The UK’s wet summer, the jet stream and climate change*

There’s no disputing it has been a very disappointing summer so far in 2012 – with the wettest June for over a century followed up by a very wet start to July.

In fact, barring a warm and dry spell towards the end of May, the weather has been persistently dull and wet since April – which was also the wettest in records dating back to 1910.

Our weather here in the UK is complex and determined by many different factors, including the position of the jet stream.

This is the narrow band of fast moving winds which runs from west to east across the Atlantic high up in the atmosphere.

How does the jet stream affect UK weather?

Weather (or low pressure) systems bearing rain and unsettled conditions move across the Atlantic on a regular basis. The jet stream guides these systems, so its position is important for UK weather.

In summer, we would expect the jet stream to be north of the UK – dragging those weather systems away from our shores to give us relatively settled weather.

So far this year it has been to the south of the UK, guiding those systems straight to us. This is the position we’d normally expect the jet stream to be in during winter, when we are more accustomed to these wet conditions.

So why is the jet stream stuck so far south?

The jet stream, like our weather, is subject to natural variability – that is the random nature of our weather which means it is different from week, month or year to the next.

We expect it to move around and it has moved to the south of the UK in summertime many times before in the past. It has, however, been particularly persistent in holding that position this year – hence the prolonged unsettled weather.

This could be due to natural variability – a bad run of coincidence, if you will – but climate scientists are conducting ongoing research to see if there are other factors at play.

Changes in sea surface temperatures due to natural cycles may be playing a part, but there is more research to be done before anyone can establish how big a role they play.

Research has also suggested that reducing amounts of Arctic sea-ice could be affecting weather patterns, but more research needs to be done to confirm this link. Currently Arctic sea-ice is at a record low for this time of year.

Is climate change playing a role?

In the long term, most climate models project drier UK summers – but it is possible there could be other influences of a changing climate which could override that signal on shorter timescales.

If low levels of Arctic sea ice were found to be affecting the track of the jet stream, for example, this could be seen as linked to the warming of our climate – but this is currently an unknown.

The Met Office Hadley Centre, working with climate research centres around the world, is making strides in determining how the odds of extreme weather happening have been influenced by climate change.

However, it is very difficult to do this type of analysis with such highly variable rainfall events, so it may take many years before we could confirm how the odds of this summer’s wet weather happening have been altered by greenhouse gases.

We do know that the warmer air is, the more moisture it can hold. We have seen a global temperature increase of more than 0.7 deg C (since pre-industrial times) and this has led to an increase of about 4-5% in atmospheric moisture.

This means that when we do get unusual weather patterns such as we’re seeing now, it’s likely there will be more rainfall than the same patterns might have produced in the past. In short, it seems when it does rain, it is heavier.

Taking into account this effect, perhaps it’s not surprising new records like those for this April and June are being set. In fact, the wettest July and November in the records dating back to 1910 happened in 2009, making a total of four record wettest months in the past four years. If wet months occurred randomly, we would expect only one record to have been broken since 2006.

For temperature, April (2011), May (2008), July (2006), September (2006) are all recent warmest records. Again, this is much more frequent than would be expected if temperatures were not rising.

What about elsewhere in the world?

Looking at the bigger picture, the jet stream may be having an impact elsewhere in the northern hemisphere.

It is stuck in a persistent pattern of waves, with one of these ‘waves’ taking it to the south of the UK.

Figure shows upper level wind patterns in early July 2012, with the northern hemisphere jet stream marked with arrows.

The figure gives a picture of the upper level winds for the first week in July, but the wavy nature of the jet stream has been persistent throughout June.

Meanders of the jet north and south can be seen across the US, the Atlantic and into Europe.

While the wet weather in the UK has been under a southward meander of the jet stream, the recent Russian floods near the Black Sea appear to have been beneath the next trough to the east.

The US heat wave is also beneath a northward meander and a ridge of high pressure.

* This article has been written in collaboration with the Walker Institute for Climate System Research, University of Reading.