Global warming disrupts weather in many ways, but Europe’s string of record-breaking hot and dry summers has defied an easy link to climate change. Climate models do show Europe warming faster than the rest of the planet, but the recent scorchers were triggered by peculiar weather conditions: masses of hot, dry air parked over the continent, blocking any incursions of cool or moist relief.
A new study suggests global warming could be responsible after all. It proposes a chain of events that starts with an infusion of meltwater from shrinking Arctic ice, which ultimately alters massive ocean currents and regional air circulation patterns. The study, published late last month in Weather and Climate Dynamics, also suggests monitoring such patterns, which are apparent during winter months, could allow forecasters to predict extreme summer heat months to years in advance. “Physically what the authors write makes sense,” says Judah Cohen, a weather scientist at Atmospheric and Environmental Research.
Arctic melting—from both floating sea ice and glaciers on Greenland and elsewhere—is adding roughly 6000 cubic kilometers of water or more to the ocean per decade, more than enough to fill the Grand Canyon. As that freshwater pours into the North Atlantic Ocean, it sits on top of heavier ocean saltwater and impedes mixing. With less heat being stirred in from below, the surface water gets colder than usual during the fall and winter months, says Marilena Oltmanns, a climate scientist at the U.K. National Oceanography Centre. This phenomenon may explain the so-called “cold blob,” a patch of sea in the North Atlantic that NASA modeling suggests is one of the few spots on Earth getting colder.
To explore how the freshwater influx might be affecting weather, Oltmanns and her colleagues needed new salinity measurements in the North Atlantic, which satellites and ocean models struggle to accurately quantify. So they developed a way to combine data from satellites, buoys, and weather stations to provide a proxy for salinity. They found that Arctic freshwater levels varied sharply from winter to winter, depending on the extent of melting during the previous summer.
Then, after studying 4 decades of ocean and atmospheric data, the team hypothesized how the freshwater and the resulting cold blob might affect European weather. North Atlantic storms draw energy from the boundary between the blob and warm waters farther south, Oltmanns says. When the freshwater-induced cold blobs were more intense, the boundary was sharper, the data showed. The result was stormier winters with more powerful westerly winds.
Because of the Coriolis effect, a phenomenon linked to Earth’s rotation, winds can sweep water laterally. As a result, the stronger westerlies move a warm ocean flow called the North Atlantic Current—an extension of the Gulf Stream—northward from roughly 45°N to 60°N. That shift can persist into subsequent summers. And like a barrier, this warm current, curling up and around the British Isles, deflects the jet stream, allowing a mass of hot, dry air, sometimes called a high-pressure blocking system, to camp out over Europe.
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