2015 saw some exceptional temperatures over the North Atlantic and Europe. During the first half of the year, sea surface temperatures in the North Atlantic waters to the west of the UK were as much as 2 degrees colder than average. Then by summer (June, July and August), Central Europe experienced one of the hottest heat waves to occur in the last 65 years. Polish capital Warsaw sweltered, with temperatures peaking at over 32°C for more than seven days; power supplies struggled to keep up with demand as air conditioning ran at full tilt. Meanwhile, temperature records were broken across Germany, and heat warnings were issued in Poland, Hungary, Switzerland and Spain.

Was it just coincidence that the North Atlantic had a cold blob and then Europe sizzled under a heat wave, or was there a connection? In order to answer this question, Aurélie Duchez from the UK’s National Oceanography Centre and her colleagues analysed the North Atlantic sea surface temperatures associated with all European heat waves since 1980.

The data showed that central European heat waves followed on from a similar sea surface temperature pattern. "We found that the warmest central European summers of 1992, 1994, 2003, 2012 and 2015 were preceded by cold surface water temperatures in the same area as the 2015 cold blob," said Duchez.

So what is creating the cold North Atlantic blob, and why might this trigger a heat wave over Europe? Studying the surface ocean and air temperatures over the North Atlantic in recent years indicated that the cold blob emerged due to rapid ocean heat loss that started back in the winter of 2014. "Our analysis suggests that this extreme ocean heat loss was driven by atmospheric circulation changes in the preceding two winters, combined with the re-emergence of cold ocean water masses," explained Duchez, whose findings are published in Environmental Research Letters (ERL) .

Meanwhile, further south a warm ocean temperature anomaly developed off the Iberian Peninsula and Morocco. These contrasting ocean regions of ocean temperature appear to have helped to pin the high-level atmospheric jet stream onto one unwavering route to the north of Europe during summer 2015. This meant that the air to the south was able to settle and stagnate, bringing high pressure and searing temperatures over central Europe for many days at a time.

Duchez cautions that a cold blob alone doesn't guarantee a heat wave. "Several conditions have to be met, like a deficit in soil moisture (low rainfall in the preceding months), a persistent high pressure centre above the heat wave region and high temperatures exceeding a certain threshold for a minimum of five days," she said. "Being able to clearly understand the drivers of these atmospheric and land conditions is an outstanding research question."

Having established a link between ocean temperature anomalies and European heatwaves, Duchez and her colleagues now want to better understand the mechanisms driving these ocean and atmospheric weather patterns. Ultimately they hope that this will lead to better forecasts of European heat waves "Our study suggests that the forecasting of European heat waves can potentially be improved by looking out for early warning signs in the ocean," Duchez said.

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