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The Big Atmospheric Block of Autumn 2023

There has been a lot of talk about the stubborn weather pattern that has affected so much of New Zealand during the last week or so. There has also been some colourful meteorological words popping up on some of the media news websites, like “atmospheric river” and “omega block”. However, there is more to all this than meets the eye. We are actually seeing the affects of long term global warming impacting our daily weather and seasonal weather patterns. Let's take a look at this in more detail.
At the heart of the stubborn or stuck weather pattern that began the last weekend of April (28th & 29th) was a large anticyclone - an area of high pressure. Having slipped quietly across the country, it then stalled just to the east of the country and created the beginnings of a massive “brick wall” that would cause the weather systems to get stuck rather like a log jam in a raging river.

Weather map for May 4th 2023

A surge of cold air from well down in the Southern Ocean raced north across the Chathams and into the mid Pacific.

It passed over much warmer than normal seas ( 2-4 C ) above normal).This warmer pool of sea helped to increase the vertical motion within this cold surge. This rising air soon developed some spin thanks to planetary pressure forces - ie, Coriolis, etc) and formed a rapidly deepening area of low pressure. With such intense forward motion, the deepening low cut off in the upper level steering flow. Think of it like a wave breaking at the beach. This made for a spinning low that was in no hurry to move.

Sea surface temperature (SST) anomoly map. Warmer that normal seas in orange and red.

The high passing across NZ then encountered this “road block spinning low” further east of NZ and put the breaks on! At the same time a new low was forming up in the sub- tropics to the north of New Zealand and was heading southeast. Ahead of the new sub-tropical low, a good feed of very warm air in the mid levels of the atmosphere was surging towards New Zealand. This warmer air began to feed into the top of the high just east. This in turn was like pouring petrol on a bonfire. Areas of high pressure are essentially like big mountains of warm air. The taller the mountain of warm air, the higher pressure at the surface.
Imagine a stack of plates. Holding a stack of three plates is not that heavy to hold, but three hundred - well that is a different story!
This is a big mountain of warm air or “upper ridge”.

500 hpa temperature map May 4th 2023 (Warmer air in yellow and orange)

The “road block spinning low” created a log jam -
this is known as an atmospheric block.
When these upper ridges cut off in the upper wind flow to form a big circle in the shape of the greek letter Omega they can also be know as an omega block.
How does this all relate to our weather? Our weekend high was about to become a high that would become a mega high 1042hpa in strength. Intense at any time of the year, especially Autumn. This also put NZ into a warm, humid and moist northerly airflow. The initial sub-tropical low brought rain and gales the northern parts of the country by late weekend but that was only the beginning. As more warm air in the mid levels spread into the upper ridge, the high just dug its heels in and refused to move. Rather like a huge rugby prop digging in at the start of a goal line scrum!

Quite often, stubborn and persistent atmospheric blocks can bring about extreme weather events because the weather is intensified and made to linger across a particular location for an extended period of time. In NZ many will remember extreme rainfall events such as the Nelson floods and more recently the Auckland floods in the Summer of 2023. They were all enhanced and intensified because of these atmospheric blocks.

So how does this all relate to changes in our planet and global warming?

In the last several hundred years the planet has increased in temperature (on average)

just over 1 degree C.

Many people do not realize that in addition to the more rapid warming at the poles, there is a growing mass of warmth across equatorial regions. This is not just at the earth’s surface, in fact there is more warmth further up into the mid and upper levels of the atmosphere.

Global mid-level (500 hpa) temperature map 6th May 2023. Equatorial warmth in orange & red.

At these levels the seasonal cold surges do not have as much affect on cooling the atmosphere, so the heat just continues to build and expand further north and south.

It is this heat that we now see extending further south into regions like Northern Australia, the Coral Sea and the Southwest Pacific.

It was indeed, this warmth that was picked up by the developing sub-tropical lows which then spread southeast into the the upper ridge of high pressure.

This is now happening more frequently and will continue to do so as the planet warms further in the future.


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