Insights & News

Drier UK Summers Projected by 2070 – Why?

9th June 2020

I’m writing this in the wake of the sunniest spring on record for the UK (a gleaming 626 hours, beating 555 hours set 1948). Meanwhile, England beat its old record by a staggering 101.7 hours!

It was also extremely dry in many places, with some counties in north-eastern England and eastern Scotland recording their driest spring on record. For the UK overall, only four historical springs have been drier.

This came in stark contrast to a soaking wet winter with widespread flooding problems.

Parching Projections

The unusual weather has reminded me of an interesting study I read a few years ago. It showed the basis for a drier trend to UK summers in the decades ahead.

I’ll get to that later. For now, let’s look at what the Met Office’s UK Climate Projections (UKCP), updated Sep 2019, are predicting:

Table detailing the UKCP18 Projected Changes for temperature and rainfall in the Summer Season by the 2070s

These statistics are extracted from this information presented on the Met Office website.

Here, we can see that even in a low greenhouse gas emissions scenario, there’s a strong lean toward drier, warmer summers in all parts of the UK. Maps on slide 9 of the UKCP18 National Climate Projections slide pack reveal that this drier signal is strongest in the south of England and far-south of Wales.

In an associated publication of findings, the projections are summarised as:

“…an increased chance of warmer, wetter winters and hotter, drier summers.”

Note the use of the word chance. We’re not talking about a steady change in conditions here, instead an increased frequency (per period e.g. decade) of dry and/or hot summers. To give you a sense of what this means, this statement is given:

“…Climate change has already increased the chance of seeing a summer as hot as 2018 to between 12-25%. With future warming, hot summers by mid-century could become even more common, near to 50%”

Imagine that – on average, half the summers per decade being at least as hot as 2018 was (which was by some measures the hottest on record). Hotter means faster drying-out of land and vegetation. So, with unusually dry summers also projected to become more frequent, drought and wildfire risk will increase a lot.

Blame the North Atlantic…?

In 2015, a research study by J. Haarsma et al. examined a set of 15 global climate projections (coupled CMIP5 models). Like the UKCP projection, these climate models predict higher mean surface pressure (MSP) across the British Isles for the summer months. In fact, it’s the strongest pattern change in the whole of the North Atlantic and Europe. Just how strong is very uncertain, but some degree of associated drying trend in UK summers is favoured.

(See caption)

Alongside this, they predict that climate warming will be relatively small over a region of the North Atlantic centred some 2,000 miles due south of Greenland (explored further in this 2017 research study by Sgubin et al.). This is due to weakening ocean currents, linked to Arctic sea ice melt*. Historical analysis of similar past years found that this shows correlation with increased MSP in a region centred just west of the UK.

This also ties in with the theoretical atmospheric response to forcing by changes in ocean temperature. Essentially, a region of anomalous cooling leads to airflow changes that ‘pile-up’ air above a region just to the east. High pressure is a manifestation of such ‘piled-up’ air – the more air there is over a location, the heavier the air is overhead, leading to increased downward pressure.

Generally, more recurrent, stronger high pressure close to the west corresponds to more frequent and prolonged dry spells of weather in the UK. Sunshine is less straightforward, but at least a small increase in sunny conditions is probable.

Could This Apply to Spring or Autumn Too?

Research studies focus on either summer or winter, rather than the seasons between. So, this section features some speculative extrapolation on my part.

In March and often April too, a strong cyclonic (anticlockwise) circulation, known as the polar vortex, tends to influence Europe’s weather. This is akin to the winter season, when the relatively cool region in the North Atlantic can instead force wetter UK weather**. This is due to a southward shift in the typical path of low pressure systems (wet and windy). That implies March may in fact trend wetter, while April becomes more varied between years, depending on how fast the polar vortex decays.

By May, the polar vortex typically decays, so weather patterns begin transitioning to ‘summer mode’. I believe the Jun-Aug MSP response could be relevant in May too – especially the 2nd half. It’s possible (but not proven) that this contributed to May 2020 being exceptionally sunny in the UK, also very dry in places.

You won’t be surprised to read that autumn is essentially spring in reverse. September tends to still have a lot of ‘summer mode’ to it. Then, the polar vortex develops during Oct-Nov. So, the UK may see more variability in October and a wetter trend to November.

Overall, this would tie in with the UKCP projections discussed at the start of this blog entry. Alongside the warmer, drier summers, they predict milder, wetter winters.

It’s clear that industries in the UK should prepare for larger contrasts in rainfall between the summer and winter seasons. More like a monsoonal climate, but less reliable and without such large differences in rainfall amounts and frequency. There will be increasing importance to efficient stockpiling of water during the Nov-Mar period.

James Peacock MSc
Head Meteorologist at MetSwift

* The main Atlantic current, the Atlantic Meridional Overturning Circulation (AMOC) transports warm, saline (salty) water to these most northern regions, where it cools and then sinks. It then returns equatorward, warms and rises – so completing the circulation.

As the climate warms, increased sea ice melt adds more freshwater to the northern North Atlantic Ocean.

Although driven by cooling and warming, the AMOC is not immune to interference by freshwater. You see, lower salt content reduces water density – and it’s possible for such reductions to outweigh the gains from cooling, reducing the rate at which water sinks. This slows the whole circulation, meaning less upper ocean heat transport from the tropics. The result is a cooler region to the south of Greenland.

** This is explored in great detail by this 2020 study by AlinaGăinuşă-Bogdan et al.

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