When the alarm rings in Tasiilaq, a settlement in southeast Greenland, its approx. 2,100 inhabitants know, that they better seek shelter quickly and be prepared to stay home for several days. For the alarm can precede one of the most dangerous stroms, called piteraqs, which translates into 'sudden and cold wind coming out of the fjord'. Meant is Sermilik Fjord, a major East Greenland fjord adjacent to Tasiilaq. Sermilik Fjord connects Helheim Glacier, one of the largest outlet glaciers in Greenland, with the ocean.
Everything unlashed is prone to fly around during a piteraq. The storms can throw boats out of the water, shift heavy containers, uncap houses and break even thick landfast ice inside Sermilik Fjord and at the coast. The older inhabitants still have the catastrophe of the winter in 1970 on their minds, when one of the strongest piteraqs occured and wind gusts of over 300 km/h caused significant destruction in their town. Not only the high wind speeds are dangerous, but also the low temperatures, that can fall beneath -20 degree. People that do not find shelter on time can freeze to death.
Despite their power, only little was known about piteraqs until recently. Since the measuring instruments at weather stations can break down under the extreme conditions, wind speeds during piteraqs are easily misjudged. Moreover, airborne measurements are impeded because of the flight restrictions under the extreme conditions. Even so, scientists were now able to systematically investigate the strong wind events using computer simulations, weather station observations and reanalysis models. Reanalysis models use the existing observations and physical laws of nature to reconstruct data even at locations where no direct observations exist. Thereby, they found that the storms are triggered by large-scale low pressure systems that have diameters of thousands of kilometers. If such a cyclone moves between Iceland and Greenland, it is associated with winds that flow from the vast ice sheet inland towards the coast. As cold air is denser than warm air, the cold air from the central ice sheet is accelerated downwards by gravity over the steep coasts. The pronounced layering with the cold air layer over the surface and the warmer air above can trigger a complex phenomenonm, called mounain waves. Similar to waves on the sea, that occur on the boundary between air and water, there are waves in the atmosphere, that move along the boundary between different layers of air. These waves also occur during piteraqs, and they can become so large that they break – similar to water waves that break when they roll onto the beach. The waves in the atmosphere, have a much larger amplitude (wave height), however, that can reach several kilometers and extend into the stratosphere. Mountain wave breaking in southeast Greenland results in a significant acceleration of the winds. Another important aspect is the morphology of the topography. Since Tasiilaq is located inside a valley the cold air flow is funneled by the mountains, which can lead to wind gusts above 300 km/h.
The impact of the icy winds is not restricted to the land surface. When they move above the water, they extract heat from the ocean. Thereby, the water is cooled and loses buoyancy and can sink to great depths. The sinking, or convection, of the surface water southeast of Greenland is an important component of the global ocean circulation as it connects the atmosphere with the deep ocean. Gases in the atmosphere, including carbon dioxide, are dissolved in the surface water and exported with it into the deeper ocean. Thus, the ocean southeast of Greenland plays an important role for our climate, and piteraqs influence this region through their strong and icy winds.