Insights & News

The Far-Reaching Stories of Storms ‘Ciara’ & ‘Dennis’

14th February 2020

Early to mid-Feb 2020 has proven tempestuous for parts of northern Europe.

During 9th-11th, storm ‘Ciara’ (as named by the UK Met Office; UKMO) brought widespread disruption, initially to the UK and then to many neighbouring countries in Europe. At least 9 fatalities have been attributed to the storm’s strong winds and, in places, flooding rains.

The core of the storm system was unusually intense as it brushed past Scotland on 9th; a top-10% event in the records.

Now, as I write, this, the next name in the UKMO’s list has been assigned – ‘Dennis’. The recipient is a storm-to-be, that’s expected to reach exceptional intensity while moving to a point a few hundred miles southwest of Iceland during 14th-15th Feb.

 

‘These extraordinary storm systems are a consequence of a sequence of far-reaching connections – in both space and time. Read on for expert insight into what led to what…’

 

Some of the best-performing forecast models have predicted a core minimum sea-level pressure (SLP) close to the all-time low record for the North Atlantic of 915 mb! Lower SLP generally indicates a more intense cyclonic storm.

A couple examples of what forecast models were predicting for storm ‘Dennis’ as of 12th Feb 2020. The red numbers indicate the minimum sea-level pressure of the cyclonic storm.

A couple examples of what forecast models were predicting for storm ‘Dennis’ as of 12th Feb 2020. The red numbers indicate the minimum sea-level pressure of the cyclonic storm.

Despite being much more distant than ‘Ciara’ was, storm ‘Dennis’ is expected to bring further strong winds and heavy rain to the UK.

With the start of 2020 seeing pervasive discussion of climate crisis, the weather can seem increasingly chaotic and perplexing – so let’s look at the science behind these storm events.

These extraordinary storm systems are a consequence of a sequence of far-reaching connections – in both space and time. Read on for a whistle-stop tour of what led to what, – covering the polar vortex, jet stream and equatorial temperature gradients.

 

‘With the start of 2020 seeing pervasive discussion of ‘climate crisis’, the weather can seem increasingly chaotic and perplexing – so let’s look at the science behind these storm events.’

 

Tropical-Arctic Connection:
T-storm Patterns Permit Stronger Polar Vortex

 

If you were to take a flight right around the equator, you’d see long stretches (thousands of miles across) of countless towering thunderstorms, separated by similarly wide expanses of relatively small clouds or even clear skies.

These thunderstorms release a lot of energy into the atmosphere, which then influences weather patterns all the way to the polar regions.

When focused in certain key areas, they drive weather patterns that drive huge atmospheric waves into the stratosphere above the Arctic. Between October and April, these can weaken or disrupt a powerful circulation known as the polar vortex.

The thunderstorms have rarely been in such locations during Dec-Feb 2020. (As a result, the stratosphere warming events have been few and far between.) With so little to hold it back, the polar vortex has reached exceptional strength.

 

‘Those patterns of thunderstorms have been rare during Dec-Feb 2020. As a result, the stratosphere warming events have been few and far between. With so little to hold it back, the polar vortex has reached exceptional strength… increasing the likelihood of exceptionally strong cyclonic storms developing’

 

A strong polar vortex leads to stronger temperature gradients between the Arctic and mid-latitudes, which increases the likelihood of exceptionally strong cyclonic storms developing.

Let’s delve deeper into the how and why of this relationship.

 

A Powerful Polar Vortex Dominates

The polar vortex’s immense rotational force extends right through the atmosphere, from top to bottom.

Its circulation has a very big say in weather patterns hemisphere-wide. (As far equatorward as California, Spain and Japan, to name a few.)

The stronger the polar vortex, the ‘neater’ this circulation is. This has the effect of keeping surface low pressure systems largely to the north of the subtropical belt (e.g. Azores), leaving that region to be dominated by areas of high pressure.

This pattern of lows staying north of the subtropics is measured by an index called the Arctic Oscillation (AO). The more dominant the high pressure in the subtropics, the more positive the AO is (and vice-versa).

The plots below show how well the polar vortex strength and AO correlate, especially when the vortex is particularly weak or strong.

Plots of Dec-Feb mean polar vortex strength, based on mean west-to-east (zonal) wind speeds in the mid- to upper-stratosphere, versus Dec-Feb mean Arctic Oscillation index. Data: NCEP/NCAR reanalysis 1, 1950-2019.

Plots of Dec-Feb mean polar vortex strength, based on mean west-to-east (zonal) wind speeds in the mid- to upper-stratosphere, versus Dec-Feb mean Arctic Oscillation index. Data: NCEP/NCAR reanalysis 1, 1950-2019.

As you can see in the comparison of strong versus weak polar vortex states below, this has a huge effect on the distribution of relatively cold and warm near-surface temperatures.

Illustration of how the Dec-Feb mean surface air temperature compares between two sets of ten years: those with the strongest polar vortex (via zonal winds proxy) and those with the weakest. Data: NCEP/NCAR reanalysis 1, 1950-2019.

Illustration of how the Dec-Feb mean surface air temperature compares between two sets of ten years: those with the strongest polar vortex (via zonal winds proxy) and those with the weakest. Data: NCEP/NCAR reanalysis 1, 1950-2019.

