The last decade has been one of extremes when it comes to summer weather. In 2003 Europe experienced its hottest summer since at least 1540, causing a health crisis and potentially contributing to 70,000 deaths across Europe. Then in 2007 the UK was hit by major flooding, with June rainfall more than twice its average, and some places receiving a month’s worth of rain in 24 hours. Roll on to 2012 and much of northern Europe was deluged with rain while southern Europe experienced drought. What is driving these dramatic swings in summer weather?

To answer this question, Tim Woollings from the University of Oxford and colleagues at the University of Reading analysed the path of Atlantic storms using datasets spanning the period 1948 to 2011. As well as tracking the path that storms took across the Atlantic, the researchers also monitored sea level pressure, sea surface temperature, and rainfall data across Europe.

Immediately they noticed that Atlantic storms tend to follow one of two distinct paths. "In one state the storm track goes straight over the UK and into Northern Europe, giving wet summers in the UK and much of Europe, from France up to southern Scandinavia. In the other state the storm track passes north of the UK, going over Iceland and into the Nordic Seas. The UK and much of central and northern Europe is dry, but slightly unexpectedly the Mediterranean – Spain, Italy, Greece – is wetter," explained Woollings.

Which track the storms end up on appears to be linked to North Atlantic ocean temperatures: when waters are warmer than average the storm track shifts south, bringing increased rainfall over the UK and northern Europe and a decrease in blocking patterns in the weather – settled periods of weather where high-pressure systems block approaching lows. Meanwhile, when water temperatures are cooler than average, the storm track hovers over Iceland and the Nordic Seas, leaving the UK and northern Europe with a greater chance of blocking patterns and the resultant dry warm weather. Their findings are published in Environmental Research Letters (ERL).

Quite why summer weather flips between these two states is not clear. "One hypothesis is that it might be related to the layout of the ocean temperatures and the land masses," said Woollings. And, as yet, it’s hard to see a clear pattern in the flips from one state to another. Over the last decade the southward-shifted storm track has tended to dominate, bringing a string of wet summers to the UK. But the summer of 2013 bucked the trend, demonstrating how random the process can be.

Most likely the Atlantic Multidecadal Oscillation has a significant influence over the decadal timescale, and this is something that Woollings and his team intend to investigate further. By understanding how variable Europe’s summer weather can be, the scientists hope to be able to prepare us all better for extreme weather of the future.

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