward to this spring and we have the complete opposite.
The warmth has been building up in the middle atmosphere from the eastern Indian Ocean and across Australia. Further east along the equator in the mid Pacific there are more clouds and storms than this time last year.
The extra warmth from the developing El Niño, as well as long term warming, which is increasing the mid-level equatorial warmth, have created a big dome of heat that has moved over New Zealand from Australia.
This happened last weekend just in time to go head on against a couple of lobes surging north from the fading winter polar vortex.
As the two air masses met earlier this week, a strong front formed with severe northwest gales on the northern side. The contrast of warm and cold was so significant that wind gusts across Aotearoa were well above normal with one gust close to the all-time wind record.
Then, in the warm northwest flow of air from Australia, warm and dry Foehn effect winds across the east of the South Island pushed the mercury to a September record for Timaru on Wednesday at 29C. The September average is 14C.
The extremes did not end there
A second polar vortex lobe that surged north into the massive heat dome brought further damaging winds in excess of 200km/h across the deep south on Wednesday.
The frontal boundary that approached the country from the southwest was also fuel injected with a warm moist flow from the tropics in the north Tasman Sea.
The massive heat dome put the anchors down to the east of the country to create a blocking high, which prevented the front from moving on.
This caused it to dump torrential rain in the same place for a longer period.
Milford Sound picked up 328mm of rain in a day.
Gore, which suffered significant flooding, received 92mm of rain in just over 12 hours on Thursday. Its normal rainfall for the entire month of September is 68mm.
Later that night, the pipeline of torrential rain began to migrate north, with the Southern Lakes region in the firing line.
Queenstown picked up over a month’s worth of rain in just a matter of hours. It too suffered significant flooding across its wider region.
Finally, that active front took a wintry turn by the end of the week as colder air began to push in across the deep south on the back side of the rain-bearing front. The dynamics in the front were so intense that the cold air pushed right under the warm pipeline of moist air on the active front.
The result?
A week that ended with heavy snow across the interior South Island after record high September temperatures just a couple of days earlier.
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