What controls the weather in the UK?
The polar jet stream influences weather in the UK. Jet streams are fast flowing winds about five to seven miles above the Earth’s surface that reach speeds of about 200 mph. They are part of the complex global wind system caused by temperature induced pressure differences. The Polar Jet usually flows from west to east bringing mild and humid air from the Atlantic Ocean to the UK. You can imagine the polar jet stream as a “barrier” between cold air in the North and warm air in the South. The jet stream does not remain in the same location but, rather, meanders like a river. This is one of the main causes for the highly variable weather in the UK.
What is the polar vortex?
We are part of the global weather system. In order to understand cold spell that happened across the UK in 2018, we have to understand the weather in the Arctic. In wintertime, the Arctic is incredibly cold due to the lack of sunshine. The temperature contrast between the Arctic and more southerly latitudes leads to great differences in air pressure and therefore, to very fast winds high up in the atmosphere. This results in a huge low pressure system spinning counterclockwise above the Arctic known as the polar vortex. From the side, you would be able to see this huge system reaching from the upper troposphere all the way into the stratosphere. The polar vortex can change its intensity as seen in the schematic.
While a stable polar vortex confines cold air to the Arctic, an unstable or wavy polar vortex may cause cold air to spill further south. This is what we experience as cold snaps. A so called sudden stratospheric warming event can lead to such an unstable polar vortex. A disturbance in the polar vortex leads to sudden air compression and warming. It rapidly weakens the polar vortex and causes it to wobble. If these wobbles split off, they can make their way towards Europe and bring cold, Arctic air across Europe. This has been the case in 2018 when the Beast of the East hit the UK. The recent cold snap in the US is also caused by an unstable polar vortex.
What role does climate change play?
The occurrence of cold conditions with an unusual amount of snow may create some skepticism about global warming. It is important to remember that one single weather event cannot necessarily be attributed to a change in climate. Sudden stratospheric warming events are an active area of research and many open questions are remaining. We do not yet understand the mechanisms that connect the stratosphere and the troposphere, which could help to forecast our weather during such extreme events. Not all sudden stratospheric warming events destabilize the polar vortex and affect midlatitude weather. We also do not yet know how these events may be influenced by climate change. What we do know is that what is happening in the Arctic does not stay in the Arctic. The rapid warming occurring in the Arctic influences atmospheric circulation patterns. Studies have found a connection between the recent loss of sea ice and the instability of the polar vortex (Kim et al. 2014). This could lead to more frequent cold winters across Europe and the UK like the one we were experiencing in 2018.
- Bridgman, H.A. & J.E. Oliver, 2006. The Global Climate System: Patterns, Processes, and Teleconnections. Cambridge University Press.
- Kim, B.W. et al., 2014. Weakening of the stratospheric polar vortex by Arctic sea-ice loss. Nature Communications, 5, 4646
- NOAA, 2019. https://www.noaa.gov/infographic/science-behind-polar-vortex