The North Atlantic Ocean dominates the climate of Northwest Europe. Across each year, winds are predominantly westerly, bringing mild to warm conditions, with spells of wet and windy weather coming and going.
Just how predominant this weather pattern is depends on the seasons. It tends to hit a minimum May-Jul, before increasing toward a peak in Dec-Jan.
So, late summer into early autumn, Northwest Europe typically expects to see an increasing number of wet and windy spells of weather.
BUT for the UK and northern France, that trend has been on steroids in 2020, with much windier than usual conditions in recent weeks. In the final week of August, storms ‘Ellen’ and ‘Frances’ battered the region with abnormally strong winds for the time of year.
Such weather often brings disruption to transport and sometimes other infrastructure such as power networks. That entails an economic cost that many businesses will be especially vulnerable to during these times of pandemic-driven restrictions.
Are these vigorous storms a harbinger of a wild Autumn 2020 for Northern Europe? Let’s explore.
Looking to the Past for Guidance on the Future
The world’s weather patterns are influenced by a variety of large-scale phenomena, known as teleconnections. These range from ocean temperature patterns to energy transfers occurring way above our heads in the stratosphere.
As an example, shown below are some observations for a selection of three ocean temperature teleconnections. The first three measures (blue lines) are all observations of the El Niño Southern Oscillation (ENSO), while the other two are standalone.
Each of these has warm and cool phases. The Atlantic Multidecadal Oscillation (AMO) is in its warm phase, while the Pacific Decadal Oscillation (PDO) and ENSO are in their cool phases.
All are expected to remain in their respective phases for autumn 2020. This can then be used to isolate observations from ‘best-fit’ historical records. Combining these produces composites that provide us with guidance on how weather patterns are likely to behave relative to ‘normal’ (i.e. the long-term average).
Let’s see what those three oceanic teleconnections suggest for the remainder of September 2020 in Europe:
The big area of much lower than average sea-level pressure (blue to purple shading) in the left-hand plot corresponds to storm systems being more frequent and stronger than usual in that area.
Those bring wet and windy weather, so it’s no surprise that the right-hand plot shows above-normal mean wind speed in that area (yellow shading). This reaches across the UK, suggesting more frequent bouts of wet and windy weather than usual. In between those, predominantly mild westerlies may allow for some warm days. Overall, a very mixed month.
Rainfall (not shown) is above average for a wide swathe from Northern Ireland to Finland. Disruptive flooding and strong wind events are more likely than usual in this setup. By contrast, Portugal, Spain and Italy are all drier than normal.
A Turn of the Tables for Oct-Nov
Compared to the September signals, there’s a fascinating contrast to behold:
For the northern half of Europe, this is an almost perfect inverse to the September pattern! So, it generally looks ‘calmer’ (and drier) than usual there, especially in the UK.
Meanwhile, Europe sees more wet and windy weather than average. In mid-late autumn, that can mean a lot of rainfall, with a risk of flooding, potentially widespread. Strong winds may also cause some disruption this season – wind gusts can widely exceed 50 mph during a strong autumn storm.
A Repeat of 2019’s Autumn Flooding?
Extreme rainfall was a feature of autumn last year for much of south-western Europe.
For example, Murcia, Spain recorded 185.8 mm during 5th-6th September, enough to cause severe flash flooding. Less than a week later, another spell of exceptional rainfall lead to widespread flooding in many parts of south-eastern Spain. There was widespread disruption to transport, including the temporary closure of the Murcia and Almeria airports. The event was of a kind locally known as a “Gota Fría Storm”.
In early November, flooding and strong winds in Italy and France led to some regional evacuations, interruptions to power supply and damage to some buildings.
Later in the month, a rare ‘medicane’ (strong cyclone bearing some resemblance to a hurricane) struck Greece during 12th-13th. Meanwhile, Venice was struck by its worst Acqua Alta (high water) flooding in over 50 years due to unusually strong winds funneling ocean water up the Adriatic Sea.
In all cases, such severe weather forced rescheduling or cancellation of sporting and entertainment events.
As I blogged on early last December, it was a soaking wet autumn 2019 overall. I delved into the teleconnections in search of explanations and identified several leading factors behind the wet conditions. They are the Western Hemisphere Warm Pool (WHWP), Atlantic Multidecadal Oscillation (AMO) and Tropical North Atlantic index (TNA).
All three have positive correlations with autumn rainfall (e.g. higher AMO, more rainfall).
The table below shows how they have behaved in the past 12 months.
Alarmingly, as we head into autumn 2020, the WHWP and TNA are both very similar to how they were last autumn, while the AMO is even more positive.
Long-range guidance comes with inevitable uncertainty, but with so many factors pointing in the same direction, it seems fair to conclude that the potential for severe weather is looking abnormally high for southern Europe during Oct-Nov 2020.
I sincerely hope this potential does not bear fruition. Now more than ever, we don’t need more economic stress.
James Peacock MSc
Head Meteorologist at MetSwift