The heatwave striking large parts of Western Europe, responsible for more than 40 deaths in France alone, is caused by a weather pattern known as an Omega Block.
The Omega Block takes its name from the shape of the Greek letter Ω, in which a bubble of warm air (an anticyclone) becomes trapped between two low-pressure systems, disrupting the jet stream. Under normal conditions, the jet stream pushes weather systems steadily from west to east. During an Omega Block event, however, this flow is disrupted, and as a result warm, stagnant air becomes locked over a region. Omega Blocks typically last between three and ten days, though in some cases they can persist for weeks.
As a result of the high pressure (anticyclone) at the centre of the system, conditions become hot and dry. The high pressure also prevents cloud formation, leading to clear skies that allow temperatures to rise further. These are the conditions currently roasting France and Spain, where temperatures have exceeded 40 degrees Celsius.
By contrast, in areas affected by the low-pressure systems (cyclones) on either side of the pattern, weather can be cool and wet. Britain sits on the boundary between the high-pressure system and cooler air to the northwest. As a result, the south and east are experiencing intense heat while the north and west are seeing cooler, wetter conditions, according to the Met Office.
Is climate change to blame?
Scientists have not yet reached consensus on how climate change affects the frequency of blocking events. Generally, however, the scientific community considers that climate change is increasing the frequency and intensity of heatwaves.
Greenhouse gas emissions from the burning of coal, oil, and natural gas have raised the planet's temperature by approximately 1.3 degrees Celsius compared to pre-industrial levels.
This means that, starting from this warmer baseline, heatwaves now record higher temperatures than they otherwise would. In Europe, heatwaves are two to four degrees warmer than they would be without human-induced heating, according to Claire Barnes, a climate and extreme weather researcher at Imperial College London. As a result, when phenomena such as Omega Blocks occur, the heat can be significantly more intense.
Source: CNA