The patterns on display are largely to do with the frequency and duration of cold air movements out of the Arctic region. When the polar vortex is weak and AO negative, these cold air outbreaks are relatively frequent and prolonged (recent examples affected Europe in 2009-10 and the USA in Jan-Feb 2019).

This strong polar vortex, with very positive AO, keeps these outbreaks to a minimum, ‘locking-in’ cold air across the Arctic.

So, what’s all this got to do with storms ‘Ciara’ and ‘Dennis’?

Steep Temp. Gradients Supercharge Cyclonic Storms

Well, they’re what’s known as extratropical cyclones, which are a type of storm that depend, in part, on… baroclinic instability for intensification. This measure is proportional to (i.e. increases with) the steepness of temperature gradient between two points.

So, the more rapidly the temperature changes between two points, the greater the potential intensity of any cyclonic storm that develops in response.

The greatest divergence can be found beneath the southwest or northeast regions of a strong section of jet stream (high-speed winds way above the surface; more detail here).

The faster the section of jet stream and better-placed the cyclone, the more a storm will intensify in response to a given amount of baroclinic instability.

Due to various laws of physics and the rotation of the Earth, the fastest jet stream winds routinely occur where there are long stretches of steep temperature gradient between cold air on the poleward side and warmer air on the other.

Due to various laws of physics and the rotation of the Earth, the fastest jet stream winds routinely occur where there are long stretches of steep temperature gradient between cold air on the poleward side and warmer air on the other.

It follows that a pattern that tends to keep it colder in the Arctic and warmer outside of it – e.g. when there’s a strongly positive AO – will support a stronger jet stream.

Feb 2020: Extreme Temperature Gradient Drives Exceptionally Strong Jet Stream

As the following plots show, this year has seen record-high temperatures across the mid-latitudes (which include most of the USA, Europe and Japan), yet Arctic temperatures have been typical of the 21st Century so far.

Plots showing the mean air temperatures between 1st Dec and 9th Feb, for years 1951-2020, for mid-latitudes (left) and within the Arctic circle (right). Notice the lower Arctic temperatures, contrasting with much higher mid-latitude temperatures, in 2019-20.

Plots showing the mean air temperatures between 1st Dec and 9th Feb, for years 1951-2020, for mid-latitudes (left) and within the Arctic circle (right). Notice the lower Arctic temperatures, contrasting with much higher mid-latitude temperatures, in 2019-20.

This has resulted in the largest mean temperature difference between the two regions since 1998.

Plot of the difference between the two previously shown statistics (Arctic mean temperatures subtracted from mid-latitude mean temperatures). It’s been a while since the difference was quite as large as in 2019-20.

Plot of the difference between the two previously shown statistics (Arctic mean temperatures subtracted from mid-latitude mean temperatures). It’s been a while since the difference was quite as large as in 2019-20.

Plot of the difference between the two previously shown statistics (Arctic mean temperatures subtracted from mid-latitude mean temperatures). It’s been a while since the difference was quite as large as in 2019-20.

It’s fair to say that we’ve become used to a weaker polar vortex in recent years, much as the temperature gradient trend implies. When identifying the weakest ten winters (Dec-Feb) for mean polar vortex strength for the earlier versus-AO analysis, seven of those ten were in the 21st Century!

‘It’s fair to say that we’ve become used to a weaker polar vortex in recent years, much as the temperature gradient trend implies. When identifying the weakest ten winters (Dec-Feb) for mean polar vortex strength for the earlier versus-AO analysis, seven of those ten were in the 21st Century!’

So, it appears that storms ‘Ciara’ and ‘Dennis’ are part of a fluctuation in the temperature gradient back toward what used to be possible back in the 19th Century.

However, it’s not that straightforward. You see, cyclonic storms can also draw upon the energy of latent heat release, which occurs when water vapour condenses into droplets. So, the more moisture-laden air there is available, the stronger the cyclone may become…

Is Climate Change an Accomplice?
Rising Temperature – Rising Moisture…

The warmer air is, the more water vapour it can contain. For every 1°C temperature rise, that capacity increases by about 7%.

Global-mean temperatures have increased by about that much since the mid-20th Century. This suggests increased available air moisture for storms like ‘Ciara’ and ‘Dennis’ to draw upon.

Just how much this adds to intensification is hard to gauge. Research studies into deep cyclones have produced mixed results; it depends a lot on the historical dataset used.

I find this lack of clear trend to be interesting, though, considering the reduction in temperature gradients illustrated earlier, and the high frequency of weak Dec-Feb mean polar vortex strength in the 21st Century so far.  Based on that alone, one would expect decreasing long-term trends in storm intensity and strong storm counts. Perhaps, increasing available moisture is counteracting these two influencers?

If this is on the money, then we’ll really have to watch out for exceptionally strong extratropical cyclones during strong polar vortex winters in the years, and decades, to come.

‘This suggests increased available air moisture for storms like ‘Ciara’ and ‘Dennis’ to draw on …If this is on the money, then we’ll have to watch out for exceptionally strong extratropical cyclones during strong polar vortex winters in years, and decades, to come.’

James Peacock MSc
Head Meteorologist at MetSwift

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