A western-European heatwave of unprecedented intensity for the month, has already been breaking national June heat records as of 26th – and it’s set to reach even more extreme highs 27th-28th.
Not only that, but it’s breaking some all-time records as well. This is occurring over a month sooner than the typical ‘peak of the heat’ for the European summer season.
For example, in France, La Rochelle set an all-time record high of 40.5°C on 27th, beating the previous record of 39.7°C set 4th August 2003. The day before, Clermont-Ferrand had soared to 40.9°C, breaking its all-time record of 40.7°C set 31st July 1983. You can follow live updates on the heatwave in France here.
As I will describe in detail, a whole sequence of important events paved the way for this exceptional heatwave; an extraordinarily large transport of hot air from Africa to western Europe.
During early and mid-June, a concentration of powerful tropical thunderstorms journeyed eastward from the Indian Ocean to the West Pacific Ocean. This organised phenomenon is known as the ‘Madden-Julian Oscillation’ (MJO).
Such thunderstorms transfer a lot of energy from the lower to the upper atmosphere. That causes large changes in the way the atmosphere moves and circulates. By knock-on effects, these changes transport the energy poleward. When this movement is from the West Pacific, the energy interacts with the jet stream (fast winds at the top of the troposphere, which is the lowest layer of our atmosphere).
This interaction causes it to extend over the USA (see Figure 1), driving cool air from the North Pacific across regions such as the Great Plains. In the warm season, this is a very unstable situation, typically leading to severe thunderstorm outbreaks. That’s precisely what happened this June.
Figure 1: Illustration and summary of the sequence of events that led to the pattern responsible for the intense western-European heatwave of late-June 2019.
Like the tropical thunderstorms, these severe Great Plains thunderstorms moved a lot of energy into the upper atmosphere. This time, however, the jet stream was very close by, meaning it was affected very quickly and strongly.
The build-up of energy aloft is associated with an increase in temperature. The jet stream always divides warm air to its south from cool air to its north, so increasing temperatures to its south has the effect of pushing it northward.
So, a broad northward arc to the jet stream developed. It reached about halfway across Canada at its most northerly point. On the right flank of this arc, the jet stream was now surging southward, so much so that to the east of the northward arc, a southward arc also took shape.
‘The jet stream was very close by, meaning it was affected very quickly & strongly … now surging southward… a southward arc also took shape.’
This pair or arcs forms what’s known as a Rossby wave. The northward arcs are known as ‘upper ridges’ and are linked to surface high pressure areas. The southward arcs are known as ‘upper troughs’ and are linked to surface low pressure systems.
The increased north and south reach (meridional component) of one Rossby wave tends to cause the next one to its east to change in the same way. In this case, the one across the North Atlantic.
By this mechanism, the thunderstorm-sourced changes would already have been enough to set up a ‘typical’ strength heatwave over western Europe. But this year, another major phenomenon has got in on the act.
‘But this year, another major phenomenon has got in on the act.’
An unusual amount of high pressure and warmth has been occurring across the Arctic since early May. You can see this by the dominance of red shading across the central area of the map below (Figure 2).
Figure 2: 500 mb Geopotential Height (GPH) analysis chart from the European Centre for Medium Range Weather Forecasts, courtesy of tropicaltidbits.com. Generally, red shading corresponds to higher GPH and higher surface pressure, and blue shading to lower GPH and lower surface pressure.
Specifically, this chart shows the situation as this week began. Notice how the red shading has joined up with that taking place across the North Atlantic and Europe.
This interaction caused the upper ridge of the North Atlantic Rossby wave to increase its meridional component (i.e. northward reach) even further. In turn, the trough to its east extended south, reaching right down to the west of Spain.
Too Much Contortion? Cut It Out!
The North Atlantic Rossby wave was now reaching so far north and south that it could no longer sustain such a configuration. Just like a river that’s meandered too sharply, the eastward inertia of the jet stream winds resulted in the trough becoming ‘cut-off’, as illustrated in Figure 3.
Figure 3: Illustration of the formation of a ‘cut-off’ trough (with surface low pressure system beneath) during 24th-27th June 2019. The dashed line shows the starting jet stream path, and the solid line the finishing one. The red arrows indicate the movement of hot air aloft, and the orange arrows movement at the surface. The blue arrow indicates surface movement of cool air, which we’ll get to in the next section.
Cut-off troughs often ‘stall’ in the same place for several days, sometimes even a week or so. To their east, air generally moves northward over time.
A combination of the far-southward position of the cut-off trough and its lengthy stall (5 days as of 28th) allowed the surface low to drive an exceptionally large amount of hot air from northern Africa to western Europe. And so, that the intense late-June western-European heatwave of 2019 was born.
How’s the UK Escaped This Torching?
I’m glad you asked (…), as now I get to describe (and illustrate; Figure 4) how even this most exceptional of heatwaves was swiped aside by our old friend, high pressure.
‘Even this most exceptional of heatwaves was swiped aside by our old friend, high pressure.’
Figure 4: Surface pressure analysis chart from the European Centre for Medium Range Weather Forecasts, courtesy of tropicaltidbits.com, with own annotations overlaid.
A strong area of surface high pressure was hanging around to the northwest of the UK at the time that the cut-off trough first took shape. As the jet stream blasted eastward along its new found path well to the north of the trough, the ridge found itself with a nice wide path across the UK along which to extend.
A High Degree of Deflection
Now here’s the kicker. Air flows clockwise around a surface high. So, when a high is across the UK, a lot of air flows from the North Atlantic across the North Sea, then turns right again to cross parts of the UK or Northwest Europe. On this particular occasion, the shape of the ridge drove the air from the North Sea right across England and Wales. This flow also deflected the hot air away with ease.
‘Now here’s the kicker… This flow also deflected the hot air away with ease.’
This has left England and Wales reliant on sunshine heating the surface, driving some mixing of the upper and lower atmosphere. This got a small fraction of the hot air involved each day 26th-28th June. It was still enough to lift temperatures into the mid-high 20s each afternoon!
Only on 29th, as the high pressure at last moves away east, will the UK briefly see a surface flow from the roasting hot western Europe. That may allow temperatures to reach the mid-30s in some eastern parts of England.
‘Only on 29th… will the UK briefly see a surface flow from the roasting hot western Europe, potentially allowing temperatures to reach the mid-30s in some eastern parts of England.’
To everyone affected by this high-end heatwave, please do watch out for your health during these exceptional days. Among other things, there’s very strong sunshine to adapt for. I wish you all the best.
Head Meteorologist at MetSwift