Close links between Europe and the Arctic
Warm, moist air masses from the lower latitudes are the main energy source for the Arctic atmosphere in winter; they have a significant influence on soil temperatures and can produce sea-ice melting. Because climate and weather models still have considerable difficulties when it comes to accurately depicting key processes involved in the transformation of these air masses, AWI researchers recently summarised the current state of knowledge and identified remaining gaps; their study has just been released in the journal Nature Geoscience.
After the very warm and, in many places, very dry spring and summer this year, it’s easy to lose sight of how and when this extreme weather began. Though the preceding winter was remarkably mild, thanks to frigid air from the Arctic latitudes, in late February winter found its way to Central Europe after all, and was followed by a frosty March. This cold-air intrusion was dubbed the “Beast from the East” in Great Britain, where it produced heavy snowfall in Scotland and southeast England, and shortly thereafter a major snowstorm that struck Ireland, Wales and southwest England. Before the cold snap hit, unusually warm air masses had reached as far as the northern tip of Greenland, producing temperatures of roughly 8 degrees Celsius in Iceland and Svalbard in the heart of winter – similarly mild conditions to those found in the Mediterranean at the same time. In the latest issue of the online journal Nature Geoscience, Dr Felix Pithan from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and his colleagues explain the effects that these intrusions of warm air have in the atmosphere, and how they are related to cold-air intrusions in the mid-latitudes.
Normally, broad expanses of Europe essentially lie in a giant belt, in which powerful westerly winds transport moist air eastward from the Atlantic. In February, however, an upper-atmospheric high-pressure area between Scandinavia and Svalbard blocked these air currents. Instead of blowing from west to east, in the western part of the area warm and moist air from southern regions now flowed to the north, while to the east, icy air from the Arctic Ocean made its way acrosss Scandinavia to Central and Western Europe, ultimately even reaching Greece, Italy and the Pyrenees. “These intrusions of warm air flowing north and cold air moving south occur regularly in the winter, and frequently influence the weather and climate,” says AWI researcher Felix Pithan, explaining the scientific background of these events.
During this time, the sun rarely if ever rises above the horizon; from November to February, the darkness of the Polar Night envelops the Arctic. As a result, the air above the sea ice and snow-covered land masses becomes much colder. When the moist and still mild air from the south encounters these deep-freeze-like (ca. minus 20 degrees Celsius) conditions, it also cools. The colder the air becomes, the less moisture it can hold. As a result, condensation soon forms tiny water droplets, which are suspended in the air and form clouds.
Though sea ice may seem quite cold to us human beings, it still contains a small amount of heat, which is released into the atmosphere and up into space during clear nights, cooling the High North even more. But the clouds formed by the mild air currents from the south don’t allow this heat to escape; they hang over the ice and snow like a thick shroud, causing surface temperatures to rise.
Within the clouds, as temperatures sink, many of the water droplets gradually transform into ice crystals – and just like the liquid water, these, too, are suspended in the air. The remaining water droplets continue to trap the heat from the surface; as such, these mixed clouds of ice crystals and water droplets also serve as an insulating blanket over the Arctic. If the clouds grow even colder, eventually the very last of their water droplets also become ice crystals. But these ice clouds allow far more of the radiant heat from the surface to pass through than the mixed clouds. As a result, the clouds grow even colder, and the remaining ice crystals flutter to the ground as snowflakes. The air becomes drier, and the clouds begin breaking up.
Out on the sea ice, the fluffy snow from the clouds has an additional effect: the liquid saltwater below the ice – even though it might be as cold as minus 1.9 degrees Celsius – still contains a small amount of heat energy that rises up through the ice. “If, however, there is a layer of powdery snow on the ice, it almost completely blocks the heat, and the air over the snow grows even colder,” explains Felix Pithan. This Arctic air, which is now bitterly cold and extremely dry, finally heads south, where it can form cold-air intrusions, like the one that brought this February to such a frigid end in Central Europe. Much more often, these Arctic air masses find their way to the ocean. Over the North Atlantic, they then form polar lows, which can spark heavy snowstorms out to sea and on the coast of Norway.
“The exact processes involved in the final transition from mixed clouds to pure ice clouds are an aspect we plan to investigate in more detail,” says Felix Pithan. The MOSAiC expedition – during which, starting in September 2019, the AWI research icebreaker Polarstern will spend an entire year drifting through the Arctic Ocean while locked in the ice – will offer an excellent opportunity to do so. The more than 600 participating researchers, who hail from 17 countries, will be resupplied at regular intervals by aeroplanes and additional icebreakers, allowing them to take a closer look at countless processes and phenomena, including the clouds over the Arctic. With the insights gleaned during the expedition, the researchers hope to not only gain a better understanding of the weather in the High North and how it impacts Central Europe, but to also refine current climate models, the majority of which lack detailed data on the formation of these ice clouds in the Arctic.
F. Pithan, G. Svensson, R. Caballero, D. Chechin, T. Cronin, A. Ekman, R. Neggers, M. Shupe, A. Solomon, M. Tjernström, M.Wendisch: Role of air-mass transformations in exchange between the Arctic and mid-latitudes. Nature Geoscience (2018). DOI: 10.1038/s41561-018-0234-1