Drip by drip, steady ice loss in the Arctic and sub-Arctic is injecting enormous quantities of meltwater into the North Atlantic Ocean. Researchers have now shown how all that fresh water ultimately drives more extreme summer weather over Europe. These results might enable better long-term weather predictions in Europe, the team concluded.
Every year, the Arctic and sub-Arctic lose several hundred cubic kilometers, on average, of both sea ice and glacial ice due to rising temperatures. The fresh water liberated by all that melting eventually enters the North Atlantic, where it aggregates into so-called freshwater anomalies. Those structures, which tend to lurk on the surface of the ocean because of their relatively low density, can measure thousands of kilometers in extent and several tens of meters deep in winter.
First Fresh Water, Then Heat Waves
Freshwater anomalies in the North Atlantic have been shown to precede European heat waves. However, the mechanism—or mechanisms—behind that linkage has long remained unknown, said Marilena Oltmanns, an ocean and climate scientist at the National Oceanography Centre in the United Kingdom. That question is increasingly relevant given rapidly warming temperatures in the Arctic and recent weather extremes—such as heat waves and droughts—that have occurred in Europe, Oltmanns and her colleagues suggested.
To track the presence of freshwater anomalies, the team used satellite-derived sea surface temperature data available since 1979. That technique worked, said Oltmanns, because temperature and salinity were strongly correlated. The researchers opted not to use salinity data to trace freshwater anomalies because such data have been available over large areas only since 2009. Furthermore, many of those measurements are known to be biased, Oltmanns said. “The biases are about the same order of magnitude as the interannual variability.”
The researchers also combined data on the ocean’s temperature, salinity, and currents with atmospheric data tracing winds, pressure, temperature, and precipitation.
Gushing from Greenland
Oltmanns and her collaborators inferred that some of the freshwater anomalies they observed were caused by runoff from Greenland. The seasonal signals that they noted were consistent with melting in the summer and then propagation of that meltwater south, via ocean currents, the following winter. Freshwater anomalies have a pronounced effect at latitudes between 25°N and 65°N, the team showed.
By virtue of residing in the uppermost layer of the ocean, freshwater anomalies function somewhat like a barrier, Oltmanns said. “The fresh water inhibits heat exchange between the deeper ocean and the air.” Thanks to their relative thermal isolation, freshwater anomalies also cool more quickly in the autumn and winter than surrounding water masses, the researchers found.
That situation sets up sharp north–south gradients in sea surface temperature, which in turn promote the development of stormy weather and a preponderance of westerly winds along a freshwater anomaly’s southern boundary, the team showed.
Eddies Go North
Those winds then drive pressure changes that form eddies that same winter, Oltmanns and her colleagues found. Those swirling bodies of water stretch in a band across the Atlantic, and their presence can shift the North Atlantic Current—an eastward-flowing extension of the warm Gulf Stream—to the north by several hundred kilometers, the team showed. “It’s at an anomalously northward location,” said Oltmanns.
“Southern Europe will become warmer and drier, no question.”
When westerly winds blow the following summer, they follow the temperature front between the now-shifted North Atlantic Current and colder subpolar waters. “The location of the freshwater anomaly in winter has a lot of implications for the location of the winds in subsequent summers,” Oltmanns said. And those same winds are deflected even farther north when they make landfall over Europe, the researchers found. All that northward deflection forms atmospheric anomalies associated with high-pressure systems, which are in turn linked to warmer and drier weather patterns.
When Oltmanns and her colleagues focused on the 10 warmest and coldest summers in Europe over the past 4 decades, they found that warmer summers tended to follow winters characterized by larger freshwater anomalies, colder freshwater anomalies, and stronger northward deflection of westerly winds. These results were published in Weather and Climate Dynamics.
Given that rates of melting in the Arctic and sub-Arctic are likely to increase in the future, it makes sense that warmer conditions in Europe will also be on the rise, Oltmanns said. That’ll be particularly true in more southerly regions, she added. “Southern Europe will become warmer and drier, no question.”
The Lure of Predictability
“European summer weather is predictable at least a winter in advance.”
These findings offer an important way forward when it comes to predicting weather in Europe, Oltmanns said. Because the drivers of conditions in June, July, and August in cities such as Barcelona, Spain, and London are set in motion many months prior, it should be possible to make long-term weather forecasts, she said. “European summer weather is predictable at least a winter in advance.”
“These results can help us understand some of the impacts of future climate change,” said Melissa Gervais, a climate dynamicist at Pennsylvania State University who was not involved in the research. And there’s incredible utility to such weather predictions, she added. “If I’m a farmer and I know I’m going to be experiencing heat and drought, I might change what crops I have,” she said. “I might think about my irrigation strategy.”
Oltmanns and her colleagues hope to expand their analysis to freshwater anomalies that occur in the North Pacific. Similar weather forcing might also be occurring over parts of the United States and Canada, she said. “There are so many open questions.”
—Katherine Kornei (@KatherineKornei), Science Writer