Met Office figures show that, with a mean temperature of 17 °C, July 2013 was the third warmest in the national record going back to 1910, behind 2006 (17.8 °C) and 1983 (17.3 °C). Much of this weather can be attributed to the movement of the jet stream which spent most of the month to the far north of UK.
This is exactly the kind of event the Potsdam Institute for Climate Research was discussing in a paper in February when they investigated 32 years of extreme weather events. These were linked to the way the jet stream has changed its normal pattern. Instead of wandering around the northern hemisphere between the Arctic and the tropics in a series of gentle and continuously moving waves caused by differences in land and sea temperatures, the jet stream appears occasionally to get stuck.
The result is that we can get different sorts of extreme weather for a few weeks. Last year it was a great deal of rain – this year it has been hot sunshine. The researchers concluded that manmade climate change was the major suspect, creating conditions that induced the jet stream to get stuck and causing weather related disasters. Their forecast was to expect more of these extremes.
The key question is what is causing the jet stream to shift in this way?” asked Professor Stephen Belcher, head of the Met Office Hadley Centre in June 2013 when he helped to organise a workshop of 26 experts to discuss the recent run of unusual seasons in Europe.
One possible answer lies in what is happening in the Arctic, which has seen temperature increases some two or three times higher than elsewhere in the world, and leading to dramatic and unprecedented loss of sea ice and melting of the massive Greenland ice sheet. Scientists know that the jet stream is driven by temperature differences between the Arctic and latitudes further south. The smaller this temperature difference, the weaker the jet stream, and the weaker the jet stream the more likely it is to meander as travels around the northern hemisphere, said Jennifer Francis of Rutgers University in New Jersey. “Certainly it all fits together. We are seeing big fluctuations in the path of the jet stream and where it gets into a meandering the north-south waves tend to stand still in one place, bringing extreme weather because whatever weather you are getting tends to hang around for a long time,” Dr Francis said.
This increasingly unpredictable weather caused by fluctuations in the jet stream is already bringing new challenges to UK gardeners. After one of the coldest winters on record followed by one of the hottest July months since 1910, it means that plants in our gardens are having to deal with many extremes. Which plants are the winners and which are loosers in a changing climate is a debate that is starting to gather momentum in horticultural circles.