Explainer-What is the 'Omega Block' causing Europe's intense heatwave?

BRUSSELS, June 23 - The intense heatwave engulfing Western Europe, resulting in more than 40 deaths in France alone, is being sustained by a weather pattern known as an omega block.

Here is what you need to know about omega blocks and whether climate change means they could become more frequent in the years ahead.

WHAT IS AN 'OMEGA BLOCK'?

An omega block takes its name from the shape of the Greek letter Ω — with a bulge of warmer, settled high pressure held between two cooler low pressure systems.

The "blocking" element refers to how the high pressure area of warm air gets stuck. Under normal conditions, the jet stream carries weather systems steadily from west to east.

But during an omega block, that flow becomes disrupted and can buckle dramatically north and south, isolating the pressure systems. Weaker steering winds and temperature contrasts in the atmosphere contribute to these slow-moving, locked patterns.

The result is that hot, still air gets lodged over the same area. Omega blocks typically last between three and 10 days, but can persist for weeks.

WHAT HAPPENS DURING AN OMEGA BLOCK?

Under the high-pressure area in the centre, conditions become hot and dry. The high pressure also suppresses cloud formation, resulting in clear, sunny skies that allow temperatures to climb.

It is conditions like these that are baking France and Spain, where temperatures have exceeded 40 degrees Celsius (104 degrees Fahrenheit).

Regions in the low-pressure areas flanking the heatwave, meanwhile, are more likely to see cooler, rainy conditions.

Britain lies on the boundary between the high-pressure system and the cooler air to the northwest - producing intense heat in the south and east, and cooler, wetter conditions in the north and west, according to the UK Met Office.

IS CLIMATE CHANGE RESPONSIBLE?

Scientists have not yet agreed upon how climate change is affecting the frequency of blocking events like this one.

However, the global scientific consensus is clear that climate change is increasing the frequency and intensity of heatwaves.

Greenhouse gas emissions, mainly from burning coal, oil and gas, have heated the planet by about 1.3 C since pre-industrial times.

That warmer baseline means heatwaves reach higher temperatures.

Europe is now experiencing heatwaves that are 2 to 4 degrees hotter than they would have been without human-caused warming, said Clair Barnes, a research associate in extreme weather and climate at Imperial College London.

As a result, when patterns like omega blocks occur, the resulting heat can be significantly more intense. REUTERS