What has caused the Beast from the East and Storm Emma?
This week (26th Feb-4th March) we have all felt the thrill of childish excitement brought on by the snow. Many of us have had snow days as trains were delayed and cancelled, roads were jammed and icy, and schools closed. Facebook and Twitter were full of everyone’s snowy landscapes. Just as the weather had seemed to calm down, we have head a second wind (pun intended). So what caused this unusually cool March weather?
Part 1: The Beast From the East
How does air circulate?
Three things determine the movement of air: gravity, the sun, and the Coriolis effect. Gravity pulls the air to the Earth. The sun warms the Earth, it is currently radiating, causing the air above it to warm and in turn expand and rise. As air rises it cools, becomes denser and sinks, creating the air circulation cycle. As the air circulates across the Earth, it follows a three-cell circulation model. The first cell is the Hadley cell, which brings warm air from the equator to around 30oN (or S of the equator); secondly, there is the Polar cell, that brings cool air from the poles to around 60o; finally, there is the Ferrell cell that transports the sinking at 30o to the rising at 60o.
Instead of the air moving directly north to south (i.e. from the poles to 60o) the spinning of the Earth causing the air to be deflected. To complete a full spin, the Earth must travel a greater distance than the poles, resulting in the equator travelling at a faster speed. When the air moves northward from the equator the change in speed causes the air to curve westward (in the southern hemisphere the air curves in an eastward direction).
When we look at the three-cell circulation model associated with air circulation we see that cold air sinks over the poles. This mass of cold air is referred to as a polar vortex. However, frequently the polar vortex can weaken and in extreme cases split into two or more vortexes. This is referred to as a ‘sudden stratospheric warming’.
What is Sudden Stratospheric Warming?
Jet streams are high up in the atmosphere, but sometimes weather patterns in the lower atmosphere can cause disruption. Resulting in the Jet Stream to ‘wobble’ or have ‘waves’. If these are strong enough they can cause the westerly wind to reverse (east to west). When this occurs the air in the stratosphere sinks and compresses into the Arctic, causing it to warm.
Daily Arctic Temperatures– the red line 2018s higher than average temperature. (Graph: Zachary Labe, 2018)
This is why the Arctic is currently seeing higher temperatures than normal; December 2017 through to January 2018 had an Arctic temperature 7oC warmer than the 1981-2000 average. The warming of the Arctic has actually resulted in the polar vortex splintering and fracturing.
When the next set of ‘wave’ hits, due to the new direction of the wind (east to west) the waves break at a lower altitude. This continues, with the waves breaking at lower and lower altitudes until the waves break in the section of the atmosphere that weather occurs in. This causes the lower atmosphere to follow the same wind direction as the higher atmosphere (the surface wind direction across the UK is East-West). This in addition to the split Vortex, results in cold air from the East to flow across northern Europe and increasing the risk of cold weather (including snow).
Part 2: Storm Emma
The UK is now being hit with a storm from the West, known as storm Emma. This has resulted in warnings being issued all across the South West of England. Emma has gathered the moisture as it has travelled over the Atlantic Ocean before hitting into the cold air mass (as described above). This amount of moisture mixing with the cold air results in the second coating of snow.
The combined snow has resulted in human impacts for the country. (At the time of writing) rail passengers have been advised against travel, 1,250 flights have been cancelled on Friday, Motorways are closed and motorists stuck in the snow. However, at the moment these impacts will continue, please find updates here.
As Climate Change has resulted in Arctic temperatures increasing and sea ice receding year upon year, what possible weather futures can we expect for the UK?
- Will this result in the Polar Vortex splitting more frequently?
- Will we see the cooling of UK temperatures and snowfall as a more frequent occurrence?
- Will the UK start investing in snow-proof infrastructure to be able to fully function during times of high snowfall? If so, what should the UK government prioritise first?
Update: Beast from the East 2
Eastern England has been given yellow warning for snow over the weekend of 15-17th March. The weather is replicating that seen previously with the ‘Beast from the East’ from two week earlier.
This is because the wind and weather patterns are following the same path (from the frozen tundra of Siberia) due to the split polar vortex caused by sudden stratospheric warming. In the original post I queried whether climate change may see this phenomena occurring more regularly. With this already being the second snowfall this spring, from sudden stratospheric warming, should we prepare for more next year?