Will we have a cold winter?
Will we have a cold winter?
The chances of a cold winter plus my winter preparations in the garden
A couple of weeks ago, frost was forecast and I was racing around the garden, moving pots of lemons and salvias into the greenhouse, taking cuttings and generally preparing for cold weather. We got away with it. And now, of course, mild and very wet weather has returned.
But a chance conversation with Dee who helps me in the garden, got me thinking about the coming winter. She mentioned that her mother had been reading some meteorological reports including those relating to the polar vortex.
What does the polar vortex have to do with our winter weather?
Quite a lot! I remember the term from my A level geography. The polar vortex is a large area of cold air and low pressure that swirls around both the Earth's poles. Think of it as a spinning dome of cold air. This cold air is trapped by strong winds that circle the pole, keeping it contained. The stronger the winds, the more the air is trapped and the colder it gets.
There is an interaction between the polar vortex and the jet stream, the jet stream being a fast moving current of strong winds that move from west to east. When the polar vortex is strong and stable, the jet stream shifts to the north and the coldest air stays in the Arctic, resulting in milder winters for North America and Europe. But when the vortex weakens, the jet stream takes a more wavy path and this lets cool polar air to move south, while in other areas, warm air moves north.
This diagram from NOAA Climate explains it well.
During winter, the polar vortex can weaken or get disrupted, which allows very cold air to move southward into areas like North America, Europe, and Asia. When this happens, it can lead to much colder than normal temperatures in these regions, sometimes causing severe winter storms. I remember visiting Savannah in Georgia in February 2023 after a large mass of Arctic air had moved down and brought extremely cold weather to the southern states, which amongst other things, ruined the camellia blooms.
The polar vortex builds in autumn as the Arctic get cooler (the sun is lower so less heat energy reaches the North Pole) and meteorologists report that the new polar vortex is build more slowly than usual and is predicted to be weaker. This means a weaker jet stream and more likelihood of colder snowy weather in North America and Europe.
Sudden Stratospheric Warming Events
Another factor is something called a Sudden Stratospheric Warming Event. You may not of heard this term before, but I bet UK readers remember the Beast from the East which hit the UK in March 2018. This and the unseasonal cold weather in the southern states of the USA in winter 2022/2023 described above are classic examples of SSWEs. They are caused by a sudden rise in the temperature of the polar stratosphere in winter which weakens the polar vortex, disrupts the jet stream and releases a mass of cold air over North America and Europe.
La Niña is coming….
And something else to consider. We are entering a La Niña phase. Unlike the El Niño, this is a cold phase and meteorologists have already reported areas of cold air over the tropical regions of the Pacific. This too can result in the collapse of the polar vortex and a Sudden Stratospheric Warming Event.
What is the outlook for 2024/2025?
Taking all these points to take into consideration, I think there is a greater chance of a colder winter this year, so I probably won’t have to worry about chill hours for my fruit trees and tulips. However, I do have to think about my tender plants.
Preparing the garden for winter
My gingers are all Hedychium coccineum 'Tara' which is described as being cold hardy. The original seeds were collected from Nepal in the 1970s by Tony Schilling of Kew Gardens. The pots will be moved into the barn, but the rest are in the ground.
I have a selection of cannas too, some in containers but most in the ground with the gingers. They are very late this year, and some are only just coming into flower. Again, these plants may have a tropical origin, but they are said to be hardy to -10C or lower.
I’ll leave the gingers and cannas in the ground under a heavy mulch. They are planted near the base of a south facing stone wall, so are in quite a sheltered position. In addition, the stones act as a heat sink during the day, radiating heat at night and helping to keep the temperature a few degrees above what it would be otherwise. In fact, the greater risk is mild, wet winters, as last winter, which caused several of my ginger rhizomes to rot.
Other plants needing protection are my Japanese bananas (Musa basjoo), although they are said to be hardy to -15C. They survived -10C in 2022 but I do prefer to wrap them, if nothing else to protect the fronds from wind damage. First, I wrap the stems in fleece and secure this in place with hessian.
So at least I am prepared!
Happy Gardening
Sally
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Talks
I am talking at a number of venues in the coming weeks. Most of these clubs allow non members to attend for a fee.
19 November Mayfield Horticultural Society by zoom
21 November University of Bristol Botanic Gardens
23 November I am talking at the inaugural LandAlive conference at the Bath and West showground on Sat 23 November. The topic is on creating a climate resilient allotment.
4 December Castle Cary Gardening Club
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Really interesting article, hadn’t realised how the polar vortex impacted the jet stream !