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Our Changing Weather

Why have recent winters changed in the UK?

 

The answer to why winters in the UK (and to a certain extent, the northern hemisphere as a whole) have changed is not a certain one. Meteorology is not an exact science and, as a result, it is theorised that there are numerous interlocking causes behind what drives both our shorter and longer-range weather patterns.

 

Lets start with what has changed. The first really notable cold spell for many years occurred in early February 2009. Winds switched to an easterly direction off of a very cold near continent, and this very cold air met some rather moist air from the Atlantic ocean tracking to the south of the UK, and where the two met, a battleground scenario ensued. The UK remained on the northern side of this battleground, and many inches of snow fell across parts of England and Wales in particular. These changes occurred as the jet stream (the rapid winds that flow up to or over 100mph at around 30,000ft above the earth’s surface) took a course much further south than usual during the month of February, allowing a blocking high pressure to set up across Scandinavia, pushing the cold west across the UK and preventing the Atlantic weather systems from moving NE across the UK.

 

There is a phenomenon behind this movement in the jet stream which had been identified by a researcher named Judah Cohen (who has written extensively about this subject), and initially picked up back in 2008 by the online amateur weather community called a Sudden Stratospheric Warming (SSW). The jet stream usually flows almost directly from west to east across the Atlantic and into the UK in a horizontal line, moving low pressure in across the UK. This is lead during the winter by an area of almost constant low pressure called the polar vortex. The polar vortex is a series of low pressure systems that encircle the polar regions during the winter as the poles cool significantly. This prevents colder arctic air from flooding south into the mid latitudes, and as the UK stays on the southern side of these low pressure systems (or, to look at it another way, the southern side of the jet stream) we remain under the influence of westerly winds from the Atlantic – making things rather milder due to the warmer ocean temperatures created by Gulf Stream in the North Atlantic. This cold dense air that forms the polar vortex stretches all the way up into the Stratosphere – the second layer up in the earth’s atmosphere (the first being the Troposphere), at a heights upwards of 30,000 ft. What happens occasionally is that the Stratosphere is warmed significantly, reverting in temperature profile from around -70c up to or even slightly above 0c. This warming effect destroys the polar vortex in the Stratosphere, reversing the wind direction from westerly (under low pressure) to easterly (under high pressure), and over time this downwells into the Troposphere, destroying our polar vortex, allowing the jet stream to meander somewhat to align itself more north-south (as opposed to the default west-east), which allows the colder air from the polar regions to flood south, with blocking high pressures setting up within the meanders of the jet stream, and if the UK ends up on the northern side of one of these meanders, then we are subjected to the colder air flooding in from the north and east.

 

What should be made clear though is that SSW’s are not the cause of every cold spell we receive in the UK, nor are they a guarantee of a cold spell for the UK and NW Europe as a whole, as it all depends on where blocking high pressures set themselves up. For example, the UK could end up on the ‘wrong side’ of one of these blocking highs, with the cold air from the north flooding south into eastern Europe, and the UK being on the opposite side of the high pressure with a warmer southerly wind direction. This sort of detail, however, is slightly predictable based around other research carried out by Judah Cohen on snow cover advance during October across Siberia (this creates an index known as the snow advance index) and all sorts of complex feedback loops are created from this, resulting in particular types of planetary scale waves being created through the late autumn/early winter period, usually leading to analogous pressure patterns. But, for example, the second coldest December in recorded history, December 2010, occurred not due to one of these SSW’s, but due to the Stratosphere being warmer than usual to begin with during the winter period (usually the Stratosphere cools rapidly through November and into December – during that particular year it was reluctant to do so).

 

So then, is there a bigger overall cause of these changes that have occurred to really, noticeably, affect our winters since 2009? Whilst again not an absolutely conclusive factor, the biggest influence on our current climate comes from the sun. The sun goes through roughly 11 year cycles, peaking during the middle of the cycle, and declining thereafter, based upon how many earth-facing sunspots there are, with more sunspots creating the peak period around the middle of the cycle. These cycles vary greatly over time, with two historical periods, called the Dalton and Maunder minimums, being notable for being the lowest recorded sunspot cycles (i.e. their peak was much lower relative to previous and following cycles, and as a result their solar minimum period was very low indeed). To give an idea of the theorised affect on winter temperatures from these low solar cycles, these two periods (during the 15th and 19th century’s) coincided with the Thames frost fairs (obviously nowadays with the river being narrower and faster flowing such freezing would be unlikely to occur – but it gives you an idea of the affect these low solar periods have on our climate).

 

During 2009/2010, rather unexpectedly, the solar minimum period lasted many months longer than expected, and yet again our colder winters coincided with this minimum period. The theory created by Japanese researchers is that a longer than usual solar cycle (such as the end of solar cycle 23 in 2009/2010) is a warning of an impending solar minimum period – indeed these longer than usual solar cycles occurred during the onset of the Dalton and Maunder minimums. Since then, the sunspot count during our current cycle (cycle 24) has been less than half that of the peak of the previous cycle during the 1990’s/2000’s, and it is now projected that cycle 25 will be the lowest since the Dalton and Maunder minimums.

 

We will get our ultimate answer as to whether this occurs sometime later in this decade, but for now, even under solar maximum conditions (with the peak of the current cycle around 2013) we have experienced more frequent snowfall and colder weather than we were use to during the 90/00’s period, and so the expectation is that as we approach the winters later in this decade, they could be as severe as those in 2009/2010 once again.

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