Sudden Stratospheric Warming (SSW) is a meteorological event that can have a significant impact on winter weather, particularly in the UK and across Europe.
Sudden stratospheric warming is a remarkable atmospheric event that captures the attention of meteorologists and weather enthusiasts alike. Its occurrence can have far-reaching impacts on our weather, making it a key focus for seasonal forecasting and research at the Met Office.
But what exactly is a sudden stratospheric warming event, and why does it matter for our day-to-day weather? As we head into late autumn and winter when Sudden Stratospheric Warming events can impact our weather in the UK, and with some models suggesting the potential for an SSW event in early December, we’ll explore the science behind SSW events, how they develop, and what they could mean for the weeks ahead.
What is an SSW?
Sudden Stratospheric Warming (SSW) is sometimes linked to the onset of cold weather in winter. The term SSW refers to what we observe, rapid warming in the stratosphere, between 10 km and 50 km altitude.
You may have heard of the jet stream which helps to steer Atlantic weather systems towards the UK. Well, there are other jet streams high up in our atmosphere in both the northern and southern hemisphere which circumnavigate the Earth from west to east. One of these, the Polar Night Jet, circles the Arctic.
Sometimes the usual westerly flow can be disrupted by natural weather patterns or disturbances in the lower part of the atmosphere, such as a large area of high pressure in the northern hemisphere. This causes the Polar Night Jet to wobble and these wobbles, or waves, break just like waves on the beach. When they break, they can be strong enough to weaken or even reverse the westerly winds and swing them to easterlies. As this happens, air in the stratosphere starts to collapse into the polar cap and compress. As it compresses it warms, hence the dramatic stratospheric warming.
How does it move down through the atmosphere?
The relevant waves can only move around the Earth’s atmosphere in westerly winds. Fluctuations in our weather send waves up through the atmosphere to the easterly winds in the stratosphere, where they travel no further, and instead break and reinforce the easterly winds, bringing the easterlies lower. This pattern continues until the easterlies have moved down to the troposphere – the lowest part of the atmosphere where our weather is.
It can take anything from a few days to a few weeks for this process to take place.
What impact does this have on the UK?
We normally expect our weather to come in from the west – with a flow of relatively mild air coming in off the Atlantic.
When an SSW occurs it can switch the direction of the winds, bringing easterly winds which alter our weather patterns, weakening areas of low pressure and moving our jet stream further south. This leads to high pressure over the North Atlantic, ‘blocking’ the flow of mild Atlantic air and dragging in cold air from the continent. Exactly how cold it might be depends on the details of where the air comes from.
SSWs don’t always result in this outcome, but a cold snap follows more often than not, so the SSW greatly increases the risk of wintry weather.
Currently we can reliably predict individual SSWs about a week or two in advance, and we can detect them occurring with satellite data and other observations. This means we have some time to see how they develop and may impact our future weather.
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