Across the European continent, the first half of this summer has been one of weather extremes, from temperatures lower than seen for decades in parts of the northwest, to severe thunderstorms in the Low Countries, to heatwaves in the southeast.
Attribution of such weather is not an exact science, but through studying historical connections between sea surface temperature patterns and European weather, some explanations can be found. Read on for the demonstration!
By some measures, June 2024 was an extraordinary month of weather in Europe. Characterised by sea-level pressure exceptionally far below average in the region between Iceland and Scandinavia, a wide swathe of the continent experienced excessive rainfall, while eastern parts were abnormally hot.
Only the far northwest and southeast escaped the very wet weather, but while the northwest saw unremarkable monthly mean temperatures, it was baking hot in the southeast. That hot and dry combination would set the stage for more to come in July – more on that later.
What can possibly explain such an unusual outcome? Applying some meteorologist experience, I’ve had a long look at the behaviour of sea surface temperatures in key regions of the planet and picked out those which show the greatest connection to what we’ve seen in June 2024.
In this map of mean sea surface temperature anomalies across the past 30 days, the following stood out to me:
- A very warm Indian Ocean
- A very negative Pacific Decadal Oscillation (PDO)
- Cool to neutral El Niño Southern Oscillation (ENSO)
- Very warm southern and (especially) central North Atlantic, associated with a very positive Atlantic Multidecadal Oscillation (AMO).
Of these, all but the warm Indian Ocean demonstrated an appreciable connection to the observed sea-level pressure pattern of June 2024. Using those three together produces a composite that you can see below right, next to the observed pattern.
Clearly, this is not a perfect match, with a large difference in the Arctic region, but the similarity for northern Europe is striking.
Analysing the individual contributions, most of the low sea-level pressure anomaly in the UK-Scandinavia region is explained by the very negative PDO. The very positive AMO and warm central North Atlantic corresponds to a high anomaly instead, in fact one that spans most of Europe, while the cool to neutral ENSO has no significant influence.
The low sea-level pressure anomaly over much of southern Europe is slight enough to sit within the ‘normal range’ a.k.a. ‘natural variability’.
I therefore suggest the very negative PDO has played a significant role in June 2024’s European weather pattern. The PDO was also very negative during the much hotter June 2023 but was seemingly outweighed by a positive ENSO state (El Niño event; anomalously warm central and eastern tropical Pacific), which encourages high pressure across and around Scandinavia.
In many ways July has continued where June left off, with the notable exception that very wet weather has extended across the UK (stopping just short of Ireland). It’s been even more anomalously hot than June for the southeast, where a severe heatwave has sadly taken hold. Some cities have reached 40°C for an unprecedented number of days running.
Using the same sea surface temperature regions as before, the resulting composite is again of great interest.
This time, it’s only the Arctic that’s much different, with both the observed and composite featuring widely below normal sea-level pressure across Europe, with a focus across the northern half. While we’re only halfway through July and the monthly mean pattern could feasibly be quite different, forecast models currently suggest there won’t be much change (see below).
As with June, this pattern is best explained by very negative PDO, but it’s a much weaker correlation this time. Even allowing for some support from a cool to neutral ENSO, it’s hard to attribute such low pressure-dominated July to those driving forces alone. Especially considering that again, the very positive AMO and warm central North Atlantic both encourage the opposite.
Perhaps this is a reminder that the calendar months are a human delineation that the natural world does not recognise: A more June-like behaviour lasting well into July 2024. An unfortunate outcome for the northwest of Europe, as it appears more in the way of dry and warm weather would otherwise have transpired there.
Barring unexpectedly large changes to the regional sea temperature patterns, the most favoured weather pattern for August is unfortunately familiar: More low pressure, hence rainfall, than usual in the northwest, with hot and dry weather concentrated on south-eastern Europe and parts of Iberia.
Before fans of dry and warm or hot UK weather despair, this is by no means a guaranteed outcome. August is the month when Atlantic tropical cyclone activity typically starts to ramp up, and these systems can force weather patterns to change their tune for anything between a few days and a fortnight.
There’s also a phenomenon called the Madden-Julian Oscillation, which is similarly influential, but tends to be scarce when ENSO is cool to neutral.
So, if you’d prefer to see more than fleeting ‘high summer’ UK weather in August 2024, best hold out hope for an Atlantic tropical storm or hurricane encouraging high pressure across north-western Europe.
Personally, I’m hoping we can at least avoid seeing yet another wetter than average month!
James Peacock MSc
Head Meteorologist at MetSwift
Featured photo by Mitchel Lensink on Unsplash
